linux-kernel - Re: [PATCH] drm/amdgpu: limit DMA size to PAGE

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Date:   Tue, 10 Apr 2018 20:49:14 +0200
From:   Christian König <christian.koenig@....com>
To:     Sinan Kaya <okaya@...eaurora.org>, amd-gfx@...ts.freedesktop.org,
        timur@...eaurora.org, sulrich@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Alex Deucher <alexander.deucher@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        Michel Dänzer <michel.daenzer@....com>,
        Tom St Denis <tom.stdenis@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Roger He <Hongbo.He@....com>, Monk Liu <Monk.Liu@....com>,
        Harish Kasiviswanathan <Harish.Kasiviswanathan@....com>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/amdgpu: limit DMA size to PAGE_SIZE for
 scatter-gather buffers

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@...eaurora.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);