Re: [PATCH 00/12] V4L2 explicit synchronization support

[Mauro Carvalho Chehab <mchehab@osg.samsung.com>]

Em Fri, 16 Jun 2017 16:39:03 +0900
Gustavo Padovan <gustavo@padovan.org> escreveu:

> From: Gustavo Padovan <gustavo.padovan@collabora.com>
> 
> Hi,
> 
> This 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.
> 
> 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.
> 
> 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 8-9 add support to mark queues as ordered. Finally patches 10 and 11
> add more fence infrastructure to support out-fences and finally patch 12 adds
> support to out-fences.
> 
> Changelog are detailed in each patch.
> 
> Please review! Thanks.

Just reviewed the series. Most patches look good.

I have one additional concern: if the changes here won't cause any
bad behaviors if fences is not available for some VB2 non V4L2 client.
I'm actually thinking on this:

	https://patchwork.linuxtv.org/patch/31613/

>From what I saw, after this patch series, someone could try to 
inconditionally open an out fences fd for a driver. Maybe this
should be denied by default, enabling such feature only if the
VB2 "client" (e. g. videobuf-v4l2) supports it.

Regards,
Mauro