On 2020-03-17 10:21 p.m., Jason Ekstrand wrote:
Explicit synchronization is the future. At least, that seems to be what
most userspace APIs are agreeing on at this point. However, most of our
Linux APIs (both userspace and kernel UAPI) are currently built around
implicit synchronization with dma-buf. While work is ongoing to change
many of the userspace APIs and protocols to an explicit synchronization
model, switching over piecemeal is difficult due to the number of
potential components involved. On the kernel side, many drivers use
dma-buf including GPU (3D/compute), display, v4l, and others. In
userspace, we have X11, several Wayland compositors, 3D drivers, compute
drivers (OpenCL etc.), media encode/decode, and the list goes on.
This patch provides a path forward by allowing userspace to manually
manage the fences attached to a dma-buf. Alternatively, one can think
of this as making dma-buf's implicit synchronization simply a carrier
for an explicit fence. This is accomplished by adding two IOCTLs to
dma-buf for importing and exporting a sync file to/from the dma-buf.
This way a userspace component which is uses explicit synchronization,
such as a Vulkan driver, can manually set the write fence on a buffer
before handing it off to an implicitly synchronized component such as a
Wayland compositor or video encoder. In this way, each of the different
components can be upgraded to an explicit synchronization model one at a
time as long as the userspace pieces connecting them are aware of it and
import/export fences at the right times.
There is a potential race condition with this API if userspace is not
careful. A typical use case for implicit synchronization is to wait for
the dma-buf to be ready, use it, and then signal it for some other
component. Because a sync_file cannot be created until it is guaranteed
to complete in finite time, userspace can only signal the dma-buf after
it has already submitted the work which uses it to the kernel and has
received a sync_file back. There is no way to atomically submit a
wait-use-signal operation. This is not, however, really a problem with
this API so much as it is a problem with explicit synchronization
itself. The way this is typically handled is to have very explicit
ownership transfer points in the API or protocol which ensure that only
one component is using it at any given time. Both X11 (via the PRESENT
extension) and Wayland provide such ownership transfer points via
explicit present and idle messages.
The decision was intentionally made in this patch to make the import and
export operations IOCTLs on the dma-buf itself rather than as a DRM
IOCTL. This makes it the import/export operation universal across all
components which use dma-buf including GPU, display, v4l, and others.
It also means that a userspace component can do the import/export
without access to the DRM fd which may be tricky to get in cases where
the client communicates with DRM via a userspace API such as OpenGL or
Vulkan. At a future date we may choose to add direct import/export APIs
to components such as drm_syncobj to avoid allocating a file descriptor
and going through two ioctls. However, that seems to be something of a
micro-optimization as import/export operations are likely to happen at a
rate of a few per frame of rendered or decoded video.
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
Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
What's the status of this? DMA_BUF_IOCTL_EXPORT_SYNC_FILE would be
useful for Wayland compositors to wait for client buffers to become
ready without being prone to getting delayed by later HW access to them,
so it would be nice to merge that at least (if
DMA_BUF_IOCTL_IMPORT_SYNC_FILE is still controversial).
--
Earthling Michel Dänzer | https://redhat.com
Libre software enthusiast | Mesa and X developer
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