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.
This patch series actually contains two new ioctls. There is the export one mentioned above as well as an RFC for an import ioctl which provides the other half. The intention is to land the export ioctl since it seems like there's no real disagreement on that one. The import ioctl, however, has a lot of debate around it so it's intended to be RFC-only for now.
Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037 IGT tests: https://patchwork.freedesktop.org/series/90490/
v10 (Jason Ekstrand, Daniel Vetter): - Add reviews/acks - Add a patch to rename _rcu to _unlocked - Split things better so import is clearly RFC status
v11 (Daniel Vetter): - Add more CCs to try and get maintainers - Add a patch to document DMA_BUF_IOCTL_SYNC - Generally better docs - Use separate structs for import/export (easier to document) - Fix an issue in the import patch
v12 (Daniel Vetter): - Better docs for DMA_BUF_IOCTL_SYNC
v12 (Christian König): - Drop the rename patch in favor of Christian's series - Add a comment to the commit message for the dma-buf sync_file export ioctl saying why we made it an ioctl on dma-buf
v13 (Jason Ekstrand): - Rebase on Christian König's fence rework
Cc: Christian König christian.koenig@amd.com Cc: Michel Dänzer michel@daenzer.net Cc: Dave Airlie airlied@redhat.com Cc: Bas Nieuwenhuizen bas@basnieuwenhuizen.nl Cc: Daniel Stone daniels@collabora.com Cc: mesa-dev@lists.freedesktop.org Cc: wayland-devel@lists.freedesktop.org
Jason Ekstrand (2): dma-buf: Add an API for exporting sync files (v13) dma-buf: Add an API for importing sync files (v8)
drivers/dma-buf/dma-buf.c | 100 +++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 57 ++++++++++++++++++++ 2 files changed, 157 insertions(+)