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...
ret = sg_alloc_table(st, obj->mm.pages->nents, GFP_KERNEL); if (ret) goto err_free;
src = obj->mm.pages->sgl; dst = st->sgl; for (i = 0; i < obj->mm.pages->nents; i++) { sg_set_page(dst, sg_page(src), src->length, 0); dst = sg_next(dst); src = sg_next(src); }
This seems to allocate the scatter gather list and fill it in manually before passing it to dma_map_sg(). I'll give it a try.
Just double checking.
Robin.
Signed-off-by: Sinan Kaya okaya@codeaurora.org