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diff --git a/Documentation/media/uapi/v4l/rw.rst b/Documentation/media/uapi/v4l/rw.rst deleted file mode 100644 index 6e498fcf32c4..000000000000 --- a/Documentation/media/uapi/v4l/rw.rst +++ /dev/null @@ -1,54 +0,0 @@ -.. Permission is granted to copy, distribute and/or modify this -.. document under the terms of the GNU Free Documentation License, -.. Version 1.1 or any later version published by the Free Software -.. Foundation, with no Invariant Sections, no Front-Cover Texts -.. and no Back-Cover Texts. A copy of the license is included at -.. Documentation/media/uapi/fdl-appendix.rst. -.. -.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections - -.. _rw: - -********** -Read/Write -********** - -Input and output devices support the :ref:`read() <func-read>` and -:ref:`write() <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 :ref:`read() <func-read>` -function, to write the :ref:`write() <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 :ref:`select() <func-select>` and -:ref:`poll() <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 :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are - the same, and :ref:`select() <func-select>` is too important to be optional. |