linux-kernel - Re: [PATCH] HID: cp2112: fix gpio value in gpio_direction

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Date:	Mon, 7 Jul 2014 17:07:59 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Benjamin Tissoires <benjamin.tissoires@...il.com>
cc:	Antonio Borneo <borneo.antonio@...il.com>,
	linux-input <linux-input@...r.kernel.org>,
	David Barksdale <dbarksdale@...ogix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] HID: cp2112: fix gpio value in gpio_direction_output

On Mon, 7 Jul 2014, Benjamin Tissoires wrote:

> > CP2112 does not offer an atomic method to set both gpio
> > direction and value.
> > Also it does not permit to set gpio value before putting
> > gpio in output. In fact, accordingly to Silicon Labs
> > AN495, Rev. 0.2, cpt. 4.4, the HID report to set gpio
> > values "does not affect any pins that are not configured
> > as outputs".
> >
> > This is confirmed on evaluation board CP2112-EK.
> > With current driver, after execute:
> >         echo in > /sys/class/gpio/gpio248/direction
> >         echo low > /sys/class/gpio/gpio248/direction
> > gpio output is still high. Only after a following
> >         echo low > /sys/class/gpio/gpio248/direction
> > gpio output gets low.
> >
> > Fix driver by changing order of operations; first set
> > direction then set value.
> >
> > The drawback of this new sequence is that we can have
> > a pulse on gpio pin when direction is changed from
> > input to output-low, but this cannot be avoided on
> > current CP2112.
> 
> In this case, does keeping the first cp2112_gpio_set() before setting
> the output direction prevents the pulse?
> If so, then you can just keep the current code, and simply add the
> cp2112_gpio_set() at the end of cp2112_gpio_direction_output().
> 
> In both case, this is:
> Reviewed-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>

I am queuing this for 3.17, thanks.

-- 
Jiri Kosina
SUSE Labs
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