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 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@xxxxxxxxxxxxxx> -- Sinan Kaya Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
