***  The Linux MTD, JFFS HOWTO ***
         (work in progress, please contribute if you have anything)

$Id: mtd-jffs-HOWTO.txt,v 1.16 2001/08/13 23:17:55 dwmw2 Exp $
Last Updated: <see CVS Id above>
Compiled/Written By: Vipin Malik (vipin@embeddedLinuxWorks.com)
Other author's contributions as noted in the text.

**ABOUT:

This document will attempt to describe setting up the MTD (Memory
Technology Devices), DOC, CFI and the JFFS (Journaling Flash File System)
under Linux versions 2.2.x and 2.4.x

This is work in progress and (hopefully) with the help of others on
the mtd and jffs mailing lists will become quite a comprehensive
document.

Please mail any comments/corrections/contributions to
vipin@embeddedLinuxWorks.com
Please DO NOT send questions to him directly, rather send them to the
mailing lists (see below).

**************************** NO WARRANTY *****************************

# This HOWTO is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
# If you break something you get to keep both parts! Follow these
# directions at YOUR OWN RISK.
# See the GNU General Public License for more details.

**********************************************************************




*** Getting Started:

If you want to seriously design a project with MTD/JFFS please
subscribe to the respective mailing lists. Both are managed by majordomo.


MTD:
  To subscribe, see http://lists.infradead.org/mailman/listinfo/linux-mtd-cvs
  or send an email to linux-mtd-request@lists.infradead.org containing the
  line "subscribe" in the body.

  DO NOT SEND SUBSCRIBE REQUESTS TO THE LIST ITSELF, which is at 
  linux-mtd@lists.infradead.org.

JFFS:
  To subscribe, send an email to majordomo@axis.com containing the line
  "subscribe jffs-dev" in the body. 

  DO NOT SEND SUBSCRIBE REQUESTS TO THE LIST ITSELF, which is at
  jffs-dev@axis.com.

The home page for the two projects are located at:
MTD/DOC/
 http://www.linux-mtd.infradead.org/
JFFS
 http://developer.axis.com/software/jffs/ 

The MTD mail archive is at:
 http://www.linux-mtd.infradead.org/list-archive/

The JFFS mail archive is at:
 http://mhonarc.axis.se/jffs-dev/threads.html

<blatant plug by author>
A general, vendor agnostic, non commercial site for Embedded Linux 
Systems is at:

 http://www.EmbeddedLinuxWorks.com

(Here you will find articles about using IDE flash disks
in embedded systems, reports of JFFS/JFFS2 power fail reliability
tests, tips on using JFFS systems in your design, details on how 
to boot the x86 Linux kernel from FLASH without using a BIOS 
and (hopefully in due course) a vibrant community of developers 
discussing issues related to embedded Linux with each other 
on the message boards ;)

** MTD Flash Device Database: **
In the above mentioned site, you will also find a MTD Flash device
database. This database contains a list of flash devices successfully
working with the MTD drivers. If you manage to get a particular flash
device (or Disk On Chip etc.) to work with any MTD driver, please take
a few minutes to enter the relevant info in this database for the
benefit of future users. Anyone can make an entry or view any info there.

Access the MTD Flash database directly at:
http://www.embeddedLinuxWorks.com/db.html

** Power fail safe embedded database  **
There is a seperate project (with its own mailing list) going on to
develop a zero latency write, power fail safe (small) embedded
database to use on JFFS2. Read more on why we need such a beast at:
http://www.embeddedLinuxWorks.com/articles/db_project.html

</blatant plug by author>


*** Getting the latest code:

The entire MTD/DOC/JFFS (and some utils) source code may be downloaded
via anonymous CVS.

Follow the following steps:
1.Make sure that you are root.
2. cd /usr/src
3. cvs -d :pserver:anoncvs@cvs.infradead.org:/home/cvs login
(password: anoncvs)
4. cvs -d :pserver:anoncvs@cvs.infradead.org:/home/cvs co mtd

This will create a dir called mtd under /usr/src

You now have two options depending on what series of the Linux Kernel
you want to work with.
There is an extra step involved with the 2.2 series kernels as they do
not have any MTD code in them.

Note: 
Check under /dev/ If you do not have devices like mtd0,1,2 and
mtdblock0,1,2 etc. run the MAKEDEV utility found under mtd/util as:
#sh /usr/src/mtd/util/MAKEDEV
This will create all the proper devices for you under /dev


** With 2.2.x series kernels:

(Note that as far as I can tell, mtd and jffs does not work as modules
under the 2.2.x series of kernels. If you want to do modules I would
recommend that you upgrade to the 2.4.x series of kernels).

Get the 2.2.17 or 2.2.18 kernel source code from your favorite source
(ftp.kernel.org) and install the kernel under /usr/src/linux-2.2.x
with /usr/src/linux being a symbolic link to your kernel source dir.

Configure the kernel to your desired options (by doing a make config
(or menuconfig or xconfig), and make sure that the kernel compiles ok.

Download the mtd patch from:
ftp://ftp.infradead.org/pub/mtd/patches

Move the patch to /usr/src/linux and do

   patch -p1 < <patch file name here>

Make sure that the patch was applied ok without any errors.
This will add the mtd functionality to your basic kernel and bring the
mtd code up-to date to the date of the patch.

You have two choices here. You may do a make config and configure in
mtd stuff with the current code or you may want to get the latest code
from the cvs patched in.

If you want the latest CVS code patched in follow the 2.4.x directions
below.

** With 2.4.x series of kernels:

If you want the latest code from CVS (available under /usr/src/mtd)
do:
1. cd /usr/src/mtd/patches
2. sh patchin.sh /usr/src/linux

This will create symbolic links from the
/usr/src/linux/drivers/mtd/<files here> to
the respective files in /usr/src/mtd/kernel/<latest files here>

The same happens with /usr/src/linux/fs/jffs and
/usr/src/linux/include/linux/mtd

Now you have the latest cvs code available with the kernel. You may
now do a make config (or menuconfig or xconfig) and config the
mtd/jffs etc. stuff in as described below.




*** Configuring MTD and friends for DOC in the Kernel:

Do not use any mtd modules with the 2.2.x series of kernels. As far as
I can tell, it does not work even if you can get it to compile ok.

Modules work ok with the 2.4.x series of kernels.

Depending on what you want to target you have some choices here,
namely:


*** 1. Disk On Chip Devices (DOC):
For these, you need to turn on (or make into modules) the following:

* MTD core support

* Debugging (set the debug level as desired)

* Select the correct DOC driver depending on the DOC that you have.
  (1000, 2000 or Millennium). Note that the CONFIG_MTD_DOC2000 option is
  a driver for both the DiskOnChip 2000 and the DiskOnChip Millenium
  devices. If you have problems with that you could try the alternative
  DiskOnChip Millennium driver, CONFIG_MTD_DOC2001. To get the DiskOnChip
  probe code to use the Millennium-specific driver, you need to edit
  the code in docprobe.c and undefine DOC_SINGLE_DRIVER near the beginning.

* Unless you are doing something out of the ordinary, it shouldn't be 
  necessary for you to enable the "Advanced detection options for 
  DiskOnChip" option. 

* If you do so, you can specify the physical address at which to probe
  for the DiskOnChip. Normally, the probe code will probe at every
  0x2000 bytes from 0xC8000 to 0xEE000. Changing the
  CONFIG_MTD_DOCPROBE_ADDRESS option will allow you to specify a
  single location to be probed. Note that your DiskOnChip is far 
  more likely to be mapped at 0xD0000 than 0xD000. Use the real physical
  address, not the segment address.

  If you leave the address blank (or just don't enable the advanced 
  options), the code will *auto probe*. This works quite well (at 
  least for me). Try it first.

* Probe High Addresses will probe in the top of the possible memory
  range rather than in the usual BIOS ROM expansion range from 640K -
  1 Meg. This has to do with LinuxBIOS. See the mailing list archive for
  some e-mails regarding this. If you don't know what I am talking
  about here, leave this option off.

* Probe for 0x55 0xaa BIOS signature. Unless you've got LinuxBIOS on your
  DiskOnChip Millennium and need it to be detected even though you've 
  replace the IPL with your chipset setup code, say yes here.

Leave everything else off, till you reach...
User Modules and Translation layers:
* Direct char device access - yes
* NFTL layer support - yes
* Write support for NFTL(beta) - yes

Note that you don't need 'MTDBLOCK' support. That is something entirely 
different - a caching block device which works directly on the flash 
chips without wear levelling.

Save everything, make dep, make bzImage, make modules, make
modules_install

Note: If you downloaded the 2.4.x series kernels and your original
installed distribution came with the 2.2.x series of kernels then you
need to download the latest modutils (from
ftp.kernel.org/utils/kernel), else make modules_install or depmod -a
will fail for the new 2.4.x kernels.

Move everything to the right place, install the kernel, run lilo and
reboot.

If you compiled the mtd stuff into the kernel (see later section if
you compiled as modules- which is what I prefer as you don't have to
keep rebooting) then look for the startup messages. In particular pay
attention to the lines when the MTD DOC header runs. It will say
something like:

"DiskOnChip found at address 0xD0000 (your address may be different)"
"nftla1"

The above shows that the DOC was detected fine and one partition was
found and assigned to /dev/nftla1. If further partitions are detected,
they will be assigned to /dev/nftla2 etc.

Note that the MTD device is /dev/mtd0 and details are available by
doing a:

#cat /proc/mtd
dev:    size   erasesize  name
mtd0: 02000000 00004000 "DiskOnChip 2000"

/dev/nftla1,2,3  are "regular" block disk partitions and you may
mke2fs on them to put a ext2 fs on it. Then they may be mounted in the
regular way.

When the DiskOnChip is detected and instead of nftla1,2,3... you get
something like:

"Could not find valid boot record"
"Could not mount NFTL device"

...first make sure you have the latest DiskOnChip and NFTL code from
the CVS repository. 

If that doesn't help you, especially if the driver has previously
exhibited strange and buggy behaviour, and if the DOS driver built
into the device no longer works, then it's possible that you have a
"hosed" (that's a technical term) disk. You need to "un-hose" it. To
help you out in that department there is a utility available under
/usr/src/mtd/util called nftl_format.

DO NOT EVER USE THE nftl_format UTILITY WITHOUT FIRST SEEKING ADVICE
ON THE MAILING LIST. It will erase all blocks on the device,
potentially losing the factory-programmed information about bad
blocks. (Someone really ought to fix it one of these days - ed)

Essentially after your disk have been detected but complains about
"Could not mount NFTL device", just run
#./nftl_format /dev/mtd0 (if your device was installed under mtd0, see
cat /proc/mtd/).

You should unload the nftl driver module before using the nftl_format
utility, and reload it afterwards. Reformatting the NFTL underneath
the driver is not a recipe for happiness. If the driver hasn't
recognised the NFTL format, then it's safe - reboot or reload the
module after running nftl_format and it should then recognise it
again.

If your device "erasesize" is 8k (0x2000), then the utility will go
ahead and format it. Just reboot and this time the drivers will
complain about an "unknown partition table".

Don't worry. Just do:
# fdisk /dev/nftla

and create some partitions on them. TaDa! You may now e2fsck and
others on these partitions. Note that if you don't want more than one
partition you don't need to muck about with partitions at all - just
make a filesystem on the whole device /dev/nftla instead of
partitioning and using /dev/nftla1.

*** IF you compiled the mtd stuff as modules (What I prefer):
Make sure that you have done a depmod -a after you reboot with the
new kernel.

Then just
#modprobe -a doc2000 nftl mtdchar mtdblock

You have now loaded the core stuff. The actual detection takes place
only when you load the docprobe module. Then do
#modprobe -a docprobe

You should then see the messages described in the section
above. Follow the directions and procedures are outlined in the
section above (where you would have compiled the mtd/DOC stuff into
the kernel).





*** 2. Raw Flash (primarily NOR) chips

This are multiple (or just one) flash IC's that may be soldered on
your board or you may be designing some in. Unlike the DOC device,
these are usually linearly memory mapped into the address space
(though not necessarily, they may also be paged).

MTD can handle x8, x16 or x32 wide memory interfaces, using x8, x16
(even x32 chips (are they such a thing)?- confirm).

At present CFI stuff seems to work quite well and these are the type
of chips on my board. Hence I will describe them first. Maybe someone
with JEDEC chips can describe that.

You must use (for all practical purposes that involve writing) JFFS on
raw flash MTD devices. This is because JFFS provides a robust writing
and wear leveling mechanism. See FAQ for more info.

If you only want the file-system to be writable while you're developing,
but will ship the units read-only, it's acceptable to use the MTDBLOCK 
device, which performs writes by reading the entire erase block, erasing it,
changing the range of bytes which were written to, and writes it back to
the flash. Obviously that's not something you want happening in production,
but for development it's OK.


*** Configuring the kernel with MTD/CFI/JFFS and friends. 
Turn off all options for MTD except those mentioned here.

* MTD support (the core stuff)
* Debugging -yes (try level 3 initially)
* Support for ROM chips in bus mapping -yes
* CFI (common flash interface) support -yes
* Specific CFI flash geometry selection -yes
* <select they FLASH chip geometry that you have on your board>
* If you have a 32 bit wide flash interface with 8bit chips, then you
  have 4 way interleaving, etc. Turning on more than one option does
  not seem to hurt anything
* CFI support for Intel.Sharp or AMD/Fujitsu as your particular case
  may be.
* Physical mapping of flash chips - set your config here or if you
  have one of the boards listed then select the board as the case may
  be.

Then under "File systems" select:
* jffs and 
* /proc file-system support right under that.
* Select a jffs debugging verbosity level. Start high then go low.

Save, make dep, make bzImage, make modules, make modules_install, move
kernel to correct spot, add lilo entries, run lilo (or your fav. boot
loader) and reboot.

If you have compiled the stuff as modules then do (as root):
# depmod -a
# modprobe -a mtdchar mtdblock cfi_cmdset_0002 map_rom cfi_probe

This loads the core modules for cfi flash. Now we probe for the actual
flash by doing:
#modprobe -a physmap

Look at the console window (Note if you are telnet'd into the machine,
then the console may be outputting on tty0 which may be the terminal
connected to the graphics card). Being able to see the console is very
important. You may also view kernel console messages at
/var/log/messages (this depends on the distribution you are
using. This is true for Red Hat).

Don't be fooled by the message:
"physmap flash device:xxxxx at yyyyyyy"

This is just reporting what parameters you have compiled into the
system (see above under "Physical mapping of flash chips".

If your flash is really detected then it will print something like:
"Physically mapped flash: Found bla-bla-bla at location 0".

If no device is found, then physmap will refuse to load as a module!
This is not a problem with compiling it as a module or with physmap or
modprobe itself. Unfortunately this is the hard part. You have to dive
into the routine "do_cfi_probe()" called from physmap.c.

Caution! physmap.c uses ioremap() to map the physical memory into an
area of logical memory. If your processor has a cache in it, then
modify physmem to use ioremap_nocache(), else you will tear your hair
out as your flash chips will never be detected.

This routine is called cfi_probe() and is in the file "cfi_probe.c"
under mtd/kernel/

Sprinkle the file with printk's to see why your chips were not
detected. If your chips are detected, then when you load physmap (by
doing a "modprobe physmap", you will see something like:

"Physically mapped flash: Found bla-bla-bla at location"

Now, the chips have been registered under mtd and you should see them
by doing a:
#cat /proc/mtd



*** Putting a jffs file system on the flash devices:

Now that you have successfully managed to detect your flash devices,
you need to put a jffs on them. Unlike mke2fs there is no utility that
will directly create a jffs file-system onto the
/dev/mtd0,1,2... device.

You have to use a utility called mkfs.jffs available under mtd/util

Get a directory ready with the stuff that you want to put under
jffs. Let's assume that it's called /home/jffsstuff

Then just do:
#/usr/src/mtd/util/mkfs.jffs -d /home/jffsstuff -o /tmp/jffs.image

This makes a jffs image file. Then do (if your flash chips are erased,
else see below):
#cp  /tmp/jffs.image /dev/mtd0,1,2... (as the case may be, most
likely /dev/mtd0).

You may also mount an erased mtdblock device directly without putting
a file system on it. This will let you fill the device interactively
under your shell control (you know- copy stuff to the mounted dir).

If your flash chips are not erased or you have been messing around
with them earlier, your cannot just copy the new image on top of the
older one. Bad things may happen. Use the program mtd/util/erase to
erase your device.

#/usr/src/mtd/util/erase /dev/mtd0,1,2,3 <offset> <erase-size>
where 
offset: try 0 if you don't know (start of mtd device), else must be in
decimal bytes, but must start at an integral erase sector boundary.


erase-size: How many "erase sectors" worth do you want to erase.
Your max erase size for your flash is:
 (total-size/your mtd device erase size- look under `cat /proc/mtd`)


Watch the messages on your console (assuming you have verbose turned
on when you configured your kernel). You should not see any errors.

When your command prompt returns, do:
#cp  /tmp/jffs.image /dev/mtd0,1,2... (as the case may be, most
likely /dev/mtd0).


Then load the jffs module in by:
#modprobe jffs

Then mount the file system by:
#mount -t jffs /dev/mtdblock0 /mnt/jffs (assuming /mnt/jffs exists, else
make it).

Note: Note the use of /dev/mtdblock0 NOT /dev/mtd0. "mount" needs a
block device interface and /dev/mtdblock0,1,2,3... are provided for
that purpose. /dev/mtd0,1,2,3 are char devices are provided for things
like copying the binary image onto the raw flash devices.




*** Making partitions with CFI flash and working with multiple banks
    of FLASH:

Unlike a "regular" block device, you cannot launch fdisk and create
partitions on /dev/mtdblock0,1,2,3...

(As far as I know) CFI flash partitions have to be created and
compiled in the physmap.c file.

The same goes for multiple banks of flash memory. (IS THIS CORRECT????
Check and correct.)

An example of creating partitions can be found in the file
mtd/kernel/sbc_mediagx.c

An example of multiple banks of flash chips being mapped into separate
/dev/mtdn devices can be found in the file mtd/kernel/octagon_5066.c
(in particular pay attention to the multiple looping of the code while
registering the mtd device in "init_oct5066()". You may also add
partitions to each bank by looking at code in mtd/kernel/sbc_mediagx.c




*** Mounting a JFFS(1 or 2) F/S as root device.
This is rather simple.

*Note: This assumes that you can some how boot your kernel. This
section does NOT deal with booting your kernel from an mtd partition
or device.
You may be doing this by booting your kernel off an IDE flash disk/CF
disk etc. using lilo.
This procedure is the same even when you want to boot the kernel
directly off flash. This time you will just burn the kernel into the
raw flash device after the "rdev" step below.

1. Make sure that you can detect your flash devices and read and write
   them though the MTD device nodes (/dev/mtdn).
2. Make sure than you can mount the required JFFS(1 or 2) f/s on your
   flash devices and copy files to it, unmount, reboot, re-mount and
   still see your files there (also do a "diff" on a couple of files
   to make sure that the data did not get corrupted).
3. Compile all the required MTD/JFFS(1/2) support into the kernel
   (using modules to mount root is left as an exercise for the
   reader).
4. Tell the kernel what your root device is going to be. Do that by:
# rdev <your flash image here> /dev/mtdblock<n>
where mtdblock<n> is where you have constructed your root fs that you
   want to mount as root on reboot.
5. Run your boot loader init program (lilo for LILO bootloader).
6. Reboot. Your jffs mtdblock<n> partition should be mounted as root.




*** Mounting a *compressed* ext2 file system stored on an mtd
    partition or device as root.
Ah! Ha! This is much more fun (and complicated).
Prerequisites:
a. You must have ramdisk support in your development system kernel at
    least as large as the final root f/s that will be mounted in your
    target. This is for compressing the root f/s only. If you already
    have a ready-to-go compressed root f/s then you can skip this
    stage.

Steps:
1. Make a "root" file system on your mtd enabled development
    system. (mtd "enabled" means that you are running a kernel that
    supports mtd and that you can write to your mtd flash devices from
    your development station). The creation of this "root" file system
    is left to the reader. There are numerous ready available root f/s
    out on the net. Use any one or create your own (this is not
    necessarily fun if you have never done this before).

2. Make an ext2 f/s in ramdisk as large as you want the final
uncompressed root f/s to be. Do that as thus:

#mke2fs /dev/ram0 <you_root_fs_size_in_1k_blocks_here>

3. Mount this empty f/s on a free dir under /mnt as:

#mount -t ext2 /dev/ram0 /mnt/ramdisk

4. Copy your "root fs" dir that you have so carefully made over to
   this ramdisk.

#cp -af /tmp/my_final_root_fs_files/* /mnt/ramdisk

5. If you have done everything right till now you should be able to
   see the required "root" dir's (that's etc, root, bin, lib, sbin...)
   if you do a:

# ls -ld /mnt/ramdisk 

6. Now unmount and compress the file system.

#umount /mnt/ramdisk

#dd if=/dev/ram0 bs=1k count=<your_root_fs_size_in_1k_blocks> | gzip \
         -9 > /tmp/compressedRootFS.gz

7. Now we have to tell your kernel that will be mounting this
   compressed file system that this is a compressed f/s and where to
   find it on the mtd device.
   Make sure that your mtd stuff is all compiled into the
   kernel. Additionally you must make the following 2 changes to the
   kernel. This applies both to the 2.2.x and 2.4.x series.

   A. In the file drivers/block/rd.c you must comment out the check
   made for ROOT_DEV to be a floppy device. This code usually looks
   like:

        if (MAJOR(ROOT_DEV) != FLOPPY_MAJOR
#ifdef CONFIG_BLK_DEV_INITRD
                && MAJOR(real_root_dev) != FLOPPY_MAJOR
#endif
        )
                return;

   You must *NOT* return here, as your ROOT_DEV will *NOT* be a floppy
   device, it will be the mtd block device.

   B. At this time, due to the link order the rd_load() call to load
   any compressed files systems into ramdisk are made before the mtd
   driver has a chance to register the mtd block device. This causes
   the rd_load() code to fail to find your root device to load your
   compressed f/s from.
   Till this issue is fixed in the kernel, you have to make another
   explicit call to rd_load() right before mount_root() in main.c
   So, just add a call to rd_load() immediately before mount_root() in
   init/main.c

   C. Now compile the kernel with mtd and ext2 support in it (not as
   modules).
   
8. Now tell your target kernel (before installing it in the target)
   that you want it to load a compressed f/s and where this compressed
   image lies.
   There are two ways to do this. The easy way (using command line
   parameters) and the difficult way.
   We will do this the difficult way. Figuring out the easy way is
   left as an exercise for the reader.
   No, I don't usually like to do things the difficult way just for
   the fun of it, there is a reason behind this.

   I'm moving towards booting a Linux kernel out of raw flash, without
   the help of a boot load. In that situation we will not have any
   means to pass any kernel command line parameters.

   Tell the target kernel that you want to load a compressed f/s and
   where your image can be found as thus:

#rdev -r <your_target_kernel_image> <offset_number_in_dec>

where offset_number_in_dec is calculated as follows:
This number is the decimal equivalent of a binary number which is made
of various bits.
Bits 0-9 specify in 1KB blocks the offset from the start of the root
device.
Bit  14 specifies if a (compressed in our case) ramdisk needs to be
loaded- obviously a yes! Why else are you reading this!
Other Bits: Set to zero.

Just as a sanity check, 17408 is the number that you plug in as the
2nd parameter to the rdev -r above for the following.
This numbers tells the kernel that the offset is 1024 1kblocks
(i.e. find and load the compressed image found at the 1 Megabyte
offset from the start of the mtd device and mount it at the root device).

Note: If this bit pattern ever changes or you are doubtful of my
sanity, please go to arch/i386/kernel/setup.c file and look at the
various #define masks there. That's where all this bit magic comes from.

9. Now tell your target kernel what your root device is going to be:

#rdev <your_target_kernel> /dev/mtdblock<0,1,2....n>

10. Now of course you need to copy your compressed f/s image to the
    proper offset in your mtd device. Making sure that your target
    device is erased do:

#dd if=/tmp/compressedRootFS.gz bs=1k of=/dev/mtd<0,1,2....n>
 seek=<num of 1k blocks, in k, here that you told your kernel in
 above>

So for the 1Meg offset boundary you would put seek=1024

Note: "dd" is going to complain about "operation not permitted" or
some such thing. Just ignore that. dd tries to truncate the o/p
device, but mtd of course in not going to let somebody like "dd"
truncate it. The copy should go on just find.

11. Sanity check (year's of experience has taught me to triple check
    every step twice ;)
  Let's make sure that you got the compressed image in ok.

12. We will look at the first few bytes of both images and make sure
    that they are ok. You can also "dd" the target image back to a
    file and do a diff on it (left as an exercise for the reader).

#dd if=/dev/mtd<0,1,2...n> bs=1k skip=1024 (or your 1k offset in k) \
       od -Ax -tx1 |less
Jot down the first few lines. (note the use of "skip" in above, NOT
       "seek").

Now let's look at your compressed root f/s file on your hard disk:
#dd if=/tmp/compressedRootFS.gz | od -Ax -tx1 | less

Compare with the stuff that you jotted down above. They should match
(did I need to say that?).

13. Install your kernel however way you are going to boot it (run lilo
    if you are going to boot using LILO) or place it where it will
    boot from any other boot loader (or directly from flash etc.).

14. Reboot. This time, you should see the ramdisk loading code run
    twice and find the compressed image the second time and VFS mount
    it as root.

Ship it and ask for a pay raise (and send me some of that too)!





*** Booting a Linux kernel without a BIOS off an mtd device and
    mounting a compressed root file system stored on that device.

This is the holy grail of embedded Linux computing :) I shall attempt
to describe how to do this here. Note that at best this can only be a
guide as one embedded system differs a *lot* for another, not only in
terms of memory maps, but type of processors, type of flash, amount of
RAM etc.

* Assumptions:
This will (may) help you if your requirements meet the following:
You want to:

1. Use the standard Linux kernel as found when you download the entire
kernel from ftp.kernel.org

2. Know how to initialize your processor and chipset. This would
include, memory map (and chip select decode registers etc.). You
should be able to read/write the RAM and flash (if NOR type) from a
"simple" init program that you or you hardware guy wrote to test the
board. (Note: If you intend to use a BIOS, then this restriction goes
away). 

3. You are way ahead of the game if your target platform supports an
IDE hard disk (note: This is just for the development phase. We will
not end up with the hard disk in the final cut). This may not be an
unreasonable requirement. You may be able to buy an "eval" or
"development" board for the target processor that has a BIOS and
supports an IDE disk and serial console at the very least.

4. Do not think that compiling the kernel about 100-200 times is too
much effort to get this working ;)


* Overview:
We will follow the following steps:
1. Setup and boot linux on the target platform using a hard disk.

2. Take a beer break, take our spouse/(girl/boy)friend out for dinner
as they will not see you for a while.

3. Setup mtd drivers so that you can read/write the flash and mount a
jffs on it. At this stage we will use modules.

4. Once we are happy and comfortable with #3 above, compile the
mtd/jffs stuff into the kernel to prepare for booting. At this stage
we will install the kernel on the hard disk and the compressed file
system on the mtdblock device and boot that. Then we will either do 5a
or 5b as you desire.

5a. Non-compressed root file system on mtd device:
    Once we are successful with #4 above we will install a jffs file
    system on mtdblock and mount that as root (this is easy). You may
    want to do this if you want to make changes to your root file
    system by (easily) copying individual files over. The drawback to
    this is the file system will span the flash device
    uncompressed. This is bad because flash is easily 3 times more
    expensive than DRAM, and you could easily have the root file
    system compressed (with gzip) on FLASH and de-compress it into
    cheaper DRAM (5b. below).

5b.Compressed root file system on mtd device:
    Or we could just skip the easy steps and install a compressed root
    file system on the mtd device and decompress this on boot to
    ramdisk (in DRAM) and mount that ramdisk as root. This is much
    better (in my mind) as DRAM is usually faster then FLASH. If your
    processor supports a DRAM controller then it probably has read
    ahead and write combining that increase the performance even more
    and which you have turned off for the FLASH regions if you want to
    write to flash. If your processor has cache, then you are
    significantly faster accessing DRAM as that area could be cached
    and for sure you want cache turned off if you are writing to FLASH
    (else writing may fail, this is the eq. in 'C' of declaring the FLASH
    memory area as "volatile").
    
    Once we have mounted the compressed root file-system we can easily
    mount a jffs mtd flash bank or partition on a dir on root to store
    config files or logs or root file updates etc.

6. Nightmare! Boot the raw kernel off flash (note: this may be a part
   of the mtd flash, but mtd has nothing to do with this, except start
   the device after a "keep-off" area for the kernel).

   This is the MOST difficult part, but is now solved. See below.

Lets get to work:

This is now (easily) possible for bzImage kernels under x86 systems.
Please see the following for complete details:

http://www.EmbeddedLinuxWorks.com/articles/rolo_guide.html








*** FAQ's:

Q. What is MTD and why do we need it?

A. From the MTD site:

"We're working on a generic Linux subsystem for memory devices,
especially Flash devices.

The aim of the system is to make it simple to provide a driver for new
hardware, by providing a generic interface between the hardware
drivers and the upper layers of the system.  

Hardware drivers need to know nothing about the storage formats used,
such as FTL, FFS2, etc., but will only need to provide simple routines
for read, write and erase. Presentation of the device's contents to
the user in an appropriate form will be handled by the upper layers of
the system."


Q. What is JFFS?

A. JFFS was designed by Axis Communications AB, Sweden
(www.axis.com). It is an open source log structured file system that
is most suitable for putting on raw flash chips.
For more info: http://developer.axis.com/software/jffs/

Some additional documentation (not reviewed and no link to it yet):
http://developer.axis.com/software/jffs/doc/jffs.shtml


David Woodhouse described jffs in a mail to the jffs mailing
list. This is what he wrote:

"JFFS is purely log-structured. The 'filesystem' is just a huge list of
'nodes' on the flash media. Each node (struct jffs_node) contains some
information about the file (aka inode) which it is part of, may also 
contain a name for that file, and possibly also some data. In the cases 
where data are present, the jffs_node will contain a field saying at what 
location in the file those data should appear. In this way, newer data can 
overwrite older data. 

Aside from the normal inode information, the jffs_node contains a field 
which says how much data to _delete_ from the file at the node's given 
offset. This is used for truncating files, etc. 

Each node also has a 'version' number, which starts at 1 with the first node
written in an file, and increases by one each time a new node is written
for that file. The (physical) ordering of those nodes really doesn't matter at
all, but just to keep the erases level, we start at the beginning and just
keep writing till we hit the end.

To recreate the contents of a file, you scan the entire media (see 
jffs_scan_flash() which is called on mount) and put the individual nodes in 
order of increasing 'version'. Interpret the instructions in each as to 
where you should insert/delete data. The current filename is that attached 
to the most recent node which contained a name field.

(Note this is not trivial. For example, if you have a file with 1024 bytes 
of data, then you write 512 bytes to offset 256 in that file, you'll end up 
with two nodes for it - one with data_offset 0 and data_length 1024, and 
another with data_offset 256, data_length 512 and removed_size 512. Your 
first node actually appears in two places in the file - locations 0-256 and 
768-1024. The current JFFS code uses struct jffs_node_ref to represent this 
and keeps a list of the partial nodes which make up each file. )

This is all fairly simple, until your big list of nodes hits the end of the
media. At that point, we have to start again at the beginning. Of the 
nodes in the first erase block, some may have been obsoleted by later 
nodes. So before we actually reach the end of the flash and fill the 
filesystem completely, we copy all nodes from that first block which are 
still valid, and erase the original block. Hopefully, that makes us some 
more space. If not, we continue to the next block, etc. This is called 
garbage collection.

Note that we must ensure that we never get into a state where we run out of
empty space between the 'head' where we're writing the new nodes, and the
'tail' where the oldest nodes are. That would mean that we can't actually 
continue with garbage collection at all, so the filesystem can be stuck 
even if there are obsolete nodes somewhere in it.

Although we currently just start at the beginning and continue to the end,
we _should_ be treating the erase blocks individually, and just keeping a
list of erase blocks in various states (free/filling/full/obsoleted/erasing/
bad). In general, blocks will proceed through that list from free->erasing 
and then obviously back to free. (They go from full to obsoleted by 
rewriting any still-valid nodes into the 'filling' node)."




Q. What is JFFS2 and how is it different from JFFS?

A. JFFS was the original file systems developed for embedded file
systems on flash devices- designed for async power down. See above Q.
JFFS2 is an enhancement to JFFS. It enhances JFFS in the following
areas:
1. Understands and handles writes to flash on an erase sector
level. This has various advantages like garbage collection on a sector
basis rather then the entire file system basis.
2. Possible to mark bad sectors and continue to use the remaining good
sectors thus enhansing the write life of the devices.
3. Less blocking time due to garbage collection (only one sector needs
to be erased at the minimum, unlike JFFS where the entire f/s data
needs to be "squished" to garbage collect).
4. Provides native data compression inside the file system design.

Note that JFFS2 is still under active debugging/development (as of
March 7th 2001). Please see the jffs developer list for current status
if this document is more than a few months out of date.



Q. Ok, give me the skinny. How production worthy are JFFS1 and JFFS2?

A. [This is the author's opinion only. Please pose specific questions
to the list if you have any concerns]

No active development work is being done on JFFS1. JFFS1 is popularly
believed to be complete. To access this state, I did some power down
tests on JFFS1. The code, as is currently checked into CVS [edit:see
below], fails within 7 power cycles (worst case, best case it has
lasted 59 power cycles). Modes of failure are various error messages
that result in a completely unusable system including loss of data on
the file system. 
Note that, my power down test emphasised power down reliability of the
system *while data was being written to the JFFS1 system*. As far as I
know there are no issues with using JFFS1 on mostly "static" file
systems where a lot of write activity is not going on or dangers of
async power down does not exist.
I personally would not consider the CVS JFFS1 code to be production
quality to be used in unattended embedded systems.

I have investigated this issue and have submited a patch (to intrep.c)
to the mailing list. In the same power down tests, the JFFS1 CVS code
patched with my intrep.c patch, manages 1100 power down cycles during
a write before failure. That is more than two orders of magnitude
increase in the reliability of the system. This patch is still being
reviewed by the list and has not been accepted yet. USE AT YOUR OWN
RISK! I will update this note when there is further activity in this
regard.

[UPDATE: Mar 16th 2001: This patch is now applied to the CVS version.
No more mount issues were observed with this new patch. *However* a
new problem was observed. After 653 power cycles, about 8 files from
the file system disappeared without a trace! There is no explanation
as of yet. These were NOT the files being written to, rather some
programs in the /bin dir. Regardless, the CVS version of JFFS1 is now
at least an order of magnitude more reliable regarding coming up
successfully after an async power down. 
/UPDATE]

<UPDATE: June 12th 2001>

I have done power fail testing on JFFS2. Please see the following
report for more details (you can also download the power fail test
program I used, from there. It's available as open source code):

http://www.EmbeddedLinuxWorks.com/articles/jffs_guide.html

</UPDATE>


The objective is to have a very stable flash file system that is
capable of an unlimited (i.e. till you stop testing :) number of async
power fails with a successful recovery the next time around.



Q. Why another file system(s). What was wrong with ext2?

A. (from Johan Adolfsson:) JFFS is aimed at providing a
crash/powerdown-safe filesystem for disk-less embedded devices. This
typically means flash memories and these have certain characteristics,
such as you can't write twice to the same location without doing a
time-expensive erase on a full sector first (typically 64kB), this
means "normal" file systems such  as ext2 won't work very well.

Additionally if only a little amount of data has changed in the sector
to be erased, then the rest of the data needs to be stored off
somewhere, the new data merged with the old and everything written
back. So potentially, you would write 64KB for every 512 bytes of data
to be written to the file system. If this data is "saved off" in RAM,
then you could loose everything if power goes down while the sector is
being erased. If it is saved off in another sector of flash, then that
sector needs to be pre erased, and now you are doing 128KBytes of
write for a 512 byte data write.

(David Woodhouse added:) Need journalling pseudo-filesystem to emulate
a block  device and to wear levelling. then need ext3 (note ext_3_) on
that. journalling  fs on top of journalling fs - not efficient. Also,
no way for ext[23] to  mark blocks as _deleted_ and no longer cared
about. Fill ext2 partition on  NFTL, empty it again, and the NFTL will
still carefully copy around the  blocks containing old deleted data.

( -- I was hoping you'd translate that into real-person-speak, not
just cut and paste it -- dwmw2 :)

Translation of above:(Vipin: -Ok here you go David- :))
The ext2 filesystem was designed for normal desktop systems. "Normal"
desktop systems have UPS's connected to them. ext2 was designed with
various goals in mind, that included speed, size of files on disks,
speed, total file system size, fragmentation issues, oh, did I mention
speed?

Unfortunately, power down robustness was not high on the design
goal. Neither was wear levelling the physical medium that the data was
stored on (hard disk platters have a significantly more read/write
life than flash chips).

What this means is that, file system meta data (or fs structure)
corruption is a very real possibility. Additionally, file system
"repair" and scanning software needs to be written and executed if the
file system is suspect.
This is of course unacceptable in embedded systems that do not have a
UPS connected to them and power may fail without warning. Even systems
that have advance warning (like a power fail warning interrupt) do not
have enough time to sync hundreds of kilobytes of data to flash disks
and unmount the disk before the plug is pulled after the advance warning.

The answer is a file system designed specifically for flash storage
devices- jffs!

But what about ext3 or other "journalling" type file systems that do
handle power fail recovery (and quite quickly too)? Unfortunately, the
raw flash device requires a wear leveling "sector erase aware"
handler. Putting another journalling file system on top of this log
structured handler is inefficient. Hence jffs being a file system for
embedded systems. (Isn't the use of the term "journalling" wrong in
reference to jffs? JFFS is really a "log structured" file system, not
a "journal" type file system where a "change journal" is written out
before the actual change is made to the file system and this journal
is a file system modify cache that can be replayed if the entire write
did not take before power went down?) 



Q. Do I have to have JFFS on MTD?

A. Yes! JFFS (at the moment) only works on any linear device supported
by the MTD layer. It does NOT work on DOC. It does NOT work on 
Compact flash. It does NOT work on IDE flash disks.

It will work on SRAM. It will work on DRAM. It will work on FRAM.
But you have to install MTD drivers for each first and then mount
the JFFS fs on the block device for them respectively.

And I believe that support is not complete for NAND flash chips
(I may be wrong here as I am not working with NAND flash and do not
keep up with those developments. Please drop me a line if you know
otherwise).

In the future JFFS (or most likely JFFS2) *may* work on DOC. It will
most likely *never* work on Compact flash or IDE flash disks.
These devices are NOT reliable in asynchronous power fail situations.
Having a reliable file system on unreliable hardware makes no 
practical sense.

Q. Does JFFS work on Compact Flash?

A. No.

Q. Does JFFS work on IDE flash disks?

A. No.

Q. Does JFFS only work on devices suported by the MTD driver layer?

A. Yes.


Q. What is DOC (disk on chip)?

A. Manufactured by M-Systems (www.m-sys.com).
Bunch of NAND flash chips connected together with a clever ASIC
which does  hardware ECC.



Q. What File systems can I have on DOC?

A. (David Woodhouse:) If you put NFTL on it to emulate a block device
(the status quo) then any  normal filesystem. JFFS ought to work too
(though that has NOT been throughly tested yet).

(Vipin Malik:)
Note that once you put ext2 (or any other "standard desktop") file
system on DOC, these file systems may suffer from reliability problems
associated with async power down. You then have to e2fsck (for ext2)
on power up. This may result in the compelete deletions of some files
(particularly those that were being written to when power
failed). Additionally e2fsck is not an automatic scanning process. It
asks you questions (that you can force an automatic "yes" answer to
with the -y flag, but then you have no control of what the scanning
utility does).

Be aware that DOC claims data integrity at the IC (chip) level- not at
the file system level. JFFS and friends (JFFS2) claim data and file
system reliability at the data and file system level. A huge
plus. JFFS on top of DOC would be a good combination of expansion
flexibility and data and file system reliability.



Q. What is Flash memory?

A. This is a non-volatile memory integrated circuit that is arranged
in "sectors". There are two different types.

NOR or code storage flash is arranged in quite large sectors of upto
(or greater than) 64KBytes each. A fully erased flash (or sector) has
all bits "erased" to a 1.

You man change a "1" to a "0" "on-the-fly" or with a very fast byte
(or word if the chip is 16 bits wide) write to it (almost like RAM but
usually slower).
However, to change a "0" back to a "1" requires that you erase the
*entire* sector.
Each NOR flash sector also has a finite number of erase cycles
(typically from 100k to 1 million).
NOR flash is usually more tolerant of physical of writes to its
sectors and new NOR flash is 100% good and usable.

NAND flash or data flash has much smaller sectors and is typically
used to store data. This type of flash is also less tolerant of
physical writes to it and new devices may have "bad blocks" that need
to be marked unusable by the driver software (think bad blocks marked
unusable on hard drives during a format operation).

Note: Both types of flash can be used with a driver layer software to
store code (obviously both can store data). The MTD driver in linux
does just that. In this case, the code is treated as "data" and copied
to RAM before it is executed.

Please see www.amd.com or www.intel.com (or any other mfg. site like
Toshiba, Samsung, SanDisk etc.) for more information.




Q. If Flash has a limited "erase" sector life to it, how can I
reliably use it to store logs etc. in an embedded system?

A. Welcome to "wear levelling". If you use flash with a driver level
software (like MTD in Linux), then as we saw in the above question,
the driver level can convert even data flash (NAND) to code flash and
execute code from it (really copy to RAM first and then execute). In
other words, the driver level provides a layer of "functional
translation" on the raw device.

JFFS implements another type of transformation called wear
levelling. Every write to the flash device (by a user program) results
in an "addition" to the data already on the raw flash device. This is
true even if your program is sitting there writing out oxfefefefe (or
whatever) to the same place in the file. This has the effect of
spreading out the writes over the entire available flash memory.

For a quick back of the envelope calculation, lets assume the
following:
1. You want to write out a small log (say 100 bytes) 1 a second
for ever.
2. Your log flash chip is 2MBytes and the entire chip is available for
log storage.
3. If you were writing to the same location every time (if you were
accessing the flash sector directly) then assuming a sector life of
1 million erases your would wear out the sector in (assuming that you
erased the sector for every write:

1million/(1 timespersec * 60secs/min * 60mins/hr * 24hrs/day) 
or in about 11 days!

If your now used the entire flash to spread out your writes then you
would have to erase a sector (assuming 64KB sectors) only once in
(2M * 1024Kbytes * 1024 bytes)/100bytes

or 20,900 writes.

In other words your are increasing the life of your storage device by
20900 times! or to 629 years!

Note1: These calculations are just an example. Please do your own
sanity check and calculations for your particular situation.

[***Edit: I was "informed" by David Woodhouse that the following is true
for JFFS2. JFFS1 does indeed move the entire data, static and all.
Additionally JFFS2 may implement wear levelling even on the static
data, moving static data to frequently used sectors to give them a
break from being written to.****]

Note2: This example assumes that your entire flash chip area (that you
are considering in your equation ) is available for log storage and
older logs are being deleted- in other words, use the amount of flash
area that is being "churned" by your logs. If your 2MByte flash is 85%
full with OS files and stuff that never get erased, then those sectors
are "blocked out" from being available to be used in the wear
leveling. The correct amount of flash to use in your calculation would
be the 15% remaining.


 

Q. Anything that I need to watch out for while using JFFS on raw
flash?

A. Yes! At present (13th Feb 2001) the garbage collecting thread in
JFFS (that's what collects all the "good" inodes and gathers them into
a new sector, then erases the old sector to free up flash space),
BLOCKS, while doing a sector erase.
Sectors can take upto 4 seconds to erase. Additionally the design of
JFFS1 is such that the entire file system log (i.e. all the valid data
on the f/s) needs to be moved during garbage collect. This would mean
moving 12 megs data on a 16M f/s to make room for another few KB of
data.
This means that any program,
either reading or writing to *that particular file system that
contains the flash chip* will also get blocked (as you can neither
read nor write to *any* sector of the flash chip even if one sector is
being erased). This means that if you want to log a data file faster
than once every (4 * num-sectors-to-erase-to-move-all-data =
large_number_of_secs) seconds you are out of luck!

There are 2 ways around this.

1. Wait for "suspend erase" feature to be implemented (David, any time
frame on this?). CFI flash chips can be suspended while being erased,
to allow reads/writes from/to other portions of the flash. This is NOT
in place yet.

[****Edit: 7th March 2001: This will probably never be implemented as
JFFS1 is being superseeded by JFFS2, which offers erase sector size
handling of the file system and (possibly) erase suspends.****]

(I have a question on this. Say our sector needs 4 seconds to
erase. Say we "suspend" the erase 1 second into the erase to read from
the flash. When we restart the erase, does the previous 1 second erase
count towards the 4 seconds or does the flash still needs 4 seconds to
erase the sector? Anyone know? - Vipin) (nope -- dwmw2)

--

Actually, support for erase suspend is already implemented in the
physical driver for Intel CFI chips and has been for some time,
although it's largely untested. The actual problem here is the locking
issues in the JFFS data structures.  I took the sledgehammer approach
and stuck a single semaphore round all JFFS operations. So even reads
from a _different_ chip in the same filesystem are blocked while the
GC is waiting for an erase to complete.  This should be fixed in JFFS2
-- dwmw2

--


2. If you are designing a custom board, put a small FRAM chip (see
www.ramtron.com) on your board. Map this chip into a /dev/mtd device
and log your "fast" logs here. Like a flash device, FRAM chips are
non-volatile on power fail (without needing a battery backup), but
unlike a flash chip, these do not have to be sector erased to turn a
"0" bit into a "1" bit. Reads and writes to these chips occur at bus
speeds. You can then use a background task to offload the logs from
this partition to the regular flash in a non latency critical and safe
manner (make sure that the logs have taken on the flash and then erase
it from the FRAM partition). Unfortunately the largest available
device (that I know of as of 13th feb 2001) is a 32KByte (a x8)
device. Hence you can only use it as a "fast" cache, rather than for
the whole JFFS file system. This of course does not solve the problem
if your reads to the flash jffs fs cannot be blocked for more than xxx*
seconds.

* xxx = see calculation above in answer to this question above.



Q. Any other advise on writing programs that use the jffs file system?

A. Here is a tip: Since every write to the jffs file system gets
synced to the raw flash chip before the "write()" command returns to
the application, and every write is implemented as a raw inode write
to the jffs file system (see jffs_raw_inode in
mtd/include/linux/jffs.h) you can improve the write speed as well as
decrease the file system space overhead if you "collect" as many
writes as possible.
What do I mean? Consider the following:

AVOID following:
 write(fd, &hdr1, sizeof(hdr));
 write(fd, &hdr2, sizeof(hdr2));
 write(fd, &hdr3, sizeof(hdr3));
 write(fd, &data, sizeof(data));

rather do:
  write(fd, &bufferThatContainsHdrs1to3andDataAbove, sizeof(<buffer on
  left>));
 


Q. What is CFI Flash memory?

A. (from Johan Adolfsson:) CFI  = Common Flash Interface, see
http://www.amd.com/products/nvd/overview/cfi.html 

This makes it possible to read info from the flash chip so you know
how to erase it etc. without having to hard-code the ID of the flash in
your software.



Q. What is JEDEC Flash memory?

A. (from Johan Adolfsson:) Each flash chip has a manufacturer ID and a
device ID that can be read and used to determine size, algorithm
etc. to use. If the chip doesn't support CFI, this is typically what
you have to use.



Q. What is this "interleave" stuff?

A. (David Woodhouse:) If you have 16-bit chips, but a 32-bit
processor, it makes sense to arrange  them side-by-side to fill the
CPU's bus. You drive them both simultaneously. That's the arrangement
we refer to as 'interleave'.

Hence if you have four x8 bit FLASH chips connected in parallel (ahem
interleave!) to a 32bit processor bus, you are 4 way interleaved. One
quick way to see how may way interleave you are is to glance at the
address bus connected to your flash chips (on the schematic). If your
processor A0 goes to A0 on your 8 bit flash chip(s), then you are
1way. If your processor A1 goes to A0 on the flash chips then you are
2 way, similarly A2 to A0 gives 4 way interleaving. (Note: There is no
3 way interleaving).

Other possibilities are... 2x 16-bit chips on 32-bit bus, 2x8-bit 
chips on 16-bit bus, ...

If you are designing your own hardware, if possible use the maximum
width of the processor data bus as you will be able to write out 4
times faster per word write to your flash, x32 compared to a x8
connection.

But you need to be aware of a tradeoff with this approach. All flash
chips used to fill the processor buss will have their sectors erased at
the same time. In other words, 4 x8 chips interleved by 4 on a 32 bit
bus, with 64KB even sectors will have an erase size of
4*64KB=256KBytes. Why should you care about this? Because, the JFFS
code needs to keep a minimum number of sectors free to continue to
garbage collect. At this time, that minimum number is 4 sectors (see Q
below). In other words, in the above example, you will never be able
to put data in 1MegBytes of your jffs flash device. You may care about
this. If you do, and write speeds are not that important to you, then
connect your x8 bit flash devices to an x8 bit processor bus (or as a
byte wide memory on your 32 bit data bus). Then you will have an erase
size of 4*64KB = 256KBytes or 4 times better.


Q. What is a reasonable fmc->min_free_size?
[David Woodhouse wrote]
Good question. The code in question currently reads...

        /* min_free_size:
           1 sector, obviously.
           + 1 x max_chunk_size, for when a nodes overlaps the end of a sector
           + 1 x max_chunk_size again, which ought to be enough to handle 
                   the case where a rename causes a name to grow, and GC has
                   to write out larger nodes than the ones it's obsoleting.
                   We should fix it so it doesn't have to write the name
                   _every_ time. Later.
           + another 2 sectors because people keep getting GC stuck and
                   we don't know why. This scares me - I want formal proof
                   of correctness of whatever number we put here. dwmw2.
        */
        fmc->min_free_size = fmc->sector_size << 2;

Theoretically, we should only require 2 * sector_size. In practice, that
sometimes wasn't enough, and we didn't reproduce the problem in-house so
didn't find out why, and I increased it to 4 * sector_size just to be on
the safe side.



Q. Can I boot my kernel from a DOC or jffs NOR flash mtd device
(with/without the help of a BIOS)?

A. Yes! At least for x86 systems & NOR FLASH (or ROM) see 
http://www.EmbeddedLinuxWorks.com/articles/rolo_guide.html
for complete details.


*** Credits:

<developers, please provide me with the credits for MTD, jffs, DOC
etc. etc. etc. for the wonderful code in MTD, DOC, JFFS,
etc. etc. etc. Who is doing/had done what etc.>
...
...
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