[RFC] Remove AGP support from Radeon/Nouveau/TTM

Michel Dänzer michel at daenzer.net
Wed May 13 10:26:53 UTC 2020

On 2020-05-13 11:28 a.m., Rui Salvaterra wrote:
> On Wed, 13 May 2020 at 08:19, Daniel Vetter <daniel at ffwll.ch> wrote:
>> i915 is even worse, we manually mess around with clflush. In
>> userspace. So really there's 2 axis for dma memory: coherent vs.
>> non-coherent (which is something the dma-api somewhat exposed), i.e.
>> do you need to clflush or not, and cached vs uncached, i.e. are the
>> PAT entries wc or wb.
> So, the PowerPC AGP GART ends up being cached and non-coherent, right
> (assuming there's no way to set the page attributes MTRR/PAT-style)?

It was uncached when I was using my last-gen PowerBook (until a few
years ago), though it's possible that broke since then. I don't remember
the details how it's done offhand though.

The only theoretical problem there was that the kernel still had a
cacheable mapping of the same memory, and any access via that (e.g.
prefetch due to access to a neighbouring page) could trigger a machine
check. But I don't remember ever hitting that. Maybe I was just lucky
all those years.

Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer

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