Re: [RFC 00/10] V4L2 explicit synchronization support

[Sakari Ailus <sakari.ailus@iki.fi>]

Hi Gustavo,

On Wed, Apr 05, 2017 at 05:24:57PM +0200, Gustavo Padovan wrote:
> Hi Sakari,
> 
> 2017-04-04 Sakari Ailus <sakari.ailus@iki.fi>:
> 
> > Hi Gustavo,
> > 
> > Thank you for the patchset. Please see my comments below.
> > 
> > On Mon, Mar 13, 2017 at 04:20:25PM -0300, Gustavo Padovan wrote:
> > > From: Gustavo Padovan <gustavo.padovan@collabora.com>
> > > 
> > > Hi,
> > > 
> > > This RFC adds support for Explicit Synchronization of shared buffers in V4L2.
> > > It uses the Sync File Framework[1] as vector to communicate the fences
> > > between kernel and userspace.
> > > 
> > > I'm sending this to start the discussion on the best approach to implement
> > > Explicit Synchronization, please check the TODO/OPEN section below.
> > > 
> > > Explicit Synchronization allows us to control the synchronization of
> > > shared buffers from userspace by passing fences to the kernel and/or 
> > > receiving them from the the kernel.
> > > 
> > > Fences passed to the kernel are named in-fences and the kernel should wait
> > > them to signal before using the buffer. On the other side, the kernel creates
> > > out-fences for every buffer it receives from userspace. This fence is sent back
> > > to userspace and it will signal when the capture, for example, has finished.
> > > 
> > > Signalling an out-fence in V4L2 would mean that the job on the buffer is done
> > > and the buffer can be used by other drivers.
> > 
> > Shouldn't you be able to add two fences to the buffer, one in and one out?
> > I.e. you'd have the buffer passed from another device to a V4L2 device and
> > on to a third device.
> > 
> > (Or, two fences per a plane, as you elaborated below.)
> 
> The out one should be created by V4L2 in this case, sent to userspace
> and then sent to third device. Another options is what we've been
> calling future fences in DRM. Where we may have a syscall to create this
> out-fence for us and then we could pass both in and out fence to the
> device. But that can be supported later along with what this RFC
> proposes.

Please excuse my ignorance on fences.

I just wanted to make sure that case was also considered. struct v4l2_buffer
will run out of space soon so we'll need a replacement anyway. The timecode
field is still available for re-use...

> 
> 
> > 
> > > 
> > > Current RFC implementation
> > > --------------------------
> > > 
> > > The current implementation is not intended to be more than a PoC to start
> > > the discussion on how Explicit Synchronization should be supported in V4L2.
> > > 
> > > The first patch proposes an userspace API for fences, then on patch 2
> > > we prepare to the addition of in-fences in patch 3, by introducing the
> > > infrastructure on vb2 to wait on an in-fence signal before queueing the buffer
> > > in the driver.
> > > 
> > > Patch 4 fix uvc v4l2 event handling and patch 5 configure q->dev for vivid
> > > drivers to enable to subscribe and dequeue events on it.
> > > 
> > > Patches 6-7 enables support to notify BUF_QUEUED events, i.e., let userspace
> > > know that particular buffer was enqueued in the driver. This is needed,
> > > because we return the out-fence fd as an out argument in QBUF, but at the time
> > > it returns we don't know to which buffer the fence will be attached thus
> > > the BUF_QUEUED event tells which buffer is associated to the fence received in
> > > QBUF by userspace.
> > > 
> > > Patches 8 and 9 add more fence infrastructure to support out-fences and finally
> > > patch 10 adds support to out-fences.
> > > 
> > > TODO/OPEN:
> > > ----------
> > > 
> > > * For this first implementation we will keep the ordering of the buffers queued
> > > in videobuf2, that means we will only enqueue buffer whose fence was signalled
> > > if that buffer is the first one in the queue. Otherwise it has to wait until it
> > > is the first one. This is not implmented yet. Later we could create a flag to
> > > allow unordered queing in the drivers from vb2 if needed.
> > > 
> > > * Should we have out-fences per-buffer or per-plane? or both? In this RFC, for
> > > simplicity they are per-buffer, but Mauro and Javier raised the option of
> > > doing per-plane fences. That could benefit mem2mem and V4L2 <-> GPU operation
> > > at least on cases when we have Capture hw that releases the Y frame before the
> > > other frames for example. When using V4L2 per-plane out-fences to communicate
> > > with KMS they would need to be merged together as currently the DRM Plane
> > > interface only supports one fence per DRM Plane.
> > > 
> > > In-fences should be per-buffer as the DRM only has per-buffer fences, but
> > > in case of mem2mem operations per-plane fences might be useful?
> > > 
> > > So should we have both ways, per-plane and per-buffer, or just one of them
> > > for now?
> > 
> > The data_offset field is only present in struct v4l2_plane, i.e. it is only
> > available through using the multi-planar API even if you just have a single
> > plane.
> 
> I didn't get why you mentioned the data_offset field. :)

I think I meant to continue this but didn't end up writing it down. :-)

What I wanted to say that the multi-plane API is a super-set of the
single-plane API and there's already a case for not all the functionality
being available through the single-plane API. At least I'm ok with adding
the per-plane fences to the multi-plane API only.

The framework could possibly do more to support the single-plane API as an
application interface so that the applications using single-plane API only
would get that as a bonus. (Just thinking out loud. Out of scope of this
patchset definitely.)

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi	XMPP: sailus@retiisi.org.uk