55 lines
2.3 KiB
ReStructuredText
55 lines
2.3 KiB
ReStructuredText
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.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/media/uapi/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _rw:
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**********
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Read/Write
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**********
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Input and output devices support the :ref:`read() <func-read>` and
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:ref:`write() <func-write>` function, respectively, when the
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``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
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:c:type:`v4l2_capability` returned by the
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:ref:`VIDIOC_QUERYCAP` ioctl is set.
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Drivers may need the CPU to copy the data, but they may also support DMA
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to or from user memory, so this I/O method is not necessarily less
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efficient than other methods merely exchanging buffer pointers. It is
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considered inferior though because no meta-information like frame
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counters or timestamps are passed. This information is necessary to
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recognize frame dropping and to synchronize with other data streams.
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However this is also the simplest I/O method, requiring little or no
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setup to exchange data. It permits command line stunts like this (the
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vidctrl tool is fictitious):
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.. code-block:: none
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$ vidctrl /dev/video --input=0 --format=YUYV --size=352x288
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$ dd if=/dev/video of=myimage.422 bs=202752 count=1
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To read from the device applications use the :ref:`read() <func-read>`
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function, to write the :ref:`write() <func-write>` function. Drivers
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must implement one I/O method if they exchange data with applications,
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but it need not be this. [#f1]_ When reading or writing is supported, the
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driver must also support the :ref:`select() <func-select>` and
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:ref:`poll() <func-poll>` function. [#f2]_
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.. [#f1]
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It would be desirable if applications could depend on drivers
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supporting all I/O interfaces, but as much as the complex memory
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mapping I/O can be inadequate for some devices we have no reason to
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require this interface, which is most useful for simple applications
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capturing still images.
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.. [#f2]
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At the driver level :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are
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the same, and :ref:`select() <func-select>` is too important to be optional.
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