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Subject: Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for
 scatter-gather buffers
To:     Christoph Hellwig <hch@lst.de>, Robin Murphy <robin.murphy@arm.com>
Cc:     Sinan Kaya <okaya@codeaurora.org>, amd-gfx@lists.freedesktop.org,
        timur@codeaurora.org, sulrich@codeaurora.org,
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        David Panariti <David.Panariti@amd.com>,
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        Roger He <Hongbo.He@amd.com>, Monk Liu <Monk.Liu@amd.com>,
        Feifei Xu <Feifei.Xu@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux-foundation.org
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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Am 12.04.2018 um 08:26 schrieb Christoph Hellwig:
> 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.

I agree that the common DRM functions should be able to deal with both, 
but we should let the driver side decide if it wants multiple addresses 
combined or not.

>
>>> 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.

Sounds like my understanding of max_seg_size is incorrect, what exactly 
should that describe?

Thanks,
Christian.