On Sunday 04 February 2007 15:23, Michel Verbraak wrote:
> Hans Verkuil schreef:
> > On Sunday 04 February 2007 13:43, Michel Verbraak wrote:
> >> I was debugging a small program of mine and found that:
> >>
> >> - when I do a poll after opening the device, and not doing a read
> >> first, it would always give me a POLLERR instead of a POLLIN.
> >>
> >> Example:
> >>
> >> videofd=open("/dev/video0", O_RDONLY);
> >>
> >> // Add video device
> >> poll_item.fd = videofd;
> >> poll_item.events = POLLIN;
> >> poll_item.revents = 0;
> >>
> >> // poll infinite
> >> retvalue = poll(&poll_item, 1, -1);
> >>
> >> poll_item.revents is allways POLLERR.
> >>
> >> When I do a read(videofd, buffer, 0); directly after the open and
> >> then do the poll I get POLLIN.
> >>
> >> I can trace this back to the fact that the bit IVTV_F_S_STREAMING
> >> is only set, in function ivtv_v4l2_read in file ivtv-fileops.c,
> >> when at least one read is done. Even if the request is to read 0
> >> (zero) bytes.
> >>
> >> I do not know if this is by design.
> >> But I would think that it should be possible to first do a poll to
> >> see if data is available to read before I even do any reading. So
> >> probably the IVTV_F_S_STREAMING bit should be set in the function
> >> ivtv_v4l2_open in file ivtv-fileops.c.
> >>
> >> This problem is in ivtv 0.10.0RC1 and I would think even in the
> >> earlier versions.
> >>
> >> I did not yet spend time to find a fix because I would first like
> >> to know if this is by design or not.
> >
> > It's by design, but I'm not certain whether it is the right design.
> > Basically just opening the device will not start a capture, it has
> > to be started explicitly, either by a read or by an IVTV_IOC_PLAY
> > ioctl. It is unclear what poll should do if no capture is in
> > progress. Wait indefinitely? Automatically start a capture? Or
> > return an error? I can't find a clear answer in the V4L2 spec.
> >
> > I'll ask on the video4linux mailinglist, see what the correct
> > policy is.
> >
> > Thanks,
> >
> > Hans
> >
> > _______________________________________________
> > ivtv-devel mailing list
> > ivtv-devel@ivtvdriver.org
> > http://ivtvdriver.org/mailman/listinfo/ivtv-devel
>
> The program I'm writing does not known it is reading from an ivtv
> device. The file handle could be a device or a file or a socket.
> I would like to keep it this way because otherwise I have to make two
> different programs or during startup I have to specify if it is an
> ivtv device or the other. During startup I only specify te location
> like /dev/video0 or /movies/somemovie.avi or 192.168.1.1:10000 and
> the program either opens the device or file or creates a tcp socket.
And some other program or thread will actually start the capture? Just
opening a v4l2 device will NEVER automatically start a capture.
Otherwise even opening a video device just to change the frequency
would have the nasty side-effect of beginning a capture.
Regards,
Hans
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