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

[Robin Murphy]

On 11/04/18 15:33, Sinan Kaya wrote:
> On 4/11/2018 8:03 AM, 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.
>
> Sure, I'll take a better fix if there is one.
>
> > > 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).
> >
> > As a short-term fix, at least do something like what i915 does and
> > constrain the table allocation to the desired segment size as well,
> > so things remain self-consistent. But still never claim that faking
> > a hardware constraint as a workaround for a driver shortcoming is
> > "the right thing to do" ;)
>
> You are asking for something like this from here, right?
>
> https://elixir.bootlin.com/linux/v4.16.1/source/drivers/gpu/drm/i915/i915_gem_dmabuf.c#L58

I just looked for callers of __sg_alloc_table_from_pages() with a limit 
other than SCATTERLIST_MAX_SEGMENT, and found 
__i915_gem_userptr_alloc_pages(), which at first glance appears to be 
doing something vaguely similar to amdgpu_ttm_tt_pin_userptr().

Robin.