On 2019-01-21 6:59 p.m., Ard Biesheuvel wrote: > On Mon, 21 Jan 2019 at 18:55, Michel Dänzer <michel@xxxxxxxxxxx> wrote: >> >> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote: >>> On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: >>> >>>> Until that happens we should just change the driver ifdefs to default >>>> the hacks to off and only enable them on setups where we 100% >>>> positively know that they actually work. And document that fact >>>> in big fat comments. >>> >>> Well, as I mentioned in my commit log as well, if we default to off >>> unless CONFIG_X86, we may break working setups on MIPS and Power where >>> the device is in fact non-cache coherent, and relies on this >>> 'optimization' to get things working. >> >> FWIW, the amdgpu driver doesn't rely on non-snooped transfers for >> correct basic operation (the scenario Christian brought up is a very >> specialized use-case), so that shouldn't be an issue. >> > > The point is that this is only true for x86. > > On other architectures, the use of non-cached mappings on the CPU side > means that you /do/ rely on non-snooped transfers, since if those > transfers turn out not to snoop inadvertently, the accesses are > incoherent with the CPU's view of memory. The driver generally only uses non-cached mappings if drm_arch/device_can_wc_memory returns true. -- 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
