SUCCESS booting off cramfs initrd
Last week I asked for help in my cramfs adventure, so now that I've had some success, I thought I'd share how I got it done.
My original goal was to boot from a cramfs filesystem residing in flash. As people explained to me, that would have required me to have a small ext2 initrd filesystem to boot from and them mount my cramfs filesystem as the root filesystem (through the /dev/mtd device, accessed through the MTD drivers). I went down this road, but found that the current MTD CFI flash drivers didn't support big-endian configurations at all. (Actually, I created a MTD map driver for my board that treated the
flash space as ROM and I was able to access it). But, I didn;t
really want to created two separate filesystems (the ext2 initrd, and the cramfs root filesystem), so I didn't continue this approach.
It later occurred to me that, due to the way I update my image in flash, the filesystem needed to live in RAM. I still wanted to use cramfs to save RAM (my ext2 filesystem was 4.5MB, too expensive for a 16MB board). But, this brought me back to trying to use a cramfs initrd as my root filesystem.
So, here's what I did to accomplish this:
- started with linux-2.4.4 kernel plus linuxppc_2_4 diffs patch
- incorporated the cramfs patch from Transmeta's Midori Linux 1.0.0-beta1 release (they use a different packaging mechanism, so you'll need to get the whole Midori release and unpack/build it to get access to the patch). I'm not sure if this step was necessary or not......but the patch makes significant improvements to the cramfs filesystem, so I wanted it (someone else may be able to answer this better).
- modified drivers/block/rd.c to make several changes:
1) detect the cramfs superblock (at offset 0 or offset 512 in the first block of the filesystem)
2) when a cramfs filesystem is detected, force the block size of the ramdisks to 4KB, which is cramfs's block size. (this step tripped me up for several days, as I didn't and still don't know much about block devices....but I did learn that the ramdisk that cramfs is on must use 4KB blocks!)
- modified David Blythe's originalendian-swap patch to mkcramfs to work over the top of Midori's cramfs patch. Basically, this provided a
"-r"
option to mkcramfs to reverse the endian-ness of the filesystem (my host is an i686 box and my target is a custom MPC860T board).
Basically, when i did all of the above, I was able to boot off of a cramfs initrd, and continue to use it as my root filesystem.
I put all of the patches that I used to accomplish this out at:
(listed in the order in which they should be applied)
http://www.geocities.com/heinss1/cramfs_midori.patch.txt
http://www.geocities.com/heinss1/mkcramfs_endian.patch.txt
http://www.geocities.com/heinss1/cramfs_initrd.patch.txt
for whoever may be interested. Hope it helps someone else get off the ground faster!
Steve
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Steve Hein ([email protected]) Engineering Diagnostics/Software
Silicon Graphics, Inc.
1168 Industrial Blvd. Phone: (715) 726-8410
Chippewa Falls, WI 54729 Fax: (715) 726-6715
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/