Deprecation of AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED

[Michel Dänzer]

On 03/07/17 10:03 PM, Marek Olšák wrote:
> On Mon, Jul 3, 2017 at 12:08 PM, Michel Dänzer <michel at daenzer.net> wrote:
>> On 30/06/17 08:43 PM, Marek Olšák wrote:
>>>
>>> I don't know what is being talked about here anymore, but I wouldn't
>>> like to use CPU_ACCESS_REQUIRED or CPU_ACCESS_REALLY_REQUIRED in
>>> userspace. The reason is that userspace doesn't and can't know whether
>>> CPU access will be required, and the frequency at which it will be
>>> required. 3 heaps {no CPU access, no flag, CPU access required} are
>>> too many. Userspace mostly doesn't use the "no flag" heap for VRAM. It
>>> uses "CPU access required" for almost everything except tiled
>>> textures, which use "no CPU access".
>>
>> FWIW, the difference between setting CPU_ACCESS_REQUIRED and not setting
>> it for a BO created in VRAM will be: If it's set, the BO is initially
>> created in CPU visible VRAM, otherwise it's most likely created in CPU
>> invisible VRAM.
>>
>> If userspace knows that a BO will likely be accessed by the CPU first,
>> setting the flag could save a move from CPU invisible to CPU visible
>> VRAM when the CPU access happens. Conversely, if a BO will likely never
>> be accessed by the CPU, not setting the flag may reduce pressure on CPU
>> visible VRAM.
>>
>> Not sure radeonsi can make this distinction though.
> 
> It can't.

Not sure why, but anyway.


> Either all mappable BOs set CPU_ACCESS_REQUIRED, or all mappable BOs
> don't set it.

FWIW, with kernels without John's fixes, the flag shouldn't be set for
all such BOs, because it would prevent them from ever going to CPU
invisible VRAM.

With John's fixes, one possibility would be to set the flag based on the
existing heuristics or some variant, and use it when a BO is newly
created from the kernel, but to ignore the flag when reusing a BO from
the cache (with John's fixes, whether or not the flag was set when a BO
was created won't directly affect its state WRT the flag when it's
reused from the cache).


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