linux-kernel - Re: pinctrl discussions @ Linaro Connect, and also requesting GPIOs

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Date:	Sat, 12 Nov 2011 15:17:42 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Thomas Abraham <thomas.abraham@...aro.org>
Cc:	Stephen Warren <swarren@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>
Subject: Re: pinctrl discussions @ Linaro Connect, and also requesting GPIOs

On Sat, Nov 12, 2011 at 12:22 PM, Thomas Abraham
<thomas.abraham@...aro.org> wrote:

> Samsung SoC's use two pin functions out of 4/8/16 pin functions to
> represent that a pin is used as a gpio. For instance, function 0 means
> pin is used as a gpio input, and function 1 means pin is used as gpio
> output. A pin cannot support both input and output mode at the same
> time.
>
> The existing pinmux_[request|free]_gpio() functions can specify that a
> pin has to be set in gpio function mode, but lack the provision to
> select input or output mode for the gpio.

OK then this needs fixing if this API is to persist.

> The alternative approach of
> using pin functions to select gpio-input or gpio-output will be used
> for samsung pinmux drivers.
>
> This means the gpio_direction_[input|output] api's would remain unused
> on samsung platforms. And existing platforms already using this api
> will have to be modified to use the pinmux api to select the direction
> of the gpio pin. And the role of gpio_request will seem redundant when
> used along with pinmux_get().

This does not seem right. Why should it not use
gpio_direction_[input|output] to select whether the pin is used
for input or output like everyone else?

This will just lead to trouble with shared HW blocks where
one use gpio_direction_[input|output] to select the direction
while another one will use some pinctrl API.

We need one of two things:

1. pinmux_[request|free]_gpio() to pass direction to the
  core and driver, or

2. A use pattern like this:
  pmx = pinmux_get(dev, "gpio-1-in");
  /* Use as input */
  pinmux_put(pmx);
  pmx = pinmux_get(dev, "gpio-1-out");
  /* Use as output */
  etc

The latter seems hopeless to maintain. So the external call
pinmux_[request|free]_gpio() needs to be kept around,
and it needs to be called *internally* by GPIO drivers to set
up pins, so as to avoid hazzle.

In my opinion atleast...

Linus Walleij
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