From: Tomasz Figa <tfiga@chromium.org>
To: linux-media@vger.kernel.org
Cc: linux-kernel@vger.kernel.org,
Mauro Carvalho Chehab <mchehab@kernel.org>,
Hans Verkuil <hverkuil@xs4all.nl>,
Sakari Ailus <sakari.ailus@linux.intel.com>,
Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
Alexandre Courbot <acourbot@chromium.org>,
Hirokazu Honda <hiroh@chromium.org>,
Tomasz Figa <tfiga@chromium.org>
Subject: [PATCH] media: Clarify the meaning of file descriptors in VIDIOC_DQBUF
Date: Wed, 12 Jun 2019 18:36:48 +0900 [thread overview]
Message-ID: <20190612093648.47412-1-tfiga@chromium.org> (raw)
When the application calls VIDIOC_DQBUF with the DMABUF memory type, the
v4l2_buffer structure (or v4l2_plane structures) are filled with DMA-buf
file descriptors. However, the current documentation does not explain
whether those are new file descriptors referring to the same DMA-bufs or
just the same integers as passed to VIDIOC_QBUF back in time. Clarify
the documentation that it's the latter.
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
---
Documentation/media/uapi/v4l/vidioc-qbuf.rst | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/Documentation/media/uapi/v4l/vidioc-qbuf.rst b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
index dbf7b445a27b..407302d80684 100644
--- a/Documentation/media/uapi/v4l/vidioc-qbuf.rst
+++ b/Documentation/media/uapi/v4l/vidioc-qbuf.rst
@@ -139,6 +139,14 @@ may continue as normal, but should be aware that data in the dequeued
buffer might be corrupted. When using the multi-planar API, the planes
array must be passed in as well.
+If the application sets the ``memory`` field to ``V4L2_MEMORY_DMABUF`` to
+dequeue a :ref:`DMABUF <dmabuf>` buffer, the driver fills the ``m.fd`` field
+with a file descriptor numerically the same as the one given to ``VIDIOC_QBUF``
+when the buffer was enqueued. No new file descriptor is created at dequeue time
+and the value is only for the application convenience. When the multi-planar
+API is used the ``m.fd`` fields of the passed array of struct
+:c:type:`v4l2_plane` are filled instead.
+
By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing
queue. When the ``O_NONBLOCK`` flag was given to the
:ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
--
2.22.0.rc2.383.gf4fbbf30c2-goog
next reply other threads:[~2019-06-12 9:37 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-12 9:36 Tomasz Figa [this message]
2019-06-18 9:13 ` [PATCH] media: Clarify the meaning of file descriptors in VIDIOC_DQBUF Paul Kocialkowski
2019-06-20 4:50 ` Alexandre Courbot
2019-06-20 8:44 ` Sakari Ailus
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