Re: Boot interface for device trees on ARM

[Nicolas Pitre <nico-vtqb6HGKxmzR7s880joybQ@public.gmane.org>]

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