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

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Message-Id: <1523384705-3976-1-git-send-email-okaya@codeaurora.org>
Date:   Tue, 10 Apr 2018 14:25:03 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     amd-gfx@...ts.freedesktop.org, timur@...eaurora.org,
        sulrich@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Sinan Kaya <okaya@...eaurora.org>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....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>,
        dri-devel@...ts.freedesktop.org (open list:DRM DRIVERS),
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers

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@...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);
-- 
2.7.4