[Arm-netbook] Booting from SD card with switch on so-dimm in other direction ...

[Jonathan Ervine]

On 23 Mar 2010, at 17:52, Luke Kenneth Casson Leighton wrote:

> On Tue, Mar 23, 2010 at 12:47 AM, Jonathan Ervine <jon.ervine at gmail.com> wrote:
> 
>>> make DAMN sure that's the right way round :)
>> 
>> I have a 2GB SD card, so I can prep this with a view to trying to restore to
>> Adam's system. We'll have to organise a convenient time to do this. I've got
>> a busy weekend coming up when computing won't exactly be high priority ...
>> Once the internal NAND is dd copied to the SD card is it expected that it
>> should just 'boot', or would the three fingered salute be required?
> 
> think about it: if this works, then the SD card will have EXACTLY
> what's on the NAND flash.  that means that it will try to reference
> the NAND flash, because that's set up as mmcblk0p2, right?  and u-boot
> is specifically hard-coded to set the bootparams to
> root=/dev/mmcblk0p2, yes?

OK - my curiousity was suitably piqued by this, so I've created the 2GB SD card from the mmcblk0, I've rebooted my system with the mouse buttons pressed down at the same time as the power button.

> so... which partition will it boot off of?  the NAND flash, yes?
> 
> which, in adam's case, has the remote possibility of having been
> corrupted (but we don't know for sure).
> 
> so, the sequence of events is _likely_ to be:
> 
> * u-boot loads from SDcard
> * kernel loads from SDcard
> * kernel boots and uses root=/dev/mmblk0p2 which we expect to be NAND flash

I get no SD message, neither blue nor red. If I continue to press and hold, the logo flickers, then the green bar starts, and then it powers off (presumably because I have the power button pressed). If I let go of the buttons when the green bar appears, (it is about 1/3 of the way along the 'track') then the boot process completes and I can log in. The mounted filesystem is definitely on the NAND as I can create a file beforehand in a 'normal' boot, and it's present after this partial power+button boot. I would surmise that your likely sequence of events is correct.

> however if that switch _truly_ inverts /dev/mmcblk0 and /dev/mmcblk1
> then of course /dev/mmcblk0p2 will be ... SDcard, yes?
> 
> so a correct answer is: absolutely no idea, let us know what happens.
> 
> the crucial and critical thing to get working is u-boot.  everything
> can be recovered if u-boot can be booted.

I'll know for sure when I have time to have a play on Adam's broken system :-)

Jon