On Tue, Mar 16, 2021 at 3:51 AM Michel Dänzer michel@daenzer.net wrote:
On 2021-03-16 12:10 a.m., Jason Ekstrand wrote:
On Mon, Mar 15, 2021 at 4:05 PM Jason Ekstrand jason@jlekstrand.net wrote:
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.
v2 (Jason Ekstrand):
- Use a wrapper dma_fence_array of all fences including the new one when importing an exclusive fence.
v3 (Jason Ekstrand):
- Lock around setting shared fences as well as exclusive
- Mark SIGNAL_SYNC_FILE as a read-write ioctl.
- Initialize ret to 0 in dma_buf_wait_sync_file
v4 (Jason Ekstrand):
- Use the new dma_resv_get_singleton helper
v5 (Jason Ekstrand):
- Rename the IOCTLs to import/export rather than wait/signal
- Drop the WRITE flag and always get/set the exclusive fence
v6 (Jason Ekstrand):
- Drop the sync_file import as it was all-around sketchy and not nearly as useful as import.
- Re-introduce READ/WRITE flag support for export
- Rework the commit message
Signed-off-by: Jason Ekstrand jason@jlekstrand.net
drivers/dma-buf/dma-buf.c | 55 ++++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 6 ++++ 2 files changed, 61 insertions(+)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index f264b70c383eb..e7f9dd62c19a9 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c
[...]
@@ -362,6 +363,57 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) return ret; }
+static long dma_buf_export_sync_file(struct dma_buf *dmabuf,
void __user *user_data)+{
struct dma_buf_sync_file arg;struct dma_fence *fence = NULL;struct sync_file *sync_file;int fd, ret;if (copy_from_user(&arg, user_data, sizeof(arg)))return -EFAULT;if (arg.flags & ~DMA_BUF_SYNC_RW)return -EINVAL;fd = get_unused_fd_flags(O_CLOEXEC);if (fd < 0)return fd;if (arg.flags & DMA_BUF_SYNC_WRITE) {ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);if (ret)goto err_put_fd;} else if (arg.flags & DMA_BUF_SYNC_READ) {fence = dma_resv_get_excl(dmabuf->resv);}if (!fence)fence = dma_fence_get_stub();sync_file = sync_file_create(fence);dma_fence_put(fence);if (!sync_file) {ret = -EINVAL;Should this be -EINVAL or -ENOMEM?
The latter makes more sense to me, since sync_file_create returning NULL is not related to invalid ioctl parameters.
I've switched to -ENOMEM. It'll be part of v8 whenever I send it out. I'd like to get some "real" review first.
--Jason