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

[Gustavo Padovan]

2017-04-03 Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>:

> Hello Mauro and Gustavo,
> 
> On 04/03/2017 07:16 AM, Mauro Carvalho Chehab wrote:
> > Hi Gustavo,
> > 
> > Em Mon, 13 Mar 2017 16:20:25 -0300
> > Gustavo Padovan <gustavo@xxxxxxxxxxx> escreveu:
> > 
> >> From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
> >>
> >> 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.
> > 
> > Thanks for your work!
> > 
> > I looked on your patchset, and I didn't notice anything really weird
> > there. So, instead on reviewing patch per patch, I prefer to discuss
> > about the requirements and API, as depending on it, the code base will
> > change a lot.
> >
> 
> Agree that's better to first set on an uAPI and then implement based on that.
>  
> > I'd like to do some tests with it on devices with mem2mem drivers.
> > My plan is to use an Exynos board for such thing, but I guess that
> > the DRM driver for it currently doesn't. I'm seeing internally if someone
> > could be sure that Exynos driver upstream will become ready for such
> > tests.
> >
> 
> Not sure if you should try to do testing before agreeing on an uAPI and
> implementation.
> 
> > Javier wrote some patches last year meant to implement implicit
> > fences support. What we noticed is that, while his mechanism worked
> > fine for pure capture and pure output devices, when we added a mem2mem
> > device, on a DMABUF+fences pipeline, e. g.:
> > 
> > 	sensor -> [m2m] -> DRM
> > 
> > End everything using fences/DMABUF, the fences mechanism caused dead
> > locks on existing userspace apps.
> >
> > A m2m device has both capture and output devnodes. Both should be
> > queued/dequeued. The capture queue is synchronized internally at the
> > driver with the output buffer[1].
> > 
> > [1] The names here are counter-intuitive: "capture" is a devnode
> > where userspace receives a video stream; "output" is a devnode where
> > userspace feeds a video stream.
> > 
> > The problem is that adding implicit fences changed the behavior of
> > the ioctls, causing gstreamer to wait forever for buffers to be ready.
> >
> 
> The problem was related to trying to make user-space unaware of the implicit
> fences support, and so it tried to QBUF a buffer that had already a pending
> fence. A workaround was to block the second QBUF ioctl if the buffer had a
> pending fence, but this caused the mentioned deadlock since GStreamer wasn't
> expecting the QBUF ioctl to block.
> 
> > I suspect that, even with explicit fences, the behavior of Q/DQ
> > will be incompatible with the current behavior (or will require some
> > dirty hacks to make it identical). 

For QBUF the only difference is that we set flags for fences and pass
and receives in and out fences. For DQBUF the behavior is exactly the
same. What incompatibles or hacks do you see?

I had the expectation that the flags would be for userspace to learn
about any different behavior.

> >
> > So, IMHO, the best would be to use a new set of ioctls, when fences are
> > used (like VIDIOC_QFENCE/DQFENCE).
> > 
> 
> For explicit you can check if there's an input-fence so is different than
> implicit, but still I agree that it would be better to have specific ioctls.

I'm pretty new to v4l2 so I don't know all use cases yet, but what I
thought was to just add extra flags to QBUF to mark when using fences
instead of having userspace  to setup completely new ioctls for fences.
The burden for userspace should be smaller with flags.

> 
> >>
> >> 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.
> >>
> >> 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.
> > 
> > The V4L2 spec doesn't warrant that the buffers will be dequeued at the
> > queue order.
> > 
> > In practice, however, most drivers will not reorder. Yet, mem2mem codec 
> > drivers may reorder the buffers at the output, as the luminance information
> > (Y) usually comes first on JPEG/MPEG-like formats.
> > 
> >> * 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.
> > 
> > That's another advantage of using a new set of ioctls for queues: with that,
> > queuing/dequeing per plane will be easier. On codec drivers, doing it per
> > plane could bring performance improvements.
> >
> 
> You don't really need to Q/DQ on a per plane basis AFAICT. Since on QBUF you
> can get a set of out-fences that can be passed to the other driver and so it
> should be able to wait per fence.
> 
> >> In-fences should be per-buffer as the DRM only has per-buffer fences, but
> 
> I'm not that familiar with DRM, but I thought DRM fences was also per plane
> and not per buffer.

DRM plane is a different thing, its a representation of a region on the
screen and there is only one buffer for each DRM plane. 

One of the questions I raised was: how to match V4L2 per-plane fences to
DRM per-buffer fences?

> 
> How this works without fences? For V4L2 there's a dma-buf fd per plane and
> so I was expecting the DRM API to also import a dma-buf fd per DRM plane.

Yes. It should do something similar behind the Framebuffer abstraction.

> 
> I only have access to an Exynos board whose display controller supports
> single plane formats, so I don't know how this works for multi planar.
> 
> >> 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 API should be flexible enough to support both usecases. We could
> > implement just per-buffer in the beginning, but, on such case, we
> > should deploy an API that will allow to later add per-plane fences
> > without breaking userspace.

I believe we can just extend the per-plane parts of QBUF for their
fences. We could even use plane[0] for the per-buffer case.

> >
> > So, I prefer that, on multiplane fences, we have one fence per plane,
> > even if, at the first implementation, all fences will be released
> > at the same time, when the buffer is fully filled. That would allow
> > us to later improve it, without changing userspace.
> 
> It's true that vb2 can't currently signal fences per plane since the interface
> between vb2 and drivers is per vb2_buffer. But the uAPI shouldn't be restricted
> by this implementation detail (that can be changed) and should support per plane
> fences IMHO.
> 
> That's for example the case with the V4L2 dma-buf API. There is a dma-buf fd per
> plane, and internally for vb2 single planar buffers use the dma-buf associated
> with plane 0.
> 
> Now when mentioning this, I noticed that in your implementation the fences are
> not associated with a dma-buf. I thought the idea was for the fences to be
> associated with a dma-buf's reservation object. If we do that, then fences will
> be per fence since the dma-buf/reservation objet are also per fence in v4l2/vb2.

Can you explain what you were thinking on the relation between fences
and reservation objects? Not sure I follow.

Gustavo