Orange Pi5 kernel

^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  1) .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  2) .. c:namespace:: V4L
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  3) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  4) .. _rw:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  5) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  6) **********
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  7) Read/Write
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  8) **********
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300  9) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) Input and output devices support the :c:func:`read()` and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) :c:func:`write()` function, respectively, when the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) ``V4L2_CAP_READWRITE`` flag in the ``capabilities`` field of struct
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) :c:type:`v4l2_capability` returned by the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) :ref:`VIDIOC_QUERYCAP` ioctl is set.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) Drivers may need the CPU to copy the data, but they may also support DMA
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) to or from user memory, so this I/O method is not necessarily less
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) efficient than other methods merely exchanging buffer pointers. It is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) considered inferior though because no meta-information like frame
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) counters or timestamps are passed. This information is necessary to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) recognize frame dropping and to synchronize with other data streams.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) However this is also the simplest I/O method, requiring little or no
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) setup to exchange data. It permits command line stunts like this (the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) vidctrl tool is fictitious):
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) .. code-block:: none
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28)     $ vidctrl /dev/video --input=0 --format=YUYV --size=352x288
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29)     $ dd if=/dev/video of=myimage.422 bs=202752 count=1
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) To read from the device applications use the :c:func:`read()`
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) function, to write the :c:func:`write()` function. Drivers
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) must implement one I/O method if they exchange data with applications,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) but it need not be this. [#f1]_ When reading or writing is supported, the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) driver must also support the :c:func:`select()` and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) :c:func:`poll()` function. [#f2]_
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) .. [#f1]
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39)    It would be desirable if applications could depend on drivers
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)    supporting all I/O interfaces, but as much as the complex memory
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41)    mapping I/O can be inadequate for some devices we have no reason to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42)    require this interface, which is most useful for simple applications
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43)    capturing still images.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44) 
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) .. [#f2]
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46)    At the driver level :c:func:`select()` and :c:func:`poll()` are
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47)    the same, and :c:func:`select()` is too important to be optional.