On Thu, 24 Oct 2024 15:54:32 +0100 Akash Goel <[email protected]> wrote:
> All CPU mappings are forced as uncached for Panthor buffer objects when > system(IO) coherency is disabled. Physical backing for Panthor BOs is > allocated by shmem, which clears the pages also after allocation. But > there is no explicit cache flush done after the clearing of pages. > So it could happen that there are dirty cachelines in the CPU cache > for the BOs, when they are accessed from the CPU side through uncached > CPU mapping, and the eviction of cachelines overwrites the data of BOs. Hm, this looks like something that should be handled at the drm_gem_shmem level when drm_gem_shmem_object::map_wc=true, as I suspect other drivers can hit the same issue (I'm thinking of panfrost and lima, but there might be others). > > This commit tries to avoid the potential overwrite scenario. > > Signed-off-by: Akash Goel <[email protected]> > --- > drivers/gpu/drm/panthor/panthor_gem.h | 10 ++++++++++ > drivers/gpu/drm/panthor/panthor_mmu.c | 5 +++++ > 2 files changed, 15 insertions(+) > > diff --git a/drivers/gpu/drm/panthor/panthor_gem.h > b/drivers/gpu/drm/panthor/panthor_gem.h > index e43021cf6d45..4b0f43f1edf1 100644 > --- a/drivers/gpu/drm/panthor/panthor_gem.h > +++ b/drivers/gpu/drm/panthor/panthor_gem.h > @@ -46,6 +46,16 @@ struct panthor_gem_object { > > /** @flags: Combination of drm_panthor_bo_flags flags. */ > u32 flags; > + > + /** > + * @cleaned: The buffer object pages have been cleaned. > + * > + * There could be dirty CPU cachelines for the pages of buffer object > + * after allocation, as shmem will zero out the pages. The cachelines > + * need to be cleaned if the pages are going to be accessed with an > + * uncached CPU mapping. > + */ > + bool cleaned; > }; > > /** > diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c > b/drivers/gpu/drm/panthor/panthor_mmu.c > index f522a116c1b1..d8cc9e7d064e 100644 > --- a/drivers/gpu/drm/panthor/panthor_mmu.c > +++ b/drivers/gpu/drm/panthor/panthor_mmu.c > @@ -1249,6 +1249,11 @@ static int panthor_vm_prepare_map_op_ctx(struct > panthor_vm_op_ctx *op_ctx, > > op_ctx->map.sgt = sgt; > > + if (bo->base.map_wc && !bo->cleaned) { > + dma_sync_sgtable_for_device(vm->ptdev->base.dev, sgt, > DMA_TO_DEVICE); > + bo->cleaned = true; > + } > + > preallocated_vm_bo = drm_gpuvm_bo_create(&vm->base, &bo->base.base); > if (!preallocated_vm_bo) { > if (!bo->base.base.import_attach)
