Subject: Re: [PATCH v3 01/15] [media] v4l: Document explicit synchronization
 behaviour
From: Hans Verkuil <hverkuil@xs4all.nl>
To: Gustavo Padovan <gustavo@padovan.org>, linux-media@vger.kernel.org
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
        Shuah Khan <shuahkh@osg.samsung.com>, linux-kernel@vger.kernel.org,
        Gustavo Padovan <gustavo.padovan@collabora.com>
References: <20170907184226.27482-1-gustavo@padovan.org>
 <20170907184226.27482-2-gustavo@padovan.org>
 <22b8926c-4a44-0f22-0717-c36d64003272@xs4all.nl>
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Date: Mon, 11 Sep 2017 13:01:49 +0200
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On 09/11/2017 12:50 PM, Hans Verkuil wrote:
> On 09/07/2017 08:42 PM, Gustavo Padovan wrote:
>> From: Gustavo Padovan <gustavo.padovan@collabora.com>
>>
>> Add section to VIDIOC_QBUF about it
>>
>> v2:
>> 	- mention that fences are files (Hans)
>> 	- rework for the new API
>>
>> Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
>> ---
>>  Documentation/media/uapi/v4l/vidioc-qbuf.rst | 31 ++++++++++++++++++++++++++++
>>  1 file changed, 31 insertions(+)
>>
>> diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
>> index 1f3612637200..fae0b1431672 100644
>> --- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst
>> +++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
>> @@ -117,6 +117,37 @@ immediately with an ``EAGAIN`` error code when no buffer is available.
>>  The struct :c:type:`v4l2_buffer` structure is specified in
>>  :ref:`buffer`.
>>  
>> +Explicit Synchronization
>> +------------------------
>> +
>> +Explicit Synchronization allows us to control the synchronization of
>> +shared buffers from userspace by passing fences to the kernel and/or
>> +receiving them from it. Fences passed to the kernel are named in-fences and
>> +the kernel should wait them to signal before using the buffer, i.e., queueing
> 
> wait them -> wait on them
> 
> (do you wait 'on' a fence or 'for' a fence? I think it's 'on' but I'm not 100% sure)
> 
>> +it to the driver. On the other side, the kernel can create out-fences for the
>> +buffers it queues to the drivers, out-fences signal when the driver is
> 
> Start a new sentence here: ...drivers. Out-fences...
> 
>> +finished with buffer, that is the buffer is ready. The fence are represented
> 
> s/that is/i.e/
> 
> s/The fence/The fences/
> 
>> +by file and passed as file descriptor to userspace.
> 
> s/by file/as a file/
> s/as file/as a file/
> 
>> +
>> +The in-fences are communicated to the kernel at the ``VIDIOC_QBUF`` ioctl
>> +using the ``V4L2_BUF_FLAG_IN_FENCE`` buffer
>> +flags and the `fence_fd` field. If an in-fence needs to be passed to the kernel,
>> +`fence_fd` should be set to the fence file descriptor number and the
>> +``V4L2_BUF_FLAG_IN_FENCE`` should be set as well. Failure to set both will
> 
> s/Failure to set both/Setting one but not the other/
> 
>> +cause ``VIDIOC_QBUF`` to return with error.
>> +
>> +To get a out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE`` flag should
>> +be set to notify it that the next queued buffer should have a fence attached to
>> +it. That means the out-fence may not be associated with the buffer in the
>> +current ``VIDIOC_QBUF`` ioctl call because the ordering in which videobuf2 core
>> +queues the buffers to the drivers can't be guaranteed. To become aware of the
>> +of the next queued buffer and the out-fence attached to it the
>> +``V4L2_EVENT_BUF_QUEUED`` event should be used. It will trigger an event
>> +for every buffer queued to the V4L2 driver.
> 
> This makes no sense.
> 
> Setting this flag means IMHO that when *this* buffer is queued up to the driver,
> then it should send the BUF_QUEUED event with an out fence.
> 
> I.e. it signals that userspace wants to have the out-fence. The requirement w.r.t.
> ordering is that the BUF_QUEUED events have to be in order, but that is something
> that the driver can ensure in the case it is doing internal re-ordering.
> 
> This requirement is something that needs to be documented here, BTW.
> 
> Anyway, the flag shouldn't refer to some 'next buffer', since that's very confusing.

Just ignore this comment. I assume v4 will implement it like this.

Regards,

	Hans

> 
>> +
>> +At streamoff the out-fences will either signal normally if the drivers wait
> 
> s/drivers wait/driver waits/
> 
>> +for the operations on the buffers to finish or signal with error if the
>> +driver cancel the pending operations.
> 
> s/cancel/cancels/
> 
> Thinking with my evil hat on:
> 
> What happens if the application dequeues the buffer (VIDIOC_DQBUF) before
> dequeuing the BUF_QUEUED event? Or if the application doesn't call VIDIOC_DQEVENT
> at all? Should any pending BUF_QUEUED event with an out fence be removed from the
> event queue if the application calls DQBUF on the corresponding buffer?
> 
> Regards,
> 
> 	Hans
> 
>>  
>>  Return Value
>>  ============
>>
>