loading the kernel and root FS separately from flash?

[Robert P. J. Day]

   what is (hopefully) a simple question, before i invest the time to
see whether it works.

   currently, i have a modified rpxlite board, with two flash chips,
each 8M -- one "system" flash, one "user" flash.  the system flash has
the boot loader (embedded planet's planet core), and the loadable
kernel+rootfs image.  this bootable image is only 2.5M in size, so
there's lots of leftover space in that system flash chip.

   with this configuration, it's obviously not possible to update files
or directories in the bootable image individually -- it's just one big
zImage.inited image.

   but is it feasible to redefine the layout of that flash chip and
take, say, 4M of it and create a JFFS2 filesystem, then put the root
filesystem in there uncompressed?  (assuming, of course, that the
rootfs fits in 4M.)  theoretically, then, i could have system flash
with the bootloader (only a couple hundred K), the bootable kernel (at
most 2M in our case), then 4M of JFFS2.  that would give me the
freedom to update the basic root filesystem if i had to.  we could
boot the kernel as we're doing now, and i'm assuming it wouldn't be
difficult to have the kernel know to copy the JFFS2-based root
filesystem into RAM, mount it, and take it from there.

   the current flash layouts are defined in the file
drivers/mtd/maps/rpxlite.c, and we've set it up so that the other 8M
"user" flash is associated with /dev/mtdblock/0.  my thought was to
just define half of system flash as JFFS2, at which point it would be
accessible as /dev/mtdblock/1 (depending on the definition order, of
course).

   is this possible?  anyone done it this way, and can tell me what
pitfalls i have to watch for?  thanks.

rday

** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/