Re: [PATCH] Revert "drm/amdgpu: remove TOPDOWN flags when allocating VRAM in large bar system"

[Felix Kuehling <felix.kuehling@amd.com>]

Am 2023-05-20 um 05:25 schrieb Arunpravin Paneer Selvam:
> This reverts commit c105518679b6e87232874ffc989ec403bee59664.
>
> This patch disables the TOPDOWN flag for APU and few dGPU cards
> which has the VRAM size equal to the BAR size.

With resizable BARs it's not that rare.


>
> When we enable the TOPDOWN flag, we get the free blocks at
> the highest available memory region and we don't split the
> lower order blocks. This change is required to keep off
> the fragmentation related issues particularly in ASIC
> which has VRAM space <= 500MiB

If TOPDOWN has these general benefits, then would it make sense to 
allocate visible memory TOPDOWN as well, on large-BAR GPUs? Without 
knowing too much about the internals of the allocator, my intuition 
tells me that using only one allocation strategy has a better chance of 
minimizing fragmentation than mixing two allocation strategies for no 
good reason.

Regards,
   Felix


>
> Hence, we are reverting this patch.
>
> Gitlab issue link - https://gitlab.freedesktop.org/drm/amd/-/issues/2270
>
> Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> index 2bd1a54ee866..ca5fc07faf6c 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> @@ -139,7 +139,7 @@ void amdgpu_bo_placement_from_domain(struct amdgpu_bo *abo, u32 domain)
>   
>   		if (flags & AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED)
>   			places[c].lpfn = visible_pfn;
> -		else if (adev->gmc.real_vram_size != adev->gmc.visible_vram_size)
> +		else
>   			places[c].flags |= TTM_PL_FLAG_TOPDOWN;
>   
>   		if (flags & AMDGPU_GEM_CREATE_VRAM_CONTIGUOUS)