[G1 Android Development] JesusFreke:
WARNING: Following these instructions may brick your phone, void your warranty and kill your dog. You don't want your dog to die do you?

Once I got root access on my G1, I've been messing around with trying to build reflash the recovery partition. That project is still in progress, but I have learned a bit about how to flash the various partitions on the G1.

First things first, you have to have root access. See this thread.

There are 6 mtd devices or partitions on the G1, mtd0-mtd5. They are located at /dev/mtd. You can use the /system/bin/flash_image tool to flash an image to any of these.

The syntax for the flash_image tool is:

# flash_image
usage: flash_image partition file.img
#

You can see a list of partition names and which device they are associated with by doing a "cat /proc/mtd".

#cat /proc/mtd
dev: size erasesize name
mtd0: 00040000 00020000 "misc"
mtd1: 00500000 00020000 "recovery"
mtd2: 00280000 00020000 "boot"
mtd3: 04380000 00020000 "system"
mtd4: 04380000 00020000 "cache"
mtd5: 04ac0000 00020000 "userdata"
#

These should all be self explanatory, except maybe "misc", which just appears to have a few string values.. Not really sure what it's used for..

Before you do any erasing or writing, it's a "really good idea" (tm) to make backups of each of these. Even if you don't plan on writing to them. I had accidentally erased the bootloader partition (typed mtd0 instead of mtd1), which I'm fairly sure would have bricked my phone if I had tried to reboot it. Ugh! Luckily, I had created a backup earlier, so I was able to restore it. (And then was nervous as heck when I tried to reboot it... "Please boot up! Please boot up!")

To create the backups:

# cat /dev/mtd/mtd0 > /sdcard/mtd0.img
# cat /dev/mtd/mtd1 > /sdcard/mtd1.img
# cat /dev/mtd/mtd2 > /sdcard/mtd2.img
# cat /dev/mtd/mtd3 > /sdcard/mtd3.img
# cat /dev/mtd/mtd4 > /sdcard/mtd4.img
# cat /dev/mtd/mtd5 > /sdcard/mtd4.img
#

Now you can use flash_image to write the new image.

#flash_image recovery /system/recovery.img
#

And that's how it's done.

Update: You normally don't need to erase the flash before you write, as long as you don't corrupt the flash first, by trying to "cat" an image directly to the mtd device, like I did :). If you get a lot of ECC errors when you use flash_image then you need to erase it

# cat /dev/zero > /dev/mtd/mtd1
write: No space left on device
#

On a related note, Based on my experience so far, the recovery partition is not critical to booting the G1. When I first tried this, I used flash_image to write a new image without erasing the partition first, and it corrupted it pretty good. When I tried to boot the G1 into recovery mode (power+home), it would go to the bootloader screen instead. But it would boot into normal mode just fine.

Additionally, I've verified that the device will boot into recovery mode if you screw up the boot partition (mtd2). So as long as you at least have a good recovery image *or* a good boot image, you should be able to get back in business. Just don't try to update both at the same time.

Even so.. be careful, and don't come crying to me when you brick your phone. Or if your dog dies. You don't want your dog to die do you?