On Thursday, June 30, 2011 22:35:15 Mauro Carvalho Chehab wrote:
> Em 30-06-2011 10:46, Hans Verkuil escreveu:
> > On Wednesday, June 29, 2011 16:35:04 Hans de Goede wrote:
> >> Hi,
> >>
> >> On 06/29/2011 03:43 PM, Hans Verkuil wrote:
> >>> On Wednesday, June 29, 2011 15:07:14 Hans de Goede wrote:
> >>
> >> <snip>
> >>
> >>> if (q->num_buffers == 0&& q->fileio == NULL) {
> >>> - if (!V4L2_TYPE_IS_OUTPUT(q->type)&& (q->io_modes& VB2_READ)) {
> >>> - ret = __vb2_init_fileio(q, 1);
> >>> - if (ret)
> >>> - return POLLERR;
> >>> - }
> >>> - if (V4L2_TYPE_IS_OUTPUT(q->type)&& (q->io_modes& VB2_WRITE)) {
> >>> - ret = __vb2_init_fileio(q, 0);
> >>> - if (ret)
> >>> - return POLLERR;
> >>> - /*
> >>> - * Write to OUTPUT queue can be done immediately.
> >>> - */
> >>> - return POLLOUT | POLLWRNORM;
> >>> - }
> >>> + if (!V4L2_TYPE_IS_OUTPUT(q->type)&& (q->io_modes& VB2_READ))
> >>> + return res | POLLIN | POLLRDNORM;
> >>> + if (V4L2_TYPE_IS_OUTPUT(q->type)&& (q->io_modes& VB2_WRITE))
> >>> + return res | POLLOUT | POLLWRNORM;
>
> It is wrong to return POLLIN/POLLOUT if the stream hasn't started yet. You should
> return it only when data is ready. Otherwise you should return 0.
>
> >>> }
> >>>
> >>> /*
> >>> * There is nothing to wait for if no buffers have already been
> > queued.
> >>> */
> >>> if (list_empty(&q->queued_list))
> >>> - return POLLERR;
> >>> + return have_events ? res : POLLERR;
> >>>
> >>
> >> This seems more accurate to me, given that in case of select the 2 influence
> >> different fd sets:
> >>
> >> return res | POLLERR;
> >
> > Hmm. The problem is that the poll(2) API will always return if POLLERR is set,
> > even if you only want to wait on POLLPRI.
>
> Yes, but this is the right thing to do: an error condition has happened. If you're
> in doubt, think that poll() is being used for a text file or a socket: if the connection
> has dropped, or there's a problem to access the file, poll() needs to return, as there is
> a condition error that needs to be handled.
>
> > That's a perfectly valid thing to
> > do. An alternative is to just not use POLLERR and return res|POLLIN or res|
> > POLLOUT depending on V4L2_TYPE_IS_OUTPUT().
>
> You should only rise POLLERR if a problem happened at the events delivery or at
> the device streaming.
>
> > Another option is to just return res (which is your suggestion below as well).
> > I think this is also a reasonable approach. It would in fact allow one thread
> > to call poll(2) and another thread to call REQBUFS/QBUF/STREAMON on the same
> > filehandle. And the other thread would return from poll(2) as soon as the
> > first frame becomes available.
> >
> > This also leads to another ambiguity with poll(): what should poll do if
> > another filehandle started streaming? So fh1 called STREAMON (and so becomes
> > the 'owner' of the stream), and you poll on fh2. If a frame becomes available,
> > should fh2 wake up? Is fh2 allowed to call DQBUF?
>
> IMO, both fh's should get the same results. This is what happens if you're
> writing into a file and two or more processes are selecting at the EOF.
Yes, but multiple filehandles are allowed to write/read from a file at the
same time. That's not true for V4L2. Only one filehandle can do I/O at a time.
I'm going to look into changing fs/select.c so that the poll driver function
can actually see the event mask provided by the application.
Regards,
Hans
>
> Anyway, changing from the current behavior may break applications.
>
> > To be honest, I think vb2 should keep track of the filehandle that started
> > streaming rather than leaving that to drivers, but that's a separate issue.
> >
> > I really wonder whether we should ever use POLLERR at all: it is extremely
> > vague how it should be interpreted, and it doesn't actually tell you what is
> > wrong. And is it really an error if you poll on a non-streaming node?
>
> See above. You need to rise it if, for example, an error occurred, and no data
> will be ready for read(), write() or DQEVENT. That's the reason why POLLERR
> exists.
>
> Cheers,
> Mauro
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