On Thu, Apr 07, 2022 at 04:17:11PM +0100, Robin Murphy wrote: >> My take is that the drivers using this API are doing it to make sure >> their HW blocks are setup in a way that is consistent with the DMA API >> they are also using, and run in constrained embedded-style >> environments that know the firmware support is present. >> >> So in the end it does not seem suitable right now for linking to >> IOMMU_CACHE.. > > That seems a pretty good summary - I think they're basically all "firmware > told Linux I'm coherent so I'd better act coherent" cases, but that still > doesn't necessarily mean that they're *forced* to respect that. Yes. And the interface is horribly misnamed for that. I'll see what I can do to clean this up as I've noticed various other not very nice things in that area. > One of the > things on my to-do list is to try adding a DMA_ATTR_NO_SNOOP that can force > DMA cache maintenance for coherent devices, primarily to hook up in > Panfrost (where there is a bit of a performance to claw back on the > coherent AmLogic SoCs by leaving certain buffers non-cacheable). This has been an explicit request from the amdgpu folks and thus been on my TODO list for quite a while as well. Note that I don't think it should be a flag to dma_alloc_attrs, but rather for dma_alloc_pages as the drivers that want non-snoop generally also want to actually be able to deal with pages.
