linux-kernel - Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE

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Date:   Wed, 11 Apr 2018 14:26:03 +0800
From:   Huang Rui <ray.huang@....com>
To:     Sinan Kaya <okaya@...eaurora.org>
CC:     <amd-gfx@...ts.freedesktop.org>, <timur@...eaurora.org>,
        <sulrich@...eaurora.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Monk Liu <Monk.Liu@....com>, Roger He <Hongbo.He@....com>,
        Jim Qu <Jim.Qu@....com>, Emily Deng <Emily.Deng@....com>,
        Feifei Xu <Feifei.Xu@....com>,
        Tom St Denis <tom.stdenis@....com>,
        David Panariti <David.Panariti@....com>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for
 scatter-gather buffers

On Tue, Apr 10, 2018 at 04:59:55PM -0400, Sinan Kaya wrote:
> 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>

Sinan, I guess Christian's suggestion is to add amdgpu_device_init function
like here:

8<-------
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
index 0e798b3..9b96771 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
@@ -2339,6 +2339,8 @@ int amdgpu_device_init(struct amdgpu_device *adev,
        /* init the mode config */
        drm_mode_config_init(adev->ddev);

+       dma_set_max_seg_size(adev->dev, PAGE_SIZE);
+
        r = amdgpu_device_ip_init(adev);
        if (r) {
                /* failed in exclusive mode due to timeout */
8<-------

After that, we don't do it in each generation.

Thanks,
Ray