diff options
Diffstat (limited to 'Documentation/DocBook/media/v4l/func-poll.xml')
| -rw-r--r-- | Documentation/DocBook/media/v4l/func-poll.xml | 35 |
1 files changed, 29 insertions, 6 deletions
diff --git a/Documentation/DocBook/media/v4l/func-poll.xml b/Documentation/DocBook/media/v4l/func-poll.xml index 85cad8bff5ba..4c73f115219b 100644 --- a/Documentation/DocBook/media/v4l/func-poll.xml +++ b/Documentation/DocBook/media/v4l/func-poll.xml @@ -29,9 +29,12 @@ can suspend execution until the driver has captured data or is ready to accept data for output.</para> <para>When streaming I/O has been negotiated this function waits -until a buffer has been filled or displayed and can be dequeued with -the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing -queue of the driver the function returns immediately.</para> +until a buffer has been filled by the capture device and can be dequeued +with the &VIDIOC-DQBUF; ioctl. For output devices this function waits +until the device is ready to accept a new buffer to be queued up with +the &VIDIOC-QBUF; ioctl for display. When buffers are already in the outgoing +queue of the driver (capture) or the incoming queue isn't full (display) +the function returns immediately.</para> <para>On success <function>poll()</function> returns the number of file descriptors that have been selected (that is, file descriptors @@ -44,10 +47,22 @@ Capture devices set the <constant>POLLIN</constant> and flags. When the function timed out it returns a value of zero, on failure it returns <returnvalue>-1</returnvalue> and the <varname>errno</varname> variable is set appropriately. When the -application did not call &VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the +application did not call &VIDIOC-STREAMON; the <function>poll()</function> function succeeds, but sets the <constant>POLLERR</constant> flag in the -<structfield>revents</structfield> field.</para> +<structfield>revents</structfield> field. When the +application has called &VIDIOC-STREAMON; for a capture device but hasn't +yet called &VIDIOC-QBUF;, the <function>poll()</function> function +succeeds and sets the <constant>POLLERR</constant> flag in the +<structfield>revents</structfield> field. For output devices this +same situation will cause <function>poll()</function> to succeed +as well, but it sets the <constant>POLLOUT</constant> and +<constant>POLLWRNORM</constant> flags in the <structfield>revents</structfield> +field.</para> + + <para>If an event occurred (see &VIDIOC-DQEVENT;) then +<constant>POLLPRI</constant> will be set in the <structfield>revents</structfield> +field and <function>poll()</function> will return.</para> <para>When use of the <function>read()</function> function has been negotiated and the driver does not capture yet, the @@ -58,10 +73,18 @@ continuously (as opposed to, for example, still images) the function may return immediately.</para> <para>When use of the <function>write()</function> function has -been negotiated the <function>poll</function> function just waits +been negotiated and the driver does not stream yet, the +<function>poll</function> function starts streaming. When that fails +it returns a <constant>POLLERR</constant> as above. Otherwise it waits until the driver is ready for a non-blocking <function>write()</function> call.</para> + <para>If the caller is only interested in events (just +<constant>POLLPRI</constant> is set in the <structfield>events</structfield> +field), then <function>poll()</function> will <emphasis>not</emphasis> +start streaming if the driver does not stream yet. This makes it +possible to just poll for events and not for buffers.</para> + <para>All drivers implementing the <function>read()</function> or <function>write()</function> function or streaming I/O must also support the <function>poll()</function> function.</para> |