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

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Message-ID: <20180412062600.GB30499@lst.de>
Date:   Thu, 12 Apr 2018 08:26:00 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Sinan Kaya <okaya@...eaurora.org>, amd-gfx@...ts.freedesktop.org,
        timur@...eaurora.org, sulrich@...eaurora.org,
        Tom St Denis <tom.stdenis@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        Emily Deng <Emily.Deng@....com>,
        David Airlie <airlied@...ux.ie>, linux-arm-msm@...r.kernel.org,
        Felix Kuehling <Felix.Kuehling@....com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        David Panariti <David.Panariti@....com>,
        Jim Qu <Jim.Qu@....com>, Huang Rui <ray.huang@....com>,
        Roger He <Hongbo.He@....com>, Monk Liu <Monk.Liu@....com>,
        Feifei Xu <Feifei.Xu@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, hch@....de
Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for
        scatter-gather buffers

On Wed, Apr 11, 2018 at 01:03:59PM +0100, Robin Murphy wrote:
> On 10/04/18 21:59, 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.
>
> So why not fix said code? It's clearly not a real hardware limitation, and 
> the map_sg() APIs have potentially returned fewer than nents since forever, 
> so there's really no excuse.

Yes, relying on dma_map_sg returning the same number of entries as passed
it is completely bogus.

>> 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.
>
> Disagree; this is a dodgy hack, since you'll now end up passing 
> scatterlists into dma_map_sg() which already violate max_seg_size to begin 
> with, and I think a conscientious DMA API implementation would be at rights 
> to fail the mapping for that reason (I know arm64 happens not to, but that 
> was a deliberate design decision to make my life easier at the time).

Agreed.