Am 24.01.19 um 10:28 schrieb Ard Biesheuvel: > On Thu, 24 Jan 2019 at 10:25, Koenig, Christian > <Christian.Koenig@xxxxxxx> wrote: >> Am 24.01.19 um 10:13 schrieb Christoph Hellwig: >>> On Wed, Jan 23, 2019 at 05:52:50PM +0100, Ard Biesheuvel wrote: >>>> But my concern is that it seems likely that non-cache coherent >>>> implementations are relying on this hack as well. There must be a >>>> reason that this hack is only disabled for PowerPC platforms if they >>>> are cache coherent, for instance, and I suspect that that reason is >>>> that the hack is the only thing ensuring that the CPU mapping >>>> attributes match the device ones used for these buffers (the vmap()ed >>>> ones), whereas the rings and other consistent data structures are >>>> using the DMA API as intended, and thus getting uncached attributes in >>>> the correct way. >>> Dave, who added that commit is on Cc together with just about everyone >>> involved in the review chain. Based on the previous explanation >>> that idea what we might want an uncached mapping for some non-coherent >>> architectures for this to work at all makes sense, but then again >>> the way to create those mappings is entirely architecture specific, >>> and also need a cache flushing before creating the mapping to work >>> properly. So my working theory is that this code never properly >>> worked on architectures without DMA coherent for PCIe at all, but >>> I'd love to be corrected by concrete examples including an explanation >>> of how it actually ends up working. >> Cache coherency is mandatory for modern GPU operation. >> >> Otherwise you can't implement a bunch of the requirements of the >> userspace APIs. >> >> In other words the applications doesn't inform the driver that the GPU >> or the CPU is accessing data, it just does it and assumes that it works. >> > Wonderful! > > In that case, do you have any objections to the patch proposed by > Christoph above? Yeah, the patch of Christoph actually goes way to far cause we have reports that this works on a bunch of other architectures. E.g. X86 64bit, PowerPC (under some conditions) and some MIPS. The only problematic here actually seems to be ARM, so you should probably just add an "#ifdef .._ARM return false;". Regards, Christian. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
