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.
