On Mon, 3 Feb 2025 11:23:53 +0000 Liviu Dudau <liviu.dudau@xxxxxxx> wrote: > On Mon, Feb 03, 2025 at 10:21:53AM +0100, Boris Brezillon wrote: > > +Akash with whom we've been discussing adding a 'REPEAT' mode to > > drm_gpuvm/panthor. > > > > On Sun, 2 Feb 2025 19:53:47 +0100 > > Danilo Krummrich <dakr@xxxxxxxxxx> wrote: > > > > > Hi Lina, > > > > > > On Sun, Feb 02, 2025 at 10:34:49PM +0900, Asahi Lina wrote: > > > > Some hardware requires dummy page mappings to efficiently implement > > > > Vulkan sparse features. These mappings consist of the same physical > > > > memory page, repeated for a large range of address space (e.g. 16GiB). > > > > > > > > Add support for this to drm_gpuvm. Currently, drm_gpuvm expects BO > > > > ranges to correspond 1:1 to virtual memory ranges that are mapped, and > > > > does math on the BO offset accordingly. To make single page mappings > > > > work, we need a way to turn off that math, keeping the BO offset always > > > > constant and pointing to the same page (typically BO offset 0). > > > > > > > > To make this work, we need to handle all the corner cases when these > > > > mappings intersect with regular mappings. The rules are simply to never > > > > mix or merge a "regular" mapping with a single page mapping. > > > > > > > > drm_gpuvm has support for a flags field in drm_gpuva objects. This is > > > > normally managed by drivers directly. We can introduce a > > > > DRM_GPUVA_SINGLE_PAGE flag to handle this. However, to make it work, > > > > sm_map and friends need to know ahead of time whether the new mapping is > > > > a single page mapping or not. Therefore, we need to add an argument to > > > > these functions so drivers can provide the flags to be filled into > > > > drm_gpuva.flags. > > > > > > > > These changes should not affect any existing drivers that use drm_gpuvm > > > > other than the API change: > > > > > > > > - imagination: Does not use flags at all > > > > - nouveau: Only uses drm_gpuva_invalidate(), which is only called on > > > > existing drm_gpuva objects (after the map steps) > > > > - panthor: Does not use flags at all > > > > - xe: Does not use drm_gpuva_init_from_op() or > > > > drm_gpuva_remap()/drm_gpuva_map() (which call it). This means that the > > > > flags field of the gpuva object is managed by the driver only, so these > > > > changes cannot clobber it. > > > > > > > > Note that the way this is implemented, drm_gpuvm does not need to know > > > > the GPU page size. It only has to never do math on the BO offset to meet > > > > the requirements. > > > > > > > > I suspect that after this change there could be some cleanup possible in > > > > the xe driver (which right now passes flags around in various > > > > driver-specific ways from the map step through to drm_gpuva objects), > > > > but I'll leave that to the Xe folks. > > > > > > > > Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx> > > > > --- > > > > Asahi Lina (4): > > > > drm/gpuvm: Add a flags argument to drm_gpuvm_sm_map[_*] > > > > drm/gpuvm: Plumb through flags into drm_gpuva_op_map > > > > drm/gpuvm: Add DRM_GPUVA_SINGLE_PAGE flag and logic > > > > drm/gpuvm: Plumb through flags into drm_gpuva_init > > > > > > Without looking into any details yet: > > > > > > This is a bit of tricky one, since we're not even close to having a user for > > > this new feature upstream yet, are we? > > > > Actually, we would be interesting in having this feature hooked up in > > panthor. One use case we have is Vulkan sparse bindings, of course. But > > we also have cases where we need to map a dummy page repeatedly on the > > FW side. The approach we've been considering is slightly different: > > pass a DRM_GPUVA_REPEAT_FLAG along with GEM range, so we can repeat a > > range of the GEM (see the below diff, which is completely untested by > > the way), but I think we'd be fine with this SINGLE_PAGE flag. > > Unless I've misunderstood the intent completely, it looks like Xe also wants > something similar although they call it CPU_ADDR_MIRROR for some reason: > > https://lore.kernel.org/r/20250129195212.745731-9-matthew.brost@xxxxxxxxx At first glance, it doesn't seem related. The Xe stuff looks more like an alloc-on-fault mechanism. SINGLE_PAGE is about mapping a dummy page repeatedly over a virtual address range so that sparse Vulkan images never get a fault when an unbound image region is accessed.
