Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755904AbdIGSmj (ORCPT ); Thu, 7 Sep 2017 14:42:39 -0400 Received: from mail-qk0-f195.google.com ([209.85.220.195]:38252 "EHLO mail-qk0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755868AbdIGSmh (ORCPT ); Thu, 7 Sep 2017 14:42:37 -0400 X-Google-Smtp-Source: AOwi7QA6I5ZrkPk4kH9k/u6FCLOA8fRlS5nRKVRb59qtnV7rSwjHFspRl+y9oqOUcYAPAEW6JDQJtA== From: Gustavo Padovan To: linux-media@vger.kernel.org Cc: Hans Verkuil , Mauro Carvalho Chehab , Shuah Khan , linux-kernel@vger.kernel.org, Gustavo Padovan Subject: [PATCH v3 01/15] [media] v4l: Document explicit synchronization behaviour Date: Thu, 7 Sep 2017 15:42:12 -0300 Message-Id: <20170907184226.27482-2-gustavo@padovan.org> X-Mailer: git-send-email 2.13.5 In-Reply-To: <20170907184226.27482-1-gustavo@padovan.org> References: <20170907184226.27482-1-gustavo@padovan.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2601 Lines: 57 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 +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 +finished with buffer, that is the buffer is ready. The fence are represented +by file and passed as file descriptor to userspace. + +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 +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. + +At streamoff the out-fences will either signal normally if the drivers wait +for the operations on the buffers to finish or signal with error if the +driver cancel the pending operations. Return Value ============ -- 2.13.5