On Wed, Aug 7, 2019 at 5:38 AM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > On Tue, Aug 06, 2019 at 09:31:55AM -0700, Rob Clark wrote: > > On Tue, Aug 6, 2019 at 7:35 AM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > > > > On Tue, Aug 06, 2019 at 07:11:41AM -0700, Rob Clark wrote: > > > > On Tue, Aug 6, 2019 at 1:48 AM Christoph Hellwig <hch@xxxxxx> wrote: > > > > > > > > > > This goes in the wrong direction. drm_cflush_* are a bad API we need to > > > > > get rid of, not add use of it. The reason for that is two-fold: > > > > > > > > > > a) it doesn't address how cache maintaince actually works in most > > > > > platforms. When talking about a cache we three fundamental operations: > > > > > > > > > > 1) write back - this writes the content of the cache back to the > > > > > backing memory > > > > > 2) invalidate - this remove the content of the cache > > > > > 3) write back + invalidate - do both of the above > > > > > > > > Agreed that drm_cflush_* isn't a great API. In this particular case > > > > (IIUC), I need wb+inv so that there aren't dirty cache lines that drop > > > > out to memory later, and so that I don't get a cache hit on > > > > uncached/wc mmap'ing. > > > > > > Is there a cacheable alias lying around (e.g. the linear map), or are > > > these addresses only mapped uncached/wc? > > > > > > If there's a cacheable alias, performing an invalidate isn't sufficient, > > > since a CPU can allocate a new (clean) entry at any point in time (e.g. > > > as a result of prefetching or arbitrary speculation). > > > > I *believe* that there are not alias mappings (that I don't control > > myself) for pages coming from > > shmem_file_setup()/shmem_read_mapping_page().. > > AFAICT, that's regular anonymous memory, so there will be a cacheable > alias in the linear/direct map. tbh, I'm not 100% sure whether there is a cacheable alias, or whether any potential linear map is torn down. My understanding is that a cacheable alias is "ok", with some caveats.. ie. that the cacheable alias is not accessed. I'm not entirely sure about pre-fetch from access to adjacent pages. We have been using shmem as a source for pages since the beginning, and I haven't seen it cause any problems in the last 6 years. (This is limited to armv7 and armv8, I'm not really sure what would happen on armv6, but that is a combo I don't have to care about.) BR, -R > > digging around at what dma_sync_sg_* does under the hood, it looks > > like it is just arch_sync_dma_for_cpu/device(), so I guess that should > > be sufficient for what I need. > > I don't think that's the case, per the example I gave above. > > Thanks, > Mark. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
