"Fixes" for page flipping under PRIME on AMD & nouveau

[Michel Dänzer]

On 18/08/16 05:20 PM, Christian König wrote:
> Am 18.08.2016 um 09:52 schrieb Michel Dänzer:
>> On 18/08/16 04:41 PM, Christian König wrote:
>>>> Afaiu the prime importing display gpu generates its own gem buffer
>>>> handle (prime_fd_to_handle) from that dmabuf, importing scather-gather
>>>> tables to access the dmabuf in system ram. As far as page flipping is
>>>> concerned, so far those gem buffers / radeon_bo's aren't treated any
>>>> different than native ones. During pageflip setup they get pinned into
>>>> VRAM, which moves (=copies) their content from the RAM dmabuf backing
>>>> store into VRAM.
>>> Your understanding isn't correct. Buffers imported using prime always
>>> stay in GTT, they can't be moved to VRAM.
>> That's the theory, but based on Mario's description it's clear that
>> there is at least one bug which either actually allows a shared buffer
>> to be moved to VRAM, or at least doesn't propagate the error correctly,
>> so the page flip operation "succeeds".
>>
>>
>>> It's the DDX which copies the buffer content from the imported prime
>>> handle into a native on which is enabled to scan out.
>> There is no such code which could explain what Mario is seeing.
> 
> How should this work then otherwise?

[...]

> On the other hand that we accidentally move a prime imported buffer to
> VRAM could be possible, but this would clearly be a rather severe bug we
> hopefully have noticed already.

That's what seems to be happening, based on Mario's description and patches.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer