Re: [PATCH] ARM: vexpress: initial device tree support

[Dave Martin <dave.martin-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Wed, Sep 21, 2011 at 11:37:54AM -0500, Rob Herring wrote:
> On 09/21/2011 09:57 AM, Grant Likely wrote:
> > On Wed, Sep 21, 2011 at 7:24 AM, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >> On 09/21/2011 04:19 AM, Dave Martin wrote:
> >>>       * arm,amba-bus -- widely used by other boards and patchsets, but
> >>>         seems not to be documented.
> >>>
> >>
> >> This should be dropped. There's not really any bus component to an amba
> >> bus. All the probing info is within the primecell peripherals.
> > 
> > No, if it is an AMBA bus, then it is entirely appropriate to declare
> > it as an amba bus, but to also be compatible with "simple-bus".  In
> > fact, it would be better to use a compatible string that specifies the
> > specific implementation of AMBA bus since there are several versions
> > of the spec.
> 
> And type of AMBA bus as the spec includes AXI, AHB, and APB. None of
> which have any sort of programmability or software view.
> 
> If this is required, then the policy should be simple-bus should never
> be allowed alone as every bus has some underlying type. Seems like
> overkill for buses like this.

The key question is _where_ to draw the line between generic and specific.
By definition, the DT can never be a comprehensive description of the
hardware -- rather a good DT is a description of those details of the hardware
which could relevant to any hypothetical OS.

The flipside is that details which were thought to be irrelevant at
design/implementation time can turn out to be relevant in practice, due
to errata and implementation issues etc.  So taking the description slightly
beyond what the OS needs to know can still have some merit.


I still don't know how to say where the line should be drawn in this particular
case though.

Cheers
---Dave