Packaged kernel erase on update

[Andy Green]

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Hi -

I had a wander around here last night

http://git.openmoko.org/?p=openmoko.git;a=blob;f=packages/linux/linux-openmoko.inc;h=69d7845f0462f49a394b2277ad6d2fd24d474583;hb=org.openmoko.asu.dev

I see that our packaged kernel update process ends up with this at the
sharp end, which is pretty reasonable.

~  24                 ${bindir}/flash_eraseall $MTD_KERNEL_PARTITION
~  25                 ${bindir}/nandwrite -p $MTD_KERNEL_PARTITION
/${KERNEL_IMAGEDEST}/${KERNEL_IMAGETYPE}-${KERNEL_VERSION}

However, if we make a little change here we can have a backup kernel in
a nice way, that can ultimately be selected from U-Boot menu.  I wrote
up how that backup kernel system works here:

http://wiki.openmoko.org/wiki/Freerunner_backup_kernel

It doesn't need any changes to U-Boot, but it does need a change to this
update script so that instead of erasing the whole kernel partition it
only erases the area that it intends to write to, using flash_erase
instead of flash_eraseall.

Is this going to break anything on, eg, GTA01, if we have it flash_erase
the first 4MB of the kernel partition?

- -Andy
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