linux-kernel - Re: [PATCH v2 3/6] pinctrl: Add a possibility to configure pins from a gpiolib based drivers

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Date:   Wed, 11 Jan 2017 14:06:56 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        "David E . Box" <david.e.box@...ux.intel.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v2 3/6] pinctrl: Add a possibility to configure pins from
 a gpiolib based drivers

On Tue, Jan 10, 2017 at 3:31 PM, Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:

> When a GPIO driver is backed by a pinctrl driver the GPIO driver often
> needs to call the pinctrl driver to configure certain things, like
> whether the pin is used as input or output. In addition to this there
> are other pin configurations applicable to GPIOs such as debounce
> timeout.
>
> To support this we introduce a new function pinctrl_gpio_set_config()
> that can be used by gpiolib based driver to pass configuration requests
> to the backing pinctrl driver.
>
> Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>

OK so this is needed.

But let's first pause and discuss this, because I have some stuff on my
mind here.

First this kernel-internal ABI from <linux/gpio/driver.h>:

struct gpio_chip {
(...)
        int                     (*set_debounce)(struct gpio_chip *chip,
                                                unsigned offset,
                                                unsigned debounce);
        int                     (*set_single_ended)(struct gpio_chip *chip,
                                                unsigned offset,
                                                enum single_ended_mode mode);
(...)

It's not going to scale. We need to replace this with something like

int (*set_config)(struct gpio_chip *chip, unsigned offset, unsigned
long config);

Where "config" takes the packed format described in
<linux/pinctrl/pinconf-generic.h>
and nothing else, anything else is just inviting disaster.

We can also later add:

int (*get_config)(struct gpio_chip *chip, unsigned offset, unsigned
long *config);

We can then  set and get arbitrary configs on GPIO lines, and the
drivers can simply implement a switch() for the configs they handle
else return -ENOTSUPP.

But right now only set_config() would be enough.

Maybe stuff needs to be split out of that header to be shared between
GPIO and pinctrl but hopefully you could just include it.

Then we change all in-kernel users of these two APIs over to set_config().

THEN we can think about cross-calling to pin control using the API
from this patch. It should be a simple matter of just passing along the
same config argument since we're using generic pin config.

It's not like it's impossible to merge this patch first, but I want to get some
order here.

Are you convenient with doing the above patch as part of this series, or
shall I do it first so you can rebase on it? (Will take some time if I
do it...)

We need this because GPIO is going to need more and more config
to be done by pinctrl on its behalf, and it will have to go all the
way to userspace in many cases, so we need this infrastructure in
place.

Yours,
Linus Walleij