[PATCH] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers

[PATCH] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers

[Sinan Kaya]

Code is expecing to observe the same number of buffers returned from
dma_map_sg() function compared to sg_alloc_table_from_pages(). This
doesn't hold true universally especially for systems with IOMMU.

IOMMU driver tries to combine buffers into a single DMA address as much
as it can. The right thing is to tell the DMA layer how much combining
IOMMU can do.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index e4bb435..02465cd 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -787,6 +787,8 @@ static int amdgpu_ttm_tt_pin_userptr(struct ttm_tt *ttm)
 	enum dma_data_direction direction = write ?
 		DMA_BIDIRECTIONAL : DMA_TO_DEVICE;
 
+	dma_set_max_seg_size(adev->dev, PAGE_SIZE);
+
 	r = sg_alloc_table_from_pages(ttm->sg, ttm->pages, ttm->num_pages, 0,
 				      ttm->num_pages << PAGE_SHIFT,
 				      GFP_KERNEL);
-- 
2.7.4

Re: [PATCH] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers

[Christian König]

Am 10.04.2018 um 20:25 schrieb Sinan Kaya:
> Code is expecing to observe the same number of buffers returned from
> dma_map_sg() function compared to sg_alloc_table_from_pages(). This
> doesn't hold true universally especially for systems with IOMMU.
>
> IOMMU driver tries to combine buffers into a single DMA address as much
> as it can. The right thing is to tell the DMA layer how much combining
> IOMMU can do.

Good catch, but wrong place to set this.

Please move it into the device initialization functions.

Regards,
Christian.

>
> Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> index e4bb435..02465cd 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> @@ -787,6 +787,8 @@ static int amdgpu_ttm_tt_pin_userptr(struct ttm_tt *ttm)
>   	enum dma_data_direction direction = write ?
>   		DMA_BIDIRECTIONAL : DMA_TO_DEVICE;
>   
> +	dma_set_max_seg_size(adev->dev, PAGE_SIZE);
> +
>   	r = sg_alloc_table_from_pages(ttm->sg, ttm->pages, ttm->num_pages, 0,
>   				      ttm->num_pages << PAGE_SHIFT,
>   				      GFP_KERNEL);