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

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Tue, Feb 3, 2015 at 5:03 PM, Michael Welling <mwelling@xxxxxxxx> wrote:
> On Sat, Jan 31, 2015 at 02:39:03PM +0100, Alexandre Courbot wrote:
>> On Sat, Jan 31, 2015 at 9:33 AM, folkert <folkert@xxxxxxxxxxxxxx> wrote:
>> > Michael,
>> >
>> >> > For timekeeping I wrote a program which waits for interrupts on
>> >> > gpio-pins and then tells the local ntp daemon the clock offset.
>> >> > I'm aware of the pps support in recent kernel but that does not work
>> >> > (yet) on all platforms (eg cubieboard 1).
>> >> >
>> >> > This has worked for quite some time but no longer.
>> >> >
>> >> > Until at least kernel 3.12 I could do:
>> >> >
>> >> > // export gpio pin
>> >> > // set direction to in
>> >> > // set direction to rising
>> >> > int fd = open("/sys.../value", O_RDONLY);
>> >> > fdset[0].fd = fd;
>> >> > fdset[0].events = POLLPRI;
>> >> > fdset[0].revents = 0;
>> >> > poll(fdset, 1, -1);
>> >> > // at this point pin went high
>> >>
>> >> 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?

Yours,
Linus Walleij
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