48 lines
1.9 KiB
ReStructuredText
Executable file
48 lines
1.9 KiB
ReStructuredText
Executable file
.. -*- coding: utf-8; mode: rst -*-
|
|
|
|
.. _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.
|