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

[Yi Qingliang <niqingliang2003@gmail.com>]

hello, I sent a email about 'can't wake problem' 4 days ago.

Is this problem related with mine?

> 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 (If it works, it's out of date.)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
>
>