linux-kernel - Re: [PATCH v4 3/3] gpio: add support for the Diolan DLN-2 USB GPIO driver

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Date:	Fri, 19 Sep 2014 09:11:39 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Octavian Purdila <octavian.purdila@...el.com>
Cc:	Johan Hovold <johan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>, wsa@...-dreams.de,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Daniel Baluta <daniel.baluta@...el.com>,
	Laurentiu Palcu <laurentiu.palcu@...el.com>,
	linux-usb@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
	linux-gpio@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH v4 3/3] gpio: add support for the Diolan DLN-2 USB GPIO
 driver

On Thu, Sep 18, 2014 at 06:54:34PM +0300, Octavian Purdila wrote:
> On Thu, Sep 18, 2014 at 3:46 PM, Johan Hovold <johan@...nel.org> wrote:
> > On Thu, Sep 18, 2014 at 03:43:07PM +0300, Octavian Purdila wrote:
> >> On Thu, Sep 18, 2014 at 1:54 PM, Johan Hovold <johan@...nel.org> wrote:
> >> > On Tue, Sep 09, 2014 at 10:24:46PM +0300, Octavian Purdila wrote:
> >
> >> > Either way, it looks like this could race with get_direction() if you
> >> > get a set_direction() while get_direction() is retrieving the direction
> >> > from the device.
> >> >
> >> > This would break gpio_get().
> >> >
> >> I don't think gpio_set_direction() and gpio_get() are allowed to race.
> >
> > I wrote that set_direction() and get_direction() could race, which in
> > turn would break gpio_get() as you would be caching the wrong
> > direction setting.
> >
> 
> OK, I now see the problem. I think doing this in get_direction() will
> fix the issue:
> 
>                 if (!test_and_set_bit(offset, dln2->pin_dir_set))
>                         set/clear_bit(offset, dln2->pin_dir);
> 
> because gpiolib calls get_direction() while requesting a pin and
> request cannot race with itself. Which means that get_direction() can
> not race with itself the first time it is called, when the set/clear
> operation will be run.
> 
> And because we know that get_direction() is called first, we can even
> remove the set/clear operation from set_direction().

Why not simply fetch the direction in request() and get rid of the
pin_dir_set bitmask?

Johan
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