Reading twd_base at run-time

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Wed, Apr 01, 2015 at 02:07:05PM +0200, Mason wrote:
> On 27/03/2015 21:53, Russell King - ARM Linux wrote:
> 
> >That's one scenario.  Here's the scenario Mark is describing - one which
> >has real-world examples:
> >
> >Hardware engineer picks address A for rev A and sets CP15 to address A.
> >Everything works.  Hardware engineer then picks address B for rev B, but
> >forgets to update CP15.  It breaks.
> 
> The hardware engineer told me that whatever arbitrary value is chosen
> for PERIPH_BASE is automatically exported through CP15 (which is how
> I thought this worked). So there is no "forgetting to update CP15"
> (for this platform, at least).

I'm sorry, it's not that I don't believe you, it's that ARM Ltd
employees already have evidence to the contary, and they should know
what's possible, they (as a company) designed the hardware and they're
the ones who have to deal with queries from _all_ the silicon vendors.
They get to know what the entire ARM ecosystem is doing, what vendors
get wrong, etc.  They're in a far better position than just one silicon
vendor to know what's possible and what isn't.

So when Mark says something has been seen, I believe him, and that
trumps what hardware engineers at individual silicon vendors claim.

> >If it's in DT, it can be fixed.  It should be there anyway as part of
> >the hardware description.  DT is a description of the hardware.
> 
> I thought DT was supposed to describe hardware that /cannot/ be probed
> or discovered at run-time?

And what about the cases where it is possible to probe the hardware on
some platforms but doing so crashes the kernel on others?  I guess you
don't care about anything but your own platform - that's the kind of
message you're putting out...

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