Boot interface for device trees on ARM

[John Rigby]

Wow, no responses for almost two days, does that mean consensus has
been reached?

On Wed, May 19, 2010 at 2:22 PM, Nicolas Pitre <nico at fluxnic.net> wrote:
> On Wed, 19 May 2010, Jamie Lokier wrote:
>
>> Nicolas Pitre wrote:
>> > It is not up to the bootloader to "adjust" to the kernel.  But rather
>> > for the kernel to cope with the bootloader's provided information.  If
>> > the bootloader passes a specific machine ID with the ATAG list then the
>> > kernel will use that, and if the bootloader passes a DT machine ID with
>> > a DT blob then the kernel will use that.  You just have to configure
>> > your kernel with both "machine types" at the same time.
>>
>> Scenario:
>>
>> You upgrade your systems to a new DT-capable kernel and DT-capable
>> bootloader.  It works great.  You ship new instances of your device with this.
>>
>> Once they're in the field, someone reports a bug that doesn't happen
>> with the older device instances.  It's not a bug you can reproduce,
>> but you suspect the newer of kernel.  So you remote-update some of the
>> newly shipped devices with an old, pre-DT kernel binary that's been
>> stable on the older devices, to see if the bug goes away.
>>
>> Problem: The newer shipped devices have a DT-capable bootloader, and
>> it can't boot old kernels, because they don't understand the DT format.
>>
>> You could remote-downgrade the bootloaders, but that's risky.  You
>> could try building the old kernel with DT support, but that adds
>> another variable to your testing.
>>
>> Anticipating this well in advance, you of course built your DT-capable
>> bootloader with the ability to boot old and new style kernels...
>
> Exact.  Quoting myself:
>
> |I think that, for the moment, it is best if the bootloader on already
> |existing subarchitectures where DT is introduced still preserve the
> |already existing ability to boot using ATAGs.  This allows for the
> |testing and validation of the DT concept against the legacy ATAG method
> |more easily.
> |
> |On new subarchitectures, it might make sense to go with DT from the
> |start instead of creating setup code for every single machine.  In that
> |case the bootloader for those machines would only need to care about DT
> |and forget about ATAGs.
>
>
> Nicolas
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