Re: [PATCH V2] drm/amdgpu: limit DMA size to PAGE_SIZE for scatter-gather buffers

[Christoph Hellwig <hch@lst.de>]

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.