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

[Christian KÃnig]

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.