Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 116 insertions, 0 deletions

diff --git a/Documentation/userspace-api/media/v4l/func-select.rst b/Documentation/userspace-api/media/v4l/func-select.rst
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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _func-select:
+
+*************
+V4L2 select()
+*************
+
+Name
+====
+
+v4l2-select - Synchronous I/O multiplexing
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <sys/time.h>
+    #include <sys/types.h>
+    #include <unistd.h>
+
+.. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout )
+
+Arguments
+=========
+
+``nfds``
+  The highest-numbered file descriptor in any of the three sets, plus 1.
+
+``readfds``
+  File descriptions to be watched if a read() call won't block.
+
+``writefds``
+  File descriptions to be watched if a write() won't block.
+
+``exceptfds``
+  File descriptions to be watched for V4L2 events.
+
+``timeout``
+  Maximum time to wait.
+
+Description
+===========
+
+With the :c:func:`select()` function applications can suspend
+execution until the driver has captured data or is ready to accept data
+for output.
+
+When streaming I/O has been negotiated this function waits until a
+buffer has been filled or displayed and can be dequeued with the
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. When buffers are already in
+the outgoing queue of the driver the function returns immediately.
+
+On success :c:func:`select()` returns the total number of bits set in
+``fd_set``. When the function timed out it returns
+a value of zero. On failure it returns -1 and the ``errno`` variable is
+set appropriately. When the application did not call
+:ref:`VIDIOC_QBUF` or
+:ref:`VIDIOC_STREAMON` yet the :c:func:`select()`
+function succeeds, setting the bit of the file descriptor in ``readfds``
+or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
+calls will fail. [#f1]_
+
+When use of the :c:func:`read()` function has been negotiated and the
+driver does not capture yet, the :c:func:`select()` function starts
+capturing. When that fails, :c:func:`select()` returns successful and
+a subsequent :c:func:`read()` call, which also attempts to start
+capturing, will return an appropriate error code. When the driver
+captures continuously (as opposed to, for example, still images) and
+data is already available the :c:func:`select()` function returns
+immediately.
+
+When use of the :c:func:`write()` function has been negotiated the
+:c:func:`select()` function just waits until the driver is ready for a
+non-blocking :c:func:`write()` call.
+
+All drivers implementing the :c:func:`read()` or :c:func:`write()`
+function or streaming I/O must also support the :c:func:`select()`
+function.
+
+For more details see the :c:func:`select()` manual page.
+
+Return Value
+============
+
+On success, :c:func:`select()` returns the number of descriptors
+contained in the three returned descriptor sets, which will be zero if
+the timeout expired. On error -1 is returned, and the ``errno`` variable
+is set appropriately; the sets and ``timeout`` are undefined. Possible
+error codes are:
+
+EBADF
+    One or more of the file descriptor sets specified a file descriptor
+    that is not open.
+
+EBUSY
+    The driver does not support multiple read or write streams and the
+    device is already in use.
+
+EFAULT
+    The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer
+    references an inaccessible memory area.
+
+EINTR
+    The call was interrupted by a signal.
+
+EINVAL
+    The ``nfds`` argument is less than zero or greater than
+    ``FD_SETSIZE``.
+
+.. [#f1]
+   The Linux kernel implements :c:func:`select()` like the
+   :c:func:`poll()` function, but :c:func:`select()` cannot
+   return a ``POLLERR``.