Re: [PATCH v3.16 2/2] v4l: event: Add subscription to list before calling "add" operation

[Yi Qingliang <niqingliang2003@gmail.com>]

Ok, thanks a lot.

On Thu, Jan 3, 2019 at 6:15 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> On 01/03/2019 01:58 AM, Yi Qingliang wrote:
> > hello, I sent a email about 'can't wake problem' 4 days ago.
> >
> > Is this problem related with mine?
>
> No, it's unrelated.
>
> I'll take a look at vb2_poll next week.
>
> Regards,
>
>         Hans
>
> >
> >> epoll and vb2_poll: can't wake_up
> >
> >> Sun, Dec 30, 2018, 6:17 PM (4 days ago)
> >> to linux-kernel
> >> Hello, I encountered a "can't wake_up" problem when use camera on imx6.
> >>
> >> if delay some time after 'streamon' the /dev/video0, then add fd
> >> through epoll_ctl, then the process can't be waken_up after some time.
> >>
> >> I checked both the epoll / vb2_poll(videobuf2_core.c) code.
> >>
> >> epoll will pass 'poll_table' structure to vb2_poll, but it only
> >> contain valid function pointer when inserting fd.
> >>
> >> in vb2_poll, if found new data in done list, it will not call 'poll_wait'.
> >> after that, every call to vb2_poll will not contain valid poll_table,
> >> which will result in all calling to poll_wait will not work.
> >>
> >> so if app can process frames quickly, and found frame data when
> >> inserting fd (i.e. poll_wait will not be called or not contain valid
> >> function pointer), it will not found valid frame in 'vb2_poll' finally
> >> at some time, then call 'poll_wait' to expect be waken up at following
> >> vb2_buffer_done, but no good luck.
> >>
> >> I also checked the 'videobuf-core.c', there is no this problem.
> >>
> >> of course, both epoll and vb2_poll are right by itself side, but the
> >> result is we can't get new frames.
> >>
> >> I think by epoll's implementation, the user should always call poll_wait.
> >>
> >> and it's better to split the two actions: 'wait' and 'poll' both for
> >> epoll framework and all epoll users, for example, v4l2.
> >>
> >> am I right?
> >
> > On Thu, Jan 3, 2019 at 4:17 AM Ben Hutchings <ben@decadent.org.uk> wrote:
> >>
> >> On Thu, 2018-11-08 at 14:03 +0200, Sakari Ailus wrote:
> >>> [ upstream commit 92539d3eda2c090b382699bbb896d4b54e9bdece ]
> >>>
> >>> Patch ad608fbcf166 changed how events were subscribed to address an issue
> >>> elsewhere. As a side effect of that change, the "add" callback was called
> >>> before the event subscription was added to the list of subscribed events,
> >>> causing the first event queued by the add callback (and possibly other
> >>> events arriving soon afterwards) to be lost.
> >>>
> >>> Fix this by adding the subscription to the list before calling the "add"
> >>> callback, and clean up afterwards if that fails.
> >> [...]
> >>
> >> I've queued this up for the next update, thanks.
> >>
> >> Ben.
> >>
> >> --
> >> Ben Hutchings
> >> Absolutum obsoletum. (If it works, it's out of date.) - Stafford Beer
> >>
> >>
>