Hi Laurent,
Laurent Pinchart wrote:
On Wednesday 02 October 2013 16:45:12 Sakari Ailus wrote:
Hi all,
This is the second RFC set after the initial patch that makes poll return
POLLERR if no events are subscribed. There are other issues as well which
these patches address.
The original RFC patch is here:
<URL:http://www.spinics.net/lists/linux-media/msg68077.html>
poll(2) and select(2) can both be used for I/O multiplexing. While both
provide slightly different semantics. man 2 select:
select() and pselect() allow a program to monitor multiple
file descriptors, waiting until one or more of the file descriptors
become "ready" for some class of I/O operation (e.g., input possible). A
file descriptor is considered ready if it is possible to perform the
corre‐ sponding I/O operation (e.g., read(2)) without blocking.
The two system calls provide slightly different semantics: poll(2) can
signal POLLERR related to a file handle but select(2) does not: instead, on
POLLERR it sets a bit corresponding to a file handle in the read and write
sets. This is somewhat confusing since with the original patch --- using
select(2) would suggest that there's something to read or write instead of
knowing no further exceptions are coming.
Thus, also denying polling a subdev file handle using select(2) will mean
the POLLERR never gets through in any form.
So the meaningful alternatives I can think of are:
1) Use POLLERR | POLLPRI. When the last event subscription is gone and
select(2) IOCTL is issued, all file descriptor sets are set for a file
handle. Users of poll(2) will directly see both of the flags, making the
case visible to the user immediately in some cases. On sub-devices this is
obvious but on V4L2 devices the user should poll(2) (or select(2)) again to
know whether there's I/O waiting to be read, written or whether buffers are
ready.
2) Use POLLPRI only. While this does not differ from any regular event at
the level of poll(2) or select(2), the POLLIN or POLLOUT flags are not
adversely affected.
In each of the cases to ascertain oneself in a generic way of whether events
cannot no longer be obtained one has to call VIDIOC_DQEVENT IOCTL, which
currently may block. A patch in the set makes VIDIOC_DQEVENT to signal EIO
error code if no events are subscribed.
The videobuf2 changes are untested at the moment since I didn't have a
device using videobuf2 at hand right now.
Comments and questions are very welcome.
What's the behaviour of select(2) and poll(2) after this patch set when
polling an fd for both read and events, when no event has been subscribed to ?
The first one. If you're using select(2), you'll get the fd-specific bit
set in all three sets. For poll(2), you'll get POLLERR and POLLPRI set
for the fd.
No poll nor select can directly tell that there are no further events;
instead they intend to say that the corresponding operations on the file
descriptor wouldn't block. Events are a little funny in this respect;
the difference of behaviour must be documented in select(2) V4L2
documentation which currently is missing entirely (I'll send a patch).
An alternative would be indeed use POLLPRI only. The only way the user
would know there are no further events coming would be to use
VIDIOC_DQEVENT then. As a matter of fact, this way the behaviour of
select(2) would better conform to what the POSIX standard specifies but
OTOH would not allow to tell about the situation using poll(2) only in
any case since it'd look like the same as any event. But I don't think
it'd be a problem either.
--
Regards,
Sakari Ailus
sakari.ailus@xxxxxxxxxxxxxxx
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