Re: Fw: [3.18.3] poll() on gpio pins broken

[folkert <folkert@xxxxxxxxxxxxxx>]

> >> >> Try using lseek before reading the data after the poll.
> >> >>
> >> >> EX.
> >> >>       if (fdset[0].revents & POLLPRI) {
> >> >>               lseek(fdset[0].fd, 0, SEEK_SET);
> >> >>               len = read(fdset[0].fd, buf, MAX_BUF);
> >> >>               .
> >> >>               .
> >> >>       }
> >> >>
> >> >> See if this helps.
> >> >
> >> > Yes, that fixed it!
> >>
> >> Still, shouldn't we consider this as a regression, especially if not
> >> using lseek worked for kernel 3.12 and before?
> >
> > Perhaps this is a side effect of the sysfs to kernfs change over to happened in
> > 3.14.
> >
> > Looking at 'kernfs_fop_poll' my guess is that 'kernfs_get_active' is returning NULL
> > immediately returning from the poll command and causing described behavior.
> >
> > http://lxr.free-electrons.com/source/fs/kernfs/file.c#L763
> 
> I think this looks plausible.
> Folkert what do you say about this?

That looks indeed to be the current situation. It worked different
before though.

I also think that this interface is cumbersome. I did not measure it(!)
but I think adding this open/seek + read construction may add all kinds
of overhead. Especially since my use-case requires the lowest latency
possible. Not to forget that conversion of the measured value to ascii
and back.


Folkert van Heusden

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