From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752147AbdIKNeN (ORCPT ); Mon, 11 Sep 2017 09:34:13 -0400 Received: from mail-qt0-f194.google.com ([209.85.216.194]:32938 "EHLO mail-qt0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750877AbdIKNeL (ORCPT ); Mon, 11 Sep 2017 09:34:11 -0400 X-Google-Smtp-Source: AOwi7QDPw9cRiPUj0uN2pUPzXB1MXpP8zG4zEdtRtcVPj8A/LvcmjI2oEgHd42K7XPa5vpE8EbxvoQ== Date: Mon, 11 Sep 2017 10:34:05 -0300 From: Gustavo Padovan To: Hans Verkuil Cc: linux-media@vger.kernel.org, Mauro Carvalho Chehab , Shuah Khan , linux-kernel@vger.kernel.org, Gustavo Padovan Subject: Re: [PATCH v3 01/15] [media] v4l: Document explicit synchronization behaviour Message-ID: <20170911133405.GB7552@jade> References: <20170907184226.27482-1-gustavo@padovan.org> <20170907184226.27482-2-gustavo@padovan.org> <22b8926c-4a44-0f22-0717-c36d64003272@xs4all.nl> <6bb8df91-4cd2-2ca5-dc4b-aea5ea14e7b1@xs4all.nl> <20170911131846.GA7552@jade> <33e4711c-87dc-de98-a9da-33470f5ee083@xs4all.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <33e4711c-87dc-de98-a9da-33470f5ee083@xs4all.nl> User-Agent: Mutt/1.8.3 (2017-05-23) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2017-09-11 Hans Verkuil : > On 09/11/2017 03:18 PM, Gustavo Padovan wrote: > > 2017-09-11 Hans Verkuil : > > > >> On 09/11/2017 12:50 PM, Hans Verkuil wrote: > >>> 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. > >> > >> Just ignore this comment. I assume v4 will implement it like this. > > > > What approach do you mean by "like this". I'm confused now. :) > > > > In fact, I was in doubt between these two different approaches here. > > Should the flag mean *this* or the *next* buffer? The buffers can still > > be reordered at the videobuf2 level, because they might be waiting on > > in-fences and the fences may signal out of order. Then I went for the > > *next* buffer approach because we don't know that buffer for sure. > > But now thinking on this again we shouldn't have problems with the > > *this* buffer approach also. > > It should mean *this* buffer. It's really weird to set this flag for one > buffer, only for it to mean 'next' buffer. > > Keep it simple: the flag just means: send me the output fence fd for this > buffer once you have it. If it is not set, then no BUF_QUEUE event is sent. > > Actually, it could mean one of two things: either if it is not set, then no > BUF_QUEUE event is sent, or if it is not set, then the fd in the BUF_QUEUE > event is -1. > > I'm leaning towards the first. I can't see any use-case for sending that > event if you are not requesting out fences. We could go with the first one but in this case it is better to rename it to V4L2_EVENT_OUT_FENCE or something like this, isn't it? Gustavo