Boot interface for device trees on ARM

[Grant Likely]

On Tue, May 18, 2010 at 7:41 PM, Jamie Lokier <jamie at shareable.org> wrote:
> David Gibson wrote:
>> On Tue, May 18, 2010 at 08:24:06AM -0400, Nicolas Pitre wrote:
>> > On Tue, 18 May 2010, David Gibson wrote:
>> > > On Tue, May 18, 2010 at 01:24:43PM +0800, Jeremy Kerr wrote:
>> > > > Nicolas Pitre wrote:
>> [snip]
>> > > The only reason you'd need a subarchitecture number or equivalent is
>> > > if you need to do things differently in the very, very early asm boot
>> > > code.  We may need a minimal form of this on PowerPC if we ever
>> > > support multiple MMU families in the same kernel binary.
>> >
>> > Exact.  For example, on ARM the machine ID is also used to figure out
>> > the MMU mapping needed to be able to simply be able to debug the very
>> > early assembly boot stage when there isn't even a stack available. While
>> > this info is stored in the machine record, it is actually
>> > subarchitecture specific and already half-digested for easy usage by
>> > that initial MMU setup.  I just don't want to imagine what the
>> > equivalent functionality with DT would look like.
>>
>> Well, it wouldn't be *that* bad - you'd need a minimal asm-only tree
>> walker to find and look up the compatible property.  Quite possible
>> but, yes, fairly awkward.
>
> I'm not entirely clear, is the DT intended to replace the command line
> for saying things like "console=XXX"?

It doesn't replace the kernel parameters line, but the kernel
parameters line can be passed to the kernel via a property in the
/chosen node.  Firmware is allowed to modify the DT (ie. to change the
kernel parameters) before passing it to the kernel.

Cheers,
g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.