[PATCH 6/9] drm/amdgpu: Set/clear CPU_ACCESS_REQUIRED flag on page fault and CS

[Michel Dänzer]

On 25/06/17 03:00 AM, Christian König wrote:
> Am 23.06.2017 um 19:39 schrieb John Brooks:
>> When the AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED flag is given by
>> userspace,
>> it should only be treated as a hint to initially place a BO somewhere CPU
>> accessible, rather than having a permanent effect on BO placement.
> 
> And that is a clear NAK from my side.
> 
> CPU_ACCESS_REQUIRED is a permanent limitation to where the buffer should
> be placed.

It really can't be more than a hint. The userspace driver cannot
reliably know ahead of time whether a BO will be accessed by the CPU at
all, let alone how often. A BO which incorrectly has this flag set
creates artificial pressure on CPU visible VRAM.


> Ignoring that and changing the IOCTL interface like this is clearly not
> acceptable.

I'd say it's more adapting the semantics of the flag to reality. :)


>> Instead of the flag being set in stone at BO creation, set the flag
>> when a page fault occurs so that it goes somewhere CPU-visible, and clear it
>> when the BO is requested by the GPU.

My idea was to only clear the flag when the BO is moved from GTT to
VRAM. That way, any BO which was ever accessed by the CPU since it was
last moved to VRAM will be preferably put in the CPU visible part of VRAM.

Not sure which way is better though.


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