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7347081e8a
There are lots of internal references in the form:
:ref:`foo <foo>`
Simplify them to be just: :ref:`foo`.
Patch generated via this small script:
for j in $(find . -name '*'); do echo $j; perl -ne 'if (m/\`(\S+)\s*\<(\S+)\>\`/) { if (!($1=~'http') && $1 eq $2) { s,\s*\<(\S+)\>,,; } } print $_' <$j >a && mv a $j; done
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
57 lines
2.3 KiB
ReStructuredText
57 lines
2.3 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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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 :c:func:`read()` and
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:c:func:`write()` function, respectively, when the
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``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
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:ref:`v4l2_capability <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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::
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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. [1]_ 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. [2]_
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.. [1]
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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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.. [2]
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At the driver level :c:func:`select()` and :c:func:`poll()` are
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the same, and :c:func:`select()` is too important to be optional.
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.. ------------------------------------------------------------------------------
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.. This file was automatically converted from DocBook-XML with the dbxml
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.. library (https://github.com/return42/sphkerneldoc). The origin XML comes
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.. from the linux kernel, refer to:
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..
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.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
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.. ------------------------------------------------------------------------------
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