[maemo-users] [maemo-users] Booting from MMC as a de-bricking method

From: Frantisek Dufka dufkaf at seznam.cz
Date: Fri Feb 16 22:38:40 EET 2007
Paule Ecimovic wrote:
> I am most concerned about 
> the bricking scenarios resulting from flashing a 770 to set it into R&D 
> mode, where the 770 sometimes bricks after rebooting.

Never hear of this, is this real? What you see on the screen when it 
boots? How many seconds exactly boot takes since poweron before it 
reboots again (i.e. in which step mentioned below it stops)?

> Are there bricking 
> scenarios in this reboot situation which involve corruption beyond MMC 
> reboot?

I'm not sure what you mean by this (and was not answered already). Let's 
explain how device boots:

1. bootloader (probably at least two of them, one loads another) 
initializes some hardware, checks if flasher is connected and finally 
loads linux kernel from /dev/mtd2. Some small bootloader is stored 
either in masked ROM or NOR flash directly on OMAP chip, the second one 
is stored in (NAND) flash - /dev/mtd0
2. linux kernel starts, initializes hardware and runs linuxrc from 
initfs partition (/dev/mtd3)
3. initfs (uclibc based system) starts dsme and bme, loads wlan and bt 
firmware, checks root device name stored in flash (=config partition 
/dev/mtd1)mounts it and continues boot from that device (via pivot_root 
[1]). This is where bootmenu modification lives too, it simply allows 
you to select different root device then stored by flasher.
4. /sbin/init starts from the root device and normal linux system boots 
via rc scripts

This means that if bootloader (/dev/mtd0) or kernel (/dev/mtd2) is 
corrupted or maybe even config partition (bootloader probably uses it 
too to store some data - MAC address? list of bad flash blocks? 
partition layout?) then the device is 'bricked' and bootmenu cannot help 
since it has no chance to run. Only reflash over usb (or serial) can help.

But normal bricking done by most users and mentioned in previous mail 
means corrupting rootfs (/dev/mtd4) and this can be solved by booting 
from anything else (=mmc) via bootmenu.

> Any chance of hard-disk based ITOS for Nokia Internet Tablet's. 

Harddisk would not solve anything. Same like when you corrupt your bios 
on PC.

> but it would allow rebooting in various modes and under various 
> privledge schemes without the threat of irreparable bricking.

Harddisk is not any better than mmc, both are lock devices used by linux 


1. http://www.die.net/doc/linux/man/man8/pivot_root.8.html

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