Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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
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+.. 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.