Adding mesa-dev and wayland-devel for broader circulation.
On Wed, Mar 17, 2021 at 5:19 PM Jason Ekstrand 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
>
> v7 (Jason Ekstrand):
> - Require at least one sync flag
> - Fix a refcounting bug: dma_resv_get_excl() doesn't take a reference
> - Use _rcu helpers since we're accessing the dma_resv read-only
>
> v8 (Jason Ekstrand):
> - Return -ENOMEM if the sync_file_create fails
> - Predicate support on IS_ENABLED(CONFIG_SYNC_FILE)
>
> Signed-off-by: Jason Ekstrand
> ---
> drivers/dma-buf/dma-buf.c| 62
> include/uapi/linux/dma-buf.h | 6
> 2 files changed, 68 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index f264b70c383eb..a5e4b0b6a049c 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -20,6 +20,7 @@
> #include
> #include
> #include
> +#include
> #include
> #include
> #include
> @@ -362,6 +363,62 @@ static long dma_buf_set_name(struct dma_buf *dmabuf,
> const char __user *buf)
> return ret;
> }
>
> +#if IS_ENABLED(CONFIG_SYNC_FILE)
> +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(, user_data, sizeof(arg)))
> + return -EFAULT;
> +
> + if (arg.flags & ~DMA_BUF_SYNC_RW)
> + return -EINVAL;
> +
> + if ((arg.flags & DMA_BUF_SYNC_RW) == 0)
> + 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_rcu(dmabuf->resv, );
> + if (ret)
> + goto err_put_fd;
> + } else if (arg.flags & DMA_BUF_SYNC_READ) {
> + fence = dma_resv_get_excl_rcu(dmabuf->resv);
> + }
> +
> + if (!fence)
> + fence = dma_fence_get_stub();
> +
> + sync_file = sync_file_create(fence);
> +
> + dma_fence_put(fence);
> +
> + if (!sync_file) {
> + ret = -ENOMEM;
> + goto err_put_fd;
> + }
> +
> + fd_install(fd, sync_file->file);
> +
> + arg.fd = fd;
> + if (copy_to_user(user_data, , sizeof(arg)))
> + return -EFAULT;
> +
> +
