Modern userspace APIs like Vulkan are built on an explicit synchronization model. This doesn't always play nicely with the implicit synchronization used in the kernel and assumed by X11 and Wayland. The client -> compositor half of the synchronization isn't too bad, at least on intel, because we can control whether or not i915 synchronizes on the buffer and whether or not it's considered written. The harder part is the compositor -> client synchronization when we get the buffer back from the compositor. We're required to be able to provide the client with a VkSemaphore and VkFence representing the point in time where the window system (compositor and/or display) finished using the buffer. With current APIs, it's very hard to do this in such a way that we don't get confused by the Vulkan driver's access of the buffer. In particular, once we tell the kernel that we're rendering to the buffer again, any CPU waits on the buffer or GPU dependencies will wait on some of the client rendering and not just the compositor. This new IOCTL solves this problem by allowing us to get a snapshot of the implicit synchronization state of a given dma-buf in the form of a sync file. It's effectively the same as a poll() or I915_GEM_WAIT only, instead of CPU waiting directly, it encapsulates the wait operation, at the current moment in time, in a sync_file so we can check/wait on it later. As long as the Vulkan driver does the sync_file export from the dma-buf before we re-introduce it for rendering, it will only contain fences from the compositor or display. This allows to accurately turn it into a VkFence or VkSemaphore without any over- synchronization. The only remaining TODO item as far as I know is that some kernel CI system is sending me nastygrams about build system problems. With this change, dma-fence now has a dependency on sync_file and I really don't know how best to solve it. Should we make sync_file no longer optional? Should I hide the new ioctl behind a #define? If so, what #define? I'm a bit lost when it comes to KConfig, I'm afraid. Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037 IGT tests: https://lists.freedesktop.org/archives/igt-dev/2021-March/029825.html Cc: Christian König <christian.koenig@xxxxxxx> Cc: Michel Dänzer <michel@xxxxxxxxxxx> Cc: Dave Airlie <airlied@xxxxxxxxxx> Cc: Bas Nieuwenhuizen <bas@xxxxxxxxxxxxxxxxxxx> Cc: Daniel Stone <daniels@xxxxxxxxxxxxx> Christian König (2): dma-buf: add dma_fence_array_for_each (v2) dma-buf: add dma_resv_get_singleton (v2) Jason Ekstrand (1): dma-buf: Add an API for exporting sync files (v6) drivers/dma-buf/dma-buf.c | 55 ++++++++++++++ drivers/dma-buf/dma-fence-array.c | 27 +++++++ drivers/dma-buf/dma-resv.c | 118 ++++++++++++++++++++++++++++++ include/linux/dma-fence-array.h | 17 +++++ include/linux/dma-resv.h | 3 + include/uapi/linux/dma-buf.h | 6 ++ 6 files changed, 226 insertions(+) -- 2.29.2 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
