Re: [PATCH 1/3] drm/amdgpu: add AMDGPU_GEM_CREATE_DISCARDABLE

[Marek Olšák <maraeo@xxxxxxxxx>]

3rd question: Is it worth using this on APUs?

Thanks,

Marek

On Wed, May 11, 2022 at 5:58 PM Marek Olšák <maraeo@xxxxxxxxx> wrote:

Will the kernel keep all discardable buffers in VRAM if VRAM is not overcommitted by discardable buffers, or will other buffers also affect the placement of discardable buffers?

Do evictions deallocate the buffer, or do they keep an allocation in GTT and only the copy is skipped?

Thanks,

Marek

On Wed, May 11, 2022 at 3:08 AM Marek Olšák <maraeo@xxxxxxxxx> wrote:

OK that sounds good.

Marek

On Wed, May 11, 2022 at 2:04 AM Christian König <ckoenig.leichtzumerken@xxxxxxxxx> wrote:

Hi Marek,

Am 10.05.22 um 22:43 schrieb Marek Olšák:

A better flag name would be:

AMDGPU_GEM_CREATE_BEST_PLACEMENT_OR_DISCARD

A bit long for my taste and I think the best placement is just a side effect.

Marek

On Tue, May 10, 2022 at 4:13 PM Marek Olšák <maraeo@xxxxxxxxx> wrote:

Does this really guarantee VRAM placement? The code doesn't say anything about that.

Yes, see the code here:

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
index 8b7ee1142d9a..1944ef37a61e 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
@@ -567,6 +567,7 @@ int amdgpu_bo_create(struct amdgpu_device *adev,
                bp->domain;
        bo->allowed_domains = bo->preferred_domains;
        if (bp->type != ttm_bo_type_kernel &&
+           !(bp->flags & AMDGPU_GEM_CREATE_DISCARDABLE) &&
            bo->allowed_domains == AMDGPU_GEM_DOMAIN_VRAM)
                bo->allowed_domains |= AMDGPU_GEM_DOMAIN_GTT;

The only case where this could be circumvented is when you try to allocate more than physically available on an APU.

E.g. you only have something like 32 MiB VRAM and request 64 MiB, then the GEM code will catch the error and fallback to GTT (IIRC).

Regards,
Christian.