Hi Sakari, 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 ? -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
