On Thu, 24 Jan 2019 at 10:31, Michel Dänzer <michel@xxxxxxxxxxx> wrote: > > On 2019-01-23 5:52 p.m., Ard Biesheuvel wrote: > > On Wed, 23 Jan 2019 at 17:44, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > >> > >> I think we just want a driver-local check for those combinations > >> where we know this hack actually works, which really just seems > >> to be x86-64 with PAT. Something like the patch below, but maybe with > >> even more strong warnings to not do something like this elsewhere: > > > > I agree that your patch seems like the right way to ensure that the WC > > optimization hack is only used where we know it works. > > > > But my concern is that it seems likely that non-cache coherent > > implementations are relying on this hack as well. > > I've been trying to tell you they can't rely on that, because the amdgpu > driver doesn't use this functionality for fundamentals such as ring > buffers used for feeding the hardware with commands. Instead, for those > it relies on snooped PCIe transfers being coherent with the CPU caches. > I understand it does not use this functionality for the ring. Instead, it uses the DMA API, no? On non-cache coherent systems, that DMA API will allocate memory and map it uncached for the CPU so that it is coherent with the non-cache coherent device. In any case, if non-cache coherent systems are unlikely to work, and unsupported in case they do, I am fine with disabling this optimization unconditionally for non-X86 architectures. > > >> -#elif defined(CONFIG_X86) && !defined(CONFIG_X86_PAT) > >> - /* Don't try to enable write-combining when it can't work, or things > >> - * may be slow > >> - * See https://bugs.freedesktop.org/show_bug.cgi?id=88758 > >> - */ > >> - > >> -#ifndef CONFIG_COMPILE_TEST > >> -#warning Please enable CONFIG_MTRR and CONFIG_X86_PAT for better performance \ > >> - thanks to write-combining > >> -#endif > >> - > >> - if (bo->flags & AMDGPU_GEM_CREATE_CPU_GTT_USWC) > >> - DRM_INFO_ONCE("Please enable CONFIG_MTRR and CONFIG_X86_PAT for " > >> - "better performance thanks to write-combining\n"); > > FWIW, please don't drop these compile and build time warnings where we > continue to take advantage of PAT. > > > -- > Earthling Michel Dänzer | http://www.amd.com > Libre software enthusiast | Mesa and X developer _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
