Re: [RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM

[Michael Ellerman]

Hi Will,

Will Deacon <will.deacon@xxxxxxx> writes:
> [+ BenH and MPE]
>
> On Mon, Jan 14, 2019 at 07:21:08PM +0000, Koenig, Christian wrote:
>> Am 14.01.19 um 20:13 schrieb Will Deacon:
...
>
>> > The Arm architecture (and others including Power afaiu) doesn't
>> > guarantee coherency when memory is accessed using mismatched cacheability
>> > attributes.
...
>
>> As far as I know Power doesn't really supports un-cached memory at all, 
>> except for a very very old and odd configuration with AGP.
>
> Hopefully Michael/Ben can elaborate here, but I was under the (possibly
> mistaken) impression that mismatched attributes could cause a machine-check
> on Power.

That's what I've always been told, but I can't actually find where it's
documented, I'll keep searching.

But you're right that mixing cached / uncached is not really supported,
and probably results in a machine check or worse.

cheers