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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
commit | 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch) | |
tree | a94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/userspace-api/media/v4l/rw.rst | |
parent | Initial commit. (diff) | |
download | linux-upstream/5.10.209.tar.xz linux-upstream/5.10.209.zip |
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | Documentation/userspace-api/media/v4l/rw.rst | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/rw.rst b/Documentation/userspace-api/media/v4l/rw.rst new file mode 100644 index 000000000..64b85fb2a --- /dev/null +++ b/Documentation/userspace-api/media/v4l/rw.rst @@ -0,0 +1,47 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _rw: + +********** +Read/Write +********** + +Input and output devices support the :c:func:`read()` and +:c:func:`write()` function, respectively, when the +``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct +:c:type:`v4l2_capability` returned by the +:ref:`VIDIOC_QUERYCAP` ioctl is set. + +Drivers may need the CPU to copy the data, but they may also support DMA +to or from user memory, so this I/O method is not necessarily less +efficient than other methods merely exchanging buffer pointers. It is +considered inferior though because no meta-information like frame +counters or timestamps are passed. This information is necessary to +recognize frame dropping and to synchronize with other data streams. +However this is also the simplest I/O method, requiring little or no +setup to exchange data. It permits command line stunts like this (the +vidctrl tool is fictitious): + +.. code-block:: none + + $ vidctrl /dev/video --input=0 --format=YUYV --size=352x288 + $ dd if=/dev/video of=myimage.422 bs=202752 count=1 + +To read from the device applications use the :c:func:`read()` +function, to write the :c:func:`write()` function. Drivers +must implement one I/O method if they exchange data with applications, +but it need not be this. [#f1]_ When reading or writing is supported, the +driver must also support the :c:func:`select()` and +:c:func:`poll()` function. [#f2]_ + +.. [#f1] + It would be desirable if applications could depend on drivers + supporting all I/O interfaces, but as much as the complex memory + mapping I/O can be inadequate for some devices we have no reason to + require this interface, which is most useful for simple applications + capturing still images. + +.. [#f2] + At the driver level :c:func:`select()` and :c:func:`poll()` are + the same, and :c:func:`select()` is too important to be optional. |