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_ge...
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.