[PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5)

[Daniel Vetter]

On Wed, Sep 30, 2020 at 11:39:06AM +0200, Michel Dänzer wrote:
> 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 at jlekstrand.net>
> 
> 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).

I think the missing bits are just the usual stuff
- igt testcases
- userspace using the new ioctls
- review of the entire pile

I don't think there's any fundamental objections aside from "no one ever
pushed this over the finish line".

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch