linux-kernel - Re: [3.18.3] poll() on gpio pins broken

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Date:	Wed, 4 Feb 2015 14:38:52 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	folkert <folkert@...heusden.com>, Johan Hovold <johan@...nel.org>,
	Sören Brinkmann <soren.brinkmann@...inx.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [3.18.3] poll() on gpio pins broken

On Thu, Jan 29, 2015 at 3:56 PM, folkert <folkert@...heusden.com> wrote:

> 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
>
> So: I would setup a gpio-pin using /sys-files, and then use poll() to
> wait for the interrupt triggered by the level of the gpio pin going
> high.
> This worked fine on all raspberry pi's I tried, a nanos g20, a beagle
> bone black and a cubieboard 1.
>
> Since kernel 3.18.3 (.4 as well) this no longer works:
>
> 1422542104.894908947] interrupt #2459, offset -0.105091s
> 1422542104.895162937] interrupt #2460, offset -0.104837s
> 1422542104.895408928] interrupt #2461, offset -0.104591s
> 1422542104.895650919] interrupt #2462, offset -0.104349s
> 1422542104.896293896] interrupt #2463, offset -0.103706s
> ...
>
> As you can see suddenly the poll returns immediately.
>
> strace learns me:
> poll([{fd=3, events=POLLPRI}], 1, -1)   = 1 ([{fd=3, revents=POLLPRI|POLLERR}])
>
> So apparently somewhere between 3.12 and 3.18 it is no longer
> allowed/possible to do poll() on a gpio-pin value device.
>
> Am I right that something broke the abi? Or have I been doing it wrong
> and did that became visible?
> Please advise.

Paging Johan and Sören into this discussion so they fully realize the
horror of the GPIO sysfs ABI...

Yours,
Linus Walleij
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