On Thu, 24 Jan 2019 at 10:31, Michel Dänzer michel@daenzer.net wrote:
On 2019-01-23 5:52 p.m., Ard Biesheuvel wrote:
On Wed, 23 Jan 2019 at 17:44, Christoph Hellwig hch@infradead.org 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