On Fri, Aug 02, 2019 at 11:38:03AM +0100, Will Deacon wrote: > > So this boils down to a terminology mismatch. The Arm architecture doesn't > have > anything called "write combine", so in Linux we instead provide what the Arm > architecture calls "Normal non-cacheable" memory for pgprot_writecombine(). > Amongst other things, this memory type permits speculation, unaligned accesses > and merging of writes. I found something in the architecture spec about > non-cachable memory, but it's written in Armglish[1]. > > pgprot_noncached(), on the other hand, provides what the architecture calls > Strongly Ordered or Device-nGnRnE memory. This is intended for mapping MMIO > (i.e. PCI config space) and therefore forbids speculation, preserves access > size, requires strict alignment and also forces write responses to come from > the endpoint. > > I think the naming mismatch is historical, but on arm64 we wanted to use the > same names as arm32 so that any drivers using these things directly would get > the same behaviour.
That all makes sense, but it totally needs a comment. I'll try to draft one based on this. I've also looked at the arm32 code a bit more, and it seems arm always (?) supported Normal non-cacheable attribute, but Linux only optionally uses it for arm v6+ because of fears of drivers missing barriers. The other really weird things is that in arm32 pgprot_dmacoherent incudes the L_PTE_XN bit, which from my understanding is the no-execture bit, but pgprot_writecombine does not. This seems to not very unintentional. So minus that the whole DMA_ATTR_WRITE_COMBІNE seems to be about flagging old arm specific drivers as having the proper barriers in places and otherwise is a no-op. Here is my tentative plan: - respin this patch with a small fix to handle the DMA_ATTR_NON_CONSISTENT (as in ignore it unless actually supported), but keep the name as-is to avoid churn. This should allow 5.3 inclusion and backports - remove DMA_ATTR_WRITE_COMBINE support from mips, probably also 5.3 material. - move all architectures but arm over to just define pgprot_dmacoherent, including a comment with the above explanation for arm64. - make DMA_ATTR_WRITE_COMBINE a no-op and schedule it for removal, thus removing the last instances of arch_dma_mmap_pgprot
