[RFC v2 0/8] drm: explicit fencing support

Tue Apr 26 06:30:24 UTC 2016

On Mon, Apr 25, 2016 at 11:21:18PM +0000, Mike Lothian wrote:
> Hi
> 
> Out of interest will this allow tear free with PRIME?

Tear free with prime on the kernel side is already supported using
reservation objects/fences attached implicitly to dma-bufs. Just needs
driver support (which for the displaying side has landed in i915 in 4.6,
nouveau/radeon did manage things correctly on the render side since a long
time).

I think tear free needs some additional changes in X, and the patches are
pending review.

This is just the explicit fencing needed by android (and wanted by
others). Same thing overall idea, but different ABI in the details.
-Daniel

> 
> Thanks
> 
> Mike
> 
> On Tue, 26 Apr 2016, 12:33 a.m. Gustavo Padovan, <gustavo at padovan.org>
> wrote:
> 
> > From: Gustavo Padovan <gustavo.padovan at collabora.co.uk>
> >
> > Hi,
> >
> > Currently the Linux Kernel only have an implicit fencing mechanism
> > where the fence are attached directly to buffers and userspace is unaware
> > of
> > what is happening. On the other hand explicit fencing which is not
> > supported
> > yet by Linux but it expose fences to the userspace to handle fencing
> > between
> > producer/consumer explicitely.
> >
> > For that we use the Android Sync Framework[1], a explicit fencing mechanism
> > that help the userspace handles fences directly. It has the concept of
> > sync_file (called sync_fence in Android) that expose the driver's fences to
> > userspace via file descriptors. File descriptors are useful because we can
> > pass
> > them around between process.
> >
> > The Sync Framework is currently in the staging tree and on the process to
> > be de-staged[2].
> >
> > With explicit fencing we have a global mechanism that optimizes the flow of
> > buffers between consumers and producers, avoid a lot of waiting. So instead
> > of waiting for a buffer to be processed by the GPU before sending it to DRM
> > in an Atomic IOCTL we can get a sync_file fd from the GPU driver at the
> > moment
> > we submit the buffer processing. The compositor then passes these fds to
> > DRM in
> > a atomic commit request, that will not be displayed until the fences
> > signal,
> > i.e, the GPU finished processing the buffer and it is ready to display. In
> > DRM
> > the fences we wait on before displaying a buffer are called in-fences.
> >
> > Vice-versa, we have out-fences, to sychronize the return of buffers to GPU
> > (producer) to be processed again. When DRM receives an atomic request with
> > a
> > special flag set it generates one fence per-crtc and attach it to a
> > per-crtc
> > sync_file.  It then returns the array of sync_file fds to userspace as an
> > atomic_ioctl out arg. With the fences available userspace can forward these
> > fences to the GPU, where it will wait the fence to signal before starting
> > to
> > process on buffer again.
> >
> > Explicit fencing with Sync Framework allows buffer suballocation. Userspace
> > get a large buffer and divides it into small ones and submit requests to
> > process them, each subbuffer gets and sync_file fd and can be processed in
> > parallel. This is not even possible with implicit fencing.
> >
> > While these are out-fences in DRM (the consumer) they become in-fences once
> > they get to the GPU (the producer).
> >
> > DRM explicit fences are opt-in, as the default will still be implicit
> > fencing.
> > To enable explicit in-fences one just need to pass a sync_file fd in the
> > FENCE_FD plane property. *In-fences are per-plane*, i.e., per framebuffer.
> >
> > For out-fences, just enabling DRM_MODE_ATOMIC_OUT_FENCE flag is enough.
> > *Out-fences are per-crtc*.
> >
> > In-fences
> > ---------
> >
> > In the first discussions on #dri-devel on IRC we decided to hide the Sync
> > Framework from DRM drivers to reduce complexity, so as soon we get the fd
> > via FENCE_FD plane property we convert the sync_file fd to a struct fence.
> > However a sync_file might contain more than one fence, so we created the
> > fence_collection concept. struct fence_collection is a subclass of struct
> > fence and stores a group of fences that needs to be waited together, in
> > other words, all the fences in the sync_file.
> >
> > Then we just use the already in place fence support to wait on those
> > fences.
> > Once the producer calls fence_signal() for all fences on wait we can
> > proceed
> > with the atomic commit and display the framebuffers. DRM drivers only
> > needs to
> > be converted to struct fence to make use of this feature.
> >
> > Out-fences
> > ----------
> >
> > Passing the DRM_MODE_ATOMIC_OUT_FENCE flag to an atomic request enables
> > out-fences. The kernel then creates a fence, attach it to a sync_file and
> > install this file on a unused fd for each crtc. Userspace get the fence
> > back
> > as an array of per-crtc sync_file fds.
> >
> > DRM core use the already in place drm_event infrastructure to help signal
> > fences, we've added a fence pointer to struct drm_pending_event. If the
> > atomic
> > update received requested an PAGE_FLIP_EVENT we just use the same
> > drm_pending_event and set our fence there, otherwise we just create an
> > event
> > with a NULL file_priv to set our fence. On vblank we just call
> > fence_signal()
> > to signal that the buffer related to this fence is *now* on the screen.
> > Note that this is exactly the opposite behaviour from Android, where the
> > fences
> > are signaled when they are not on display anymore, so free to be reused.
> >
> > No changes are required to DRM drivers to have out-fences support, apart
> > from
> > atomic support of course.
> >
> > Kernel tree
> > -----------
> >
> > For those who want all patches on this RFC are in my tree. The tree
> > includes
> > all sync frameworks patches needed at the moment:
> >
> >
> > https://git.kernel.org/cgit/linux/kernel/git/padovan/linux.git/log/?h=fences
> >
> > I also hacked some poor some fake fences support to modetest here:
> >
> > https://git.collabora.com/cgit/user/padovan/libdrm.git/log/?h=atomic
> >
> >
> > v2 changes
> > ----------
> >
> > The changes from RFC v1 to RFC v2 are in the patches description.
> >
> > TODO
> > ----
> >
> >  * upstream Android Sync ABI changes
> >  * de-stage Android Sync Framework
> >  * Upstream sync selftests (Emilio Lopez)
> >  * create tests for DRM fences
> >
> > Regards,
> >
> >         Gustavo
> > ---
> >
> > [1] https://source.android.com/devices/graphics/implement.html#vsync
> > [2]
> > https://git.kernel.org/cgit/linux/kernel/git/padovan/linux.git/log/?h=sync
> >
> > Gustavo Padovan (8):
> >   dma-buf/fence: add fence_collection fences
> >   Documentation: add fence-collection to kernel DocBook
> >   dma-buf/sync_file: add sync_file_fences_get()
> >   drm/fence: allow fence waiting to be interrupted by userspace
> >   drm/fence: add in-fences support
> >   drm/fence: add fence to drm_pending_event
> >   drm/fence: add fence timeline to drm_crtc
> >   drm/fence: add out-fences support
> >
> >  Documentation/DocBook/device-drivers.tmpl |   2 +
> >  drivers/dma-buf/Makefile                  |   2 +-
> >  drivers/dma-buf/fence-collection.c        | 159 ++++++++++++++++++++++++
> >  drivers/dma-buf/fence.c                   |   2 +-
> >  drivers/dma-buf/sync_file.c               |  60 +++++++++
> >  drivers/gpu/drm/Kconfig                   |   1 +
> >  drivers/gpu/drm/drm_atomic.c              | 196
> > ++++++++++++++++++++++++++++--
> >  drivers/gpu/drm/drm_atomic_helper.c       |  24 +++-
> >  drivers/gpu/drm/drm_crtc.c                |  36 ++++++
> >  drivers/gpu/drm/drm_fops.c                |   8 +-
> >  include/drm/drmP.h                        |   2 +
> >  include/drm/drm_atomic_helper.h           |   4 +-
> >  include/drm/drm_crtc.h                    |  30 +++++
> >  include/linux/fence-collection.h          |  73 +++++++++++
> >  include/linux/fence.h                     |   9 ++
> >  include/linux/sync_file.h                 |   3 +
> >  include/uapi/drm/drm_mode.h               |  11 +-
> >  17 files changed, 600 insertions(+), 22 deletions(-)
> >  create mode 100644 drivers/dma-buf/fence-collection.c
> >  create mode 100644 include/linux/fence-collection.h
> >
> > --
> > 2.5.5
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >

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