ARM machine specific DT probing

[Jeremy Kerr]

Hi Nicolas,

> What Jeremy did is to add a probe_dt method in the mdesc structure, and 
> then the core is calling them in sequence until one of them returns 
> success.
> 
> now, the "compatible" property is explained here:
> 
> http://devicetree.org/Device_Tree_Usage#Understanding_the_compatible_Property
> 
> From my understanding, this could allow for a kernel that doesn't yet 
> support the specifics of a particular board to still be able to work 
> using basic common support.  But for this to work, wouldn't it be 
> necessary for the core code to try to find the best match itself rather 
> than letting each machine's  probe_dt decide on their own?

At present, the probe_dt function is a binary match/no-match indicator,
so the core code just selects the first match it finds. This means that
we'll need one mdesc per machine; I'm intending to keep it simple to
start with.

My intention in the longer-term is to allow probe_dt to indicate
less-specific matches (eg, match on the SoC family), and change the core
code to not break out of the loop on the first match, but continue to
look for a better match. This way, we can have one mdesc to support a
SoC family, but still allow that to be overridden if there's a more
specific (eg, single machine) mdesc compiled in.

The reason I do this in the machine-specific code (rather than the core
code checking the compatible property) is to allow the probe_dt function
to check arbitrary data in the device tree. Perhaps we have two
variations of a machine - both with the same compatible property, but
distinguishable in some other way, and we need a separate mdesc for
whatever reason.

Cheers,


Jeremy