From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751305AbdIKKu3 (ORCPT ); Mon, 11 Sep 2017 06:50:29 -0400 Received: from lb1-smtp-cloud7.xs4all.net ([194.109.24.24]:54680 "EHLO lb1-smtp-cloud7.xs4all.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751007AbdIKKu2 (ORCPT ); Mon, 11 Sep 2017 06:50:28 -0400 Subject: Re: [PATCH v3 01/15] [media] v4l: Document explicit synchronization behaviour To: Gustavo Padovan , linux-media@vger.kernel.org Cc: Mauro Carvalho Chehab , Shuah Khan , linux-kernel@vger.kernel.org, Gustavo Padovan References: <20170907184226.27482-1-gustavo@padovan.org> <20170907184226.27482-2-gustavo@padovan.org> From: Hans Verkuil Message-ID: <22b8926c-4a44-0f22-0717-c36d64003272@xs4all.nl> Date: Mon, 11 Sep 2017 12:50:22 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170907184226.27482-2-gustavo@padovan.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-CMAE-Envelope: MS4wfNqYDnQMCpQCP4zepwuGt0zCfn6kzgBhB6ttWxVsmTm4UPMPZjObjT713V4EicYcukkHANWK6g0LvavaAwt2G34WMyFL4MbAnG0nAhuHGguzem2lPutv EVnIZI7dKsJJPzokcsSk5Wms8bN+lzeRggREKi3iarjjj0rECS+99WWFPCfUdo093hrpHeKPSSOyShSktSlfHf6Fsjm9bgfe2RCN/KWWBB/lkbjYtwt3/eDK EORm782PQKMXJ+TTa0ro0e1mxpR9X6AWIo3NwSODkyyNMKYn8iqOallQtmNVpfZiHuahCsdzAjj7ge1/oWVKqQ== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/07/2017 08:42 PM, Gustavo Padovan wrote: > From: Gustavo Padovan > > Add section to VIDIOC_QBUF about it > > v2: > - mention that fences are files (Hans) > - rework for the new API > > Signed-off-by: Gustavo Padovan > --- > 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. > + > +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 > ============ >