From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michael Welling <mwelling@ieee.org>
Subject: Re: Fw: [3.18.3] poll() on gpio pins broken
Date: Tue, 3 Feb 2015 03:03:26 -0600
Message-ID: <20150203090325.GA29693@deathray>
References: <20150129152902.GX21469@belle.intranet.vanheusden.com>
 <20150130234552.GA20407@sysresccd>
 <20150131083300.GZ21469@belle.intranet.vanheusden.com>
 <CAAVeFuJ6+W-CpXKCE2uY=T7RUVjA4d2PML708xH8dPQwij5NTg@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <linux-gpio-owner@vger.kernel.org>
Received: from mail-ig0-f170.google.com ([209.85.213.170]:41142 "EHLO
	mail-ig0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751434AbbBCJDe (ORCPT
	<rfc822;linux-gpio@vger.kernel.org>); Tue, 3 Feb 2015 04:03:34 -0500
Received: by mail-ig0-f170.google.com with SMTP id l13so23092626iga.1
        for <linux-gpio@vger.kernel.org>; Tue, 03 Feb 2015 01:03:33 -0800 (PST)
Content-Disposition: inline
In-Reply-To: <CAAVeFuJ6+W-CpXKCE2uY=T7RUVjA4d2PML708xH8dPQwij5NTg@mail.gmail.com>
Sender: linux-gpio-owner@vger.kernel.org
List-Id: linux-gpio@vger.kernel.org
To: Alexandre Courbot <gnurou@gmail.com>
Cc: folkert <folkert@vanheusden.com>, Linus Walleij <linus.walleij@linaro.org>, "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>

On Sat, Jan 31, 2015 at 02:39:03PM +0100, Alexandre Courbot wrote:
> On Sat, Jan 31, 2015 at 9:33 AM, folkert <folkert@vanheusden.com> 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

> 
> Linus, what do you think?