PINCTRL (PIN CONTROL) subsystem
This document outlines the pin control subsystem in Linux

This subsystem deals with:

- Enumerating and naming controllable pins

- Multiplexing of pins, pads, fingers (etc) see below for details

- Configuration of pins, pads, fingers (etc), such as software-controlled
  biasing and driving mode specific pins, such as pull-up/down, open drain,
  load capacitance etc.

Top-level interface
===================

Definition of PIN CONTROLLER:

- A pin controller is a piece of hardware, usually a set of registers, that
  can control PINs. It may be able to multiplex, bias, set load capacitance,
  set drive strength etc for individual pins or groups of pins.

Definition of PIN:

- PINS are equal to pads, fingers, balls or whatever packaging input or
  output line you want to control and these are denoted by unsigned integers
  in the range 0..maxpin. This numberspace is local to each PIN CONTROLLER, so
  there may be several such number spaces in a system. This pin space may
  be sparse - i.e. there may be gaps in the space with numbers where no
  pin exists.

When a PIN CONTROLLER is instantiated, it will register a descriptor to the
pin control framework, and this descriptor contains an array of pin descriptors
describing the pins handled by this specific pin controller.

Here is an example of a PGA (Pin Grid Array) chip seen from underneath:

        A   B   C   D   E   F   G   H

   8    o   o   o   o   o   o   o   o

   7    o   o   o   o   o   o   o   o

   6    o   o   o   o   o   o   o   o

   5    o   o   o   o   o   o   o   o

   4    o   o   o   o   o   o   o   o

   3    o   o   o   o   o   o   o   o

   2    o   o   o   o   o   o   o   o

   1    o   o   o   o   o   o   o   o

To register a pin controller and name all the pins on this package we can do
this in our driver:

#include <linux/pinctrl/pinctrl.h>

const struct pinctrl_pin_desc foo_pins[] = {
      PINCTRL_PIN(0, "A8"),
      PINCTRL_PIN(1, "B8"),
      PINCTRL_PIN(2, "C8"),
      ...
      PINCTRL_PIN(61, "F1"),
      PINCTRL_PIN(62, "G1"),
      PINCTRL_PIN(63, "H1"),
};

static struct pinctrl_desc foo_desc = {
	.name = "foo",
	.pins = foo_pins,
	.npins = ARRAY_SIZE(foo_pins),
	.maxpin = 63,
	.owner = THIS_MODULE,
};

int __init foo_probe(void)
{
	struct pinctrl_dev *pctl;

	pctl = pinctrl_register(&foo_desc, <PARENT>, NULL);
	if (IS_ERR(pctl))
		pr_err("could not register foo pin driver\n");
}

To enable the pinctrl subsystem and the subgroups for PINMUX and PINCONF and
selected drivers, you need to select them from your machine's Kconfig entry,
since these are so tightly integrated with the machines they are used on.
See for example arch/arm/mach-u300/Kconfig for an example.

Pins usually have fancier names than this. You can find these in the dataheet
for your chip. Notice that the core pinctrl.h file provides a fancy macro
called PINCTRL_PIN() to create the struct entries. As you can see I enumerated
the pins from 0 in the upper left corner to 63 in the lower right corner.
This enumeration was arbitrarily chosen, in practice you need to think
through your numbering system so that it matches the layout of registers
and such things in your driver, or the code may become complicated. You must
also consider matching of offsets to the GPIO ranges that may be handled by
the pin controller.

For a padring with 467 pads, as opposed to actual pins, I used an enumeration
like this, walking around the edge of the chip, which seems to be industry
standard too (all these pads had names, too):


     0 ..... 104
   466        105
     .        .
     .        .
   358        224
    357 .... 225


Pin groups
==========

Many controllers need to deal with groups of pins, so the pin controller
subsystem has a mechanism for enumerating groups of pins and retrieving the
actual enumerated pins that are part of a certain group.

For example, say that we have a group of pins dealing with an SPI interface
on { 0, 8, 16, 24 }, and a group of pins dealing with an I2C interface on pins
on { 24, 25 }.

These two groups are presented to the pin control subsystem by implementing
some generic pinctrl_ops like this:

#include <linux/pinctrl/pinctrl.h>

struct foo_group {
	const char *name;
	const unsigned int *pins;
	const unsigned num_pins;
};

static const unsigned int spi0_pins[] = { 0, 8, 16, 24 };
static const unsigned int i2c0_pins[] = { 24, 25 };

static const struct foo_group foo_groups[] = {
	{
		.name = "spi0_grp",
		.pins = spi0_pins,
		.num_pins = ARRAY_SIZE(spi0_pins),
	},
	{
		.name = "i2c0_grp",
		.pins = i2c0_pins,
		.num_pins = ARRAY_SIZE(i2c0_pins),
	},
};


static int foo_list_groups(struct pinctrl_dev *pctldev, unsigned selector)
{
	if (selector >= ARRAY_SIZE(foo_groups))
		return -EINVAL;
	return 0;
}

static const char *foo_get_group_name(struct pinctrl_dev *pctldev,
				       unsigned selector)
{
	return foo_groups[selector].name;
}

static int foo_get_group_pins(struct pinctrl_dev *pctldev, unsigned selector,
			       unsigned ** const pins,
			       unsigned * const num_pins)
{
	*pins = (unsigned *) foo_groups[selector].pins;
	*num_pins = foo_groups[selector].num_pins;
	return 0;
}

static struct pinctrl_ops foo_pctrl_ops = {
	.list_groups = foo_list_groups,
	.get_group_name = foo_get_group_name,
	.get_group_pins = foo_get_group_pins,
};


static struct pinctrl_desc foo_desc = {
       ...
       .pctlops = &foo_pctrl_ops,
};

The pin control subsystem will call the .list_groups() function repeatedly
beginning on 0 until it returns non-zero to determine legal selectors, then
it will call the other functions to retrieve the name and pins of the group.
Maintaining the data structure of the groups is up to the driver, this is
just a simple example - in practice you may need more entries in your group
structure, for example specific register ranges associated with each group
and so on.


Pin configuration
=================

Pins can sometimes be software-configured in an various ways, mostly related
to their electronic properties when used as inputs or outputs. For example you
may be able to make an output pin high impedance, or "tristate" meaning it is
effectively disconnected. You may be able to connect an input pin to VDD or GND
using a certain resistor value - pull up and pull down - so that the pin has a
stable value when nothing is driving the rail it is connected to, or when it's
unconnected.

For example, a platform may do this:

#include <linux/pinctrl/consumer.h>

ret = pin_config_set("foo-dev", "FOO_GPIO_PIN", PLATFORM_X_PULL_UP);

To pull up a pin to VDD. The pin configuration driver implements callbacks for
changing pin configuration in the pin controller ops like this:

#include <linux/pinctrl/pinctrl.h>
#include <linux/pinctrl/pinconf.h>
#include "platform_x_pindefs.h"

static int foo_pin_config_get(struct pinctrl_dev *pctldev,
		    unsigned offset,
		    unsigned long *config)
{
	struct my_conftype conf;

	... Find setting for pin @ offset ...

	*config = (unsigned long) conf;
}

static int foo_pin_config_set(struct pinctrl_dev *pctldev,
		    unsigned offset,
		    unsigned long config)
{
	struct my_conftype *conf = (struct my_conftype *) config;

	switch (conf) {
		case PLATFORM_X_PULL_UP:
		...
		}
	}
}

static int foo_pin_config_group_get (struct pinctrl_dev *pctldev,
		    unsigned selector,
		    unsigned long *config)
{
	...
}

static int foo_pin_config_group_set (struct pinctrl_dev *pctldev,
		    unsigned selector,
		    unsigned long config)
{
	...
}

static struct pinconf_ops foo_pconf_ops = {
	.pin_config_get = foo_pin_config_get,
	.pin_config_set = foo_pin_config_set,
	.pin_config_group_get = foo_pin_config_group_get,
	.pin_config_group_set = foo_pin_config_group_set,
};

/* Pin config operations are handled by some pin controller */
static struct pinctrl_desc foo_desc = {
	...
	.confops = &foo_pconf_ops,
};

Since some controllers have special logic for handling entire groups of pins
they can exploit the special whole-group pin control function. The
pin_config_group_set() callback is allowed to return the error code -EAGAIN,
for groups it does not want to handle, or if it just wants to do some
group-level handling and then fall through to iterate over all pins, in which
case each individual pin will be treated by separate pin_config_set() calls as
well.


Interaction with the GPIO subsystem
===================================

The GPIO drivers may want to perform operations of various types on the same
physical pins that are also registered as pin controller pins.

Since the pin controller subsystem have its pinspace local to the pin
controller we need a mapping so that the pin control subsystem can figure out
which pin controller handles control of a certain GPIO pin. Since a single
pin controller may be muxing several GPIO ranges (typically SoCs that have
one set of pins but internally several GPIO silicon blocks, each modeled as
a struct gpio_chip) any number of GPIO ranges can be added to a pin controller
instance like this:

struct gpio_chip chip_a;
struct gpio_chip chip_b;

static struct pinctrl_gpio_range gpio_range_a = {
	.name = "chip a",
	.id = 0,
	.base = 32,
	.pin_base = 32,
	.npins = 16,
	.gc = &chip_a;
};

static struct pinctrl_gpio_range gpio_range_b = {
	.name = "chip b",
	.id = 0,
	.base = 48,
	.pin_base = 64,
	.npins = 8,
	.gc = &chip_b;
};

{
	struct pinctrl_dev *pctl;
	...
	pinctrl_add_gpio_range(pctl, &gpio_range_a);
	pinctrl_add_gpio_range(pctl, &gpio_range_b);
}

So this complex system has one pin controller handling two different
GPIO chips. "chip a" has 16 pins and "chip b" has 8 pins. The "chip a" and
"chip b" have different .pin_base, which means a start pin number of the
GPIO range.

The GPIO range of "chip a" starts from the GPIO base of 32 and actual
pin range also starts from 32. However "chip b" has different starting
offset for the GPIO range and pin range. The GPIO range of "chip b" starts
from GPIO number 48, while the pin range of "chip b" starts from 64.

We can convert a gpio number to actual pin number using this "pin_base".
They are mapped in the global GPIO pin space at:

chip a:
 - GPIO range : [32 .. 47]
 - pin range  : [32 .. 47]
chip b:
 - GPIO range : [48 .. 55]
 - pin range  : [64 .. 71]

When GPIO-specific functions in the pin control subsystem are called, these
ranges will be used to look up the appropriate pin controller by inspecting
and matching the pin to the pin ranges across all controllers. When a
pin controller handling the matching range is found, GPIO-specific functions
will be called on that specific pin controller.

For all functionalities dealing with pin biasing, pin muxing etc, the pin
controller subsystem will subtract the range's .base offset from the passed
in gpio number, and add the ranges's .pin_base offset to retrive a pin number.
After that, the subsystem passes it on to the pin control driver, so the driver
will get an pin number into its handled number range. Further it is also passed
the range ID value, so that the pin controller knows which range it should
deal with.

PINMUX interfaces
=================

These calls use the pinmux_* naming prefix.  No other calls should use that
prefix.


What is pinmuxing?
==================

PINMUX, also known as padmux, ballmux, alternate functions or mission modes
is a way for chip vendors producing some kind of electrical packages to use
a certain physical pin (ball, pad, finger, etc) for multiple mutually exclusive
functions, depending on the application. By "application" in this context
we usually mean a way of soldering or wiring the package into an electronic
system, even though the framework makes it possible to also change the function
at runtime.

Here is an example of a PGA (Pin Grid Array) chip seen from underneath:

        A   B   C   D   E   F   G   H
      +---+
   8  | o | o   o   o   o   o   o   o
      |   |
   7  | o | o   o   o   o   o   o   o
      |   |
   6  | o | o   o   o   o   o   o   o
      +---+---+
   5  | o | o | o   o   o   o   o   o
      +---+---+               +---+
   4    o   o   o   o   o   o | o | o
                              |   |
   3    o   o   o   o   o   o | o | o
                              |   |
   2    o   o   o   o   o   o | o | o
      +-------+-------+-------+---+---+
   1  | o   o | o   o | o   o | o | o |
      +-------+-------+-------+---+---+

This is not tetris. The game to think of is chess. Not all PGA/BGA packages
are chessboard-like, big ones have "holes" in some arrangement according to
different design patterns, but we're using this as a simple example. Of the
pins you see some will be taken by things like a few VCC and GND to feed power
to the chip, and quite a few will be taken by large ports like an external
memory interface. The remaining pins will often be subject to pin multiplexing.

The example 8x8 PGA package above will have pin numbers 0 thru 63 assigned to
its physical pins. It will name the pins { A1, A2, A3 ... H6, H7, H8 } using
pinctrl_register_pins() and a suitable data set as shown earlier.

In this 8x8 BGA package the pins { A8, A7, A6, A5 } can be used as an SPI port
(these are four pins: CLK, RXD, TXD, FRM). In that case, pin B5 can be used as
some general-purpose GPIO pin. However, in another setting, pins { A5, B5 } can
be used as an I2C port (these are just two pins: SCL, SDA). Needless to say,
we cannot use the SPI port and I2C port at the same time. However in the inside
of the package the silicon performing the SPI logic can alternatively be routed
out on pins { G4, G3, G2, G1 }.

On the botton row at { A1, B1, C1, D1, E1, F1, G1, H1 } we have something
special - it's an external MMC bus that can be 2, 4 or 8 bits wide, and it will
consume 2, 4 or 8 pins respectively, so either { A1, B1 } are taken or
{ A1, B1, C1, D1 } or all of them. If we use all 8 bits, we cannot use the SPI
port on pins { G4, G3, G2, G1 } of course.

This way the silicon blocks present inside the chip can be multiplexed "muxed"
out on different pin ranges. Often contemporary SoC (systems on chip) will
contain several I2C, SPI, SDIO/MMC, etc silicon blocks that can be routed to
different pins by pinmux settings.

Since general-purpose I/O pins (GPIO) are typically always in shortage, it is
common to be able to use almost any pin as a GPIO pin if it is not currently
in use by some other I/O port.


Pinmux conventions
==================

The purpose of the pinmux functionality in the pin controller subsystem is to
abstract and provide pinmux settings to the devices you choose to instantiate
in your machine configuration. It is inspired by the clk, GPIO and regulator
subsystems, so devices will request their mux setting, but it's also possible
to request a single pin for e.g. GPIO.

Definitions:

- FUNCTIONS can be switched in and out by a driver residing with the pin
  control subsystem in the drivers/pinctrl/* directory of the kernel. The
  pin control driver knows the possible functions. In the example above you can
  identify three pinmux functions, one for spi, one for i2c and one for mmc.

- FUNCTIONS are assumed to be enumerable from zero in a one-dimensional array.
  In this case the array could be something like: { spi0, i2c0, mmc0 }
  for the three available functions.

- FUNCTIONS have PIN GROUPS as defined on the generic level - so a certain
  function is *always* associated with a certain set of pin groups, could
  be just a single one, but could also be many. In the example above the
  function i2c is associated with the pins { A5, B5 }, enumerated as
  { 24, 25 } in the controller pin space.

  The Function spi is associated with pin groups { A8, A7, A6, A5 }
  and { G4, G3, G2, G1 }, which are enumerated as { 0, 8, 16, 24 } and
  { 38, 46, 54, 62 } respectively.

  Group names must be unique per pin controller, no two groups on the same
  controller may have the same name.

- The combination of a FUNCTION and a PIN GROUP determine a certain function
  for a certain set of pins. The knowledge of the functions and pin groups
  and their machine-specific particulars are kept inside the pinmux driver,
  from the outside only the enumerators are known, and the driver core can:

  - Request the name of a function with a certain selector (>= 0)
  - A list of groups associated with a certain function
  - Request that a certain group in that list to be activated for a certain
    function

  As already described above, pin groups are in turn self-descriptive, so
  the core will retrieve the actual pin range in a certain group from the
  driver.

- FUNCTIONS and GROUPS on a certain PIN CONTROLLER are MAPPED to a certain
  device by the board file, device tree or similar machine setup configuration
  mechanism, similar to how regulators are connected to devices, usually by
  name. Defining a pin controller, function and group thus uniquely identify
  the set of pins to be used by a certain device. (If only one possible group
  of pins is available for the function, no group name need to be supplied -
  the core will simply select the first and only group available.)

  In the example case we can define that this particular machine shall
  use device spi0 with pinmux function fspi0 group gspi0 and i2c0 on function
  fi2c0 group gi2c0, on the primary pin controller, we get mappings
  like these:

  {
    {"map-spi0", spi0, pinctrl0, fspi0, gspi0},
    {"map-i2c0", i2c0, pinctrl0, fi2c0, gi2c0}
  }

  Every map must be assigned a symbolic name, pin controller and function.
  The group is not compulsory - if it is omitted the first group presented by
  the driver as applicable for the function will be selected, which is
  useful for simple cases.

  The device name is present in map entries tied to specific devices. Maps
  without device names are referred to as SYSTEM pinmuxes, such as can be taken
  by the machine implementation on boot and not tied to any specific device.

  It is possible to map several groups to the same combination of device,
  pin controller and function. This is for cases where a certain function on
  a certain pin controller may use different sets of pins in different
  configurations.

- PINS for a certain FUNCTION using a certain PIN GROUP on a certain
  PIN CONTROLLER are provided on a first-come first-serve basis, so if some
  other device mux setting or GPIO pin request has already taken your physical
  pin, you will be denied the use of it. To get (activate) a new setting, the
  old one has to be put (deactivated) first.

Sometimes the documentation and hardware registers will be oriented around
pads (or "fingers") rather than pins - these are the soldering surfaces on the
silicon inside the package, and may or may not match the actual number of
pins/balls underneath the capsule. Pick some enumeration that makes sense to
you. Define enumerators only for the pins you can control if that makes sense.

Assumptions:

We assume that the number of possible function maps to pin groups is limited by
the hardware. I.e. we assume that there is no system where any function can be
mapped to any pin, like in a phone exchange. So the available pins groups for
a certain function will be limited to a few choices (say up to eight or so),
not hundreds or any amount of choices. This is the characteristic we have found
by inspecting available pinmux hardware, and a necessary assumption since we
expect pinmux drivers to present *all* possible function vs pin group mappings
to the subsystem.


Pinmux drivers
==============

The pinmux core takes care of preventing conflicts on pins and calling
the pin controller driver to execute different settings.

It is the responsibility of the pinmux driver to impose further restrictions
(say for example infer electronic limitations due to load etc) to determine
whether or not the requested function can actually be allowed, and in case it
is possible to perform the requested mux setting, poke the hardware so that
this happens.

Pinmux drivers are required to supply a few callback functions, some are
optional. Usually the enable() and disable() functions are implemented,
writing values into some certain registers to activate a certain mux setting
for a certain pin.

A simple driver for the above example will work by setting bits 0, 1, 2, 3 or 4
into some register named MUX to select a certain function with a certain
group of pins would work something like this:

#include <linux/pinctrl/pinctrl.h>
#include <linux/pinctrl/pinmux.h>

struct foo_group {
	const char *name;
	const unsigned int *pins;
	const unsigned num_pins;
};

static const unsigned spi0_0_pins[] = { 0, 8, 16, 24 };
static const unsigned spi0_1_pins[] = { 38, 46, 54, 62 };
static const unsigned i2c0_pins[] = { 24, 25 };
static const unsigned mmc0_1_pins[] = { 56, 57 };
static const unsigned mmc0_2_pins[] = { 58, 59 };
static const unsigned mmc0_3_pins[] = { 60, 61, 62, 63 };

static const struct foo_group foo_groups[] = {
	{
		.name = "spi0_0_grp",
		.pins = spi0_0_pins,
		.num_pins = ARRAY_SIZE(spi0_0_pins),
	},
	{
		.name = "spi0_1_grp",
		.pins = spi0_1_pins,
		.num_pins = ARRAY_SIZE(spi0_1_pins),
	},
	{
		.name = "i2c0_grp",
		.pins = i2c0_pins,
		.num_pins = ARRAY_SIZE(i2c0_pins),
	},
	{
		.name = "mmc0_1_grp",
		.pins = mmc0_1_pins,
		.num_pins = ARRAY_SIZE(mmc0_1_pins),
	},
	{
		.name = "mmc0_2_grp",
		.pins = mmc0_2_pins,
		.num_pins = ARRAY_SIZE(mmc0_2_pins),
	},
	{
		.name = "mmc0_3_grp",
		.pins = mmc0_3_pins,
		.num_pins = ARRAY_SIZE(mmc0_3_pins),
	},
};


static int foo_list_groups(struct pinctrl_dev *pctldev, unsigned selector)
{
	if (selector >= ARRAY_SIZE(foo_groups))
		return -EINVAL;
	return 0;
}

static const char *foo_get_group_name(struct pinctrl_dev *pctldev,
				       unsigned selector)
{
	return foo_groups[selector].name;
}

static int foo_get_group_pins(struct pinctrl_dev *pctldev, unsigned selector,
			       unsigned ** const pins,
			       unsigned * const num_pins)
{
	*pins = (unsigned *) foo_groups[selector].pins;
	*num_pins = foo_groups[selector].num_pins;
	return 0;
}

static struct pinctrl_ops foo_pctrl_ops = {
	.list_groups = foo_list_groups,
	.get_group_name = foo_get_group_name,
	.get_group_pins = foo_get_group_pins,
};

struct foo_pmx_func {
	const char *name;
	const char * const *groups;
	const unsigned num_groups;
};

static const char * const spi0_groups[] = { "spi0_1_grp" };
static const char * const i2c0_groups[] = { "i2c0_grp" };
static const char * const mmc0_groups[] = { "mmc0_1_grp", "mmc0_2_grp",
					"mmc0_3_grp" };

static const struct foo_pmx_func foo_functions[] = {
	{
		.name = "spi0",
		.groups = spi0_groups,
		.num_groups = ARRAY_SIZE(spi0_groups),
	},
	{
		.name = "i2c0",
		.groups = i2c0_groups,
		.num_groups = ARRAY_SIZE(i2c0_groups),
	},
	{
		.name = "mmc0",
		.groups = mmc0_groups,
		.num_groups = ARRAY_SIZE(mmc0_groups),
	},
};

int foo_list_funcs(struct pinctrl_dev *pctldev, unsigned selector)
{
	if (selector >= ARRAY_SIZE(foo_functions))
		return -EINVAL;
	return 0;
}

const char *foo_get_fname(struct pinctrl_dev *pctldev, unsigned selector)
{
	return foo_functions[selector].name;
}

static int foo_get_groups(struct pinctrl_dev *pctldev, unsigned selector,
			  const char * const **groups,
			  unsigned * const num_groups)
{
	*groups = foo_functions[selector].groups;
	*num_groups = foo_functions[selector].num_groups;
	return 0;
}

int foo_enable(struct pinctrl_dev *pctldev, unsigned selector,
		unsigned group)
{
	u8 regbit = (1 << selector + group);

	writeb((readb(MUX)|regbit), MUX)
	return 0;
}

void foo_disable(struct pinctrl_dev *pctldev, unsigned selector,
		unsigned group)
{
	u8 regbit = (1 << selector + group);

	writeb((readb(MUX) & ~(regbit)), MUX)
	return 0;
}

struct pinmux_ops foo_pmxops = {
	.list_functions = foo_list_funcs,
	.get_function_name = foo_get_fname,
	.get_function_groups = foo_get_groups,
	.enable = foo_enable,
	.disable = foo_disable,
};

/* Pinmux operations are handled by some pin controller */
static struct pinctrl_desc foo_desc = {
	...
	.pctlops = &foo_pctrl_ops,
	.pmxops = &foo_pmxops,
};

In the example activating muxing 0 and 1 at the same time setting bits
0 and 1, uses one pin in common so they would collide.

The beauty of the pinmux subsystem is that since it keeps track of all
pins and who is using them, it will already have denied an impossible
request like that, so the driver does not need to worry about such
things - when it gets a selector passed in, the pinmux subsystem makes
sure no other device or GPIO assignment is already using the selected
pins. Thus bits 0 and 1 in the control register will never be set at the
same time.

All the above functions are mandatory to implement for a pinmux driver.


Pin control interaction with the GPIO subsystem
===============================================

The public pinmux API contains two functions named pinctrl_request_gpio()
and pinctrl_free_gpio(). These two functions shall *ONLY* be called from
gpiolib-based drivers as part of their gpio_request() and
gpio_free() semantics. Likewise the pinctrl_gpio_direction_[input|output]
shall only be called from within respective gpio_direction_[input|output]
gpiolib implementation.

NOTE that platforms and individual drivers shall *NOT* request GPIO pins to be
controlled e.g. muxed in. Instead, implement a proper gpiolib driver and have
that driver request proper muxing and other control for its pins.

The function list could become long, especially if you can convert every
individual pin into a GPIO pin independent of any other pins, and then try
the approach to define every pin as a function.

In this case, the function array would become 64 entries for each GPIO
setting and then the device functions.

For this reason there are two functions a pin control driver can implement
to enable only GPIO on an individual pin: .gpio_request_enable() and
.gpio_disable_free().

This function will pass in the affected GPIO range identified by the pin
controller core, so you know which GPIO pins are being affected by the request
operation.

If your driver needs to have an indication from the framework of whether the
GPIO pin shall be used for input or output you can implement the
.gpio_set_direction() function. As described this shall be called from the
gpiolib driver and the affected GPIO range, pin offset and desired direction
will be passed along to this function.

Alternatively to using these special functions, it is fully allowed to use
named functions for each GPIO pin, the pinctrl_request_gpio() will attempt to
obtain the function "gpioN" where "N" is the global GPIO pin number if no
special GPIO-handler is registered.


Pinmux board/machine configuration
==================================

Boards and machines define how a certain complete running system is put
together, including how GPIOs and devices are muxed, how regulators are
constrained and how the clock tree looks. Of course pinmux settings are also
part of this.

A pinmux config for a machine looks pretty much like a simple regulator
configuration, so for the example array above we want to enable i2c and
spi on the second function mapping:

#include <linux/pinctrl/machine.h>

static const struct pinctrl_map __initdata mapping[] = {
	{
		.ctrl_dev_name = "pinctrl-foo",
		.function = "spi0",
		.dev_name = "foo-spi.0",
	},
	{
		.ctrl_dev_name = "pinctrl-foo",
		.function = "i2c0",
		.dev_name = "foo-i2c.0",
	},
	{
		.ctrl_dev_name = "pinctrl-foo",
		.function = "mmc0",
		.dev_name = "foo-mmc.0",
	},
};

The dev_name here matches to the unique device name that can be used to look
up the device struct (just like with clockdev or regulators). The function name
must match a function provided by the pinmux driver handling this pin range.

As you can see we may have several pin controllers on the system and thus
we need to specify which one of them that contain the functions we wish
to map.

You register this pinmux mapping to the pinmux subsystem by simply:

       ret = pinctrl_register_mappings(mapping, ARRAY_SIZE(mapping));

Since the above construct is pretty common there is a helper macro to make
it even more compact which assumes you want to use pinctrl-foo and position
0 for mapping, for example:

static struct pinctrl_map __initdata mapping[] = {
       PIN_MAP("I2CMAP", "pinctrl-foo", "i2c0", "foo-i2c.0"),
};


Complex mappings
================

As it is possible to map a function to different groups of pins an optional
.group can be specified like this:

...
{
	.name = "spi0-pos-A",
	.ctrl_dev_name = "pinctrl-foo",
	.function = "spi0",
	.group = "spi0_0_grp",
	.dev_name = "foo-spi.0",
},
{
	.name = "spi0-pos-B",
	.ctrl_dev_name = "pinctrl-foo",
	.function = "spi0",
	.group = "spi0_1_grp",
	.dev_name = "foo-spi.0",
},
...

This example mapping is used to switch between two positions for spi0 at
runtime, as described further below under the heading "Runtime pinmuxing".

Further it is possible to match several groups of pins to the same function
for a single device, say for example in the mmc0 example above, where you can
additively expand the mmc0 bus from 2 to 4 to 8 pins. If we want to use all
three groups for a total of 2+2+4 = 8 pins (for an 8-bit MMC bus as is the
case), we define a mapping like this:

...
{
	.name = "2bit"
	.ctrl_dev_name = "pinctrl-foo",
	.function = "mmc0",
	.group = "mmc0_1_grp",
	.dev_name = "foo-mmc.0",
},
{
	.name = "4bit"
	.ctrl_dev_name = "pinctrl-foo",
	.function = "mmc0",
	.group = "mmc0_1_grp",
	.dev_name = "foo-mmc.0",
},
{
	.name = "4bit"
	.ctrl_dev_name = "pinctrl-foo",
	.function = "mmc0",
	.group = "mmc0_2_grp",
	.dev_name = "foo-mmc.0",
},
{
	.name = "8bit"
	.ctrl_dev_name = "pinctrl-foo",
	.group = "mmc0_1_grp",
	.dev_name = "foo-mmc.0",
},
{
	.name = "8bit"
	.ctrl_dev_name = "pinctrl-foo",
	.function = "mmc0",
	.group = "mmc0_2_grp",
	.dev_name = "foo-mmc.0",
},
{
	.name = "8bit"
	.ctrl_dev_name = "pinctrl-foo",
	.function = "mmc0",
	.group = "mmc0_3_grp",
	.dev_name = "foo-mmc.0",
},
...

The result of grabbing this mapping from the device with something like
this (see next paragraph):

	p = pinctrl_get(&device, "8bit");

Will be that you activate all the three bottom records in the mapping at
once. Since they share the same name, pin controller device, funcion and
device, and since we allow multiple groups to match to a single device, they
all get selected, and they all get enabled and disable simultaneously by the
pinmux core.


Pinmux requests from drivers
============================

Generally it is discouraged to let individual drivers get and enable pin
control. So if possible, handle the pin control in platform code or some other
place where you have access to all the affected struct device * pointers. In
some cases where a driver needs to e.g. switch between different mux mappings
at runtime this is not possible.

A driver may request a certain control state to be activated, usually just the
default state like this:

#include <linux/pinctrl/consumer.h>

struct foo_state {
       struct pinctrl *p;
       ...
};

foo_probe()
{
	/* Allocate a state holder named "state" etc */
	struct pinctrl p;

	p = pinctrl_get(&device, NULL);
	if IS_ERR(p)
		return PTR_ERR(p);
	pinctrl_enable(p);

	state->p = p;
}

foo_remove()
{
	pinctrl_disable(state->p);
	pinctrl_put(state->p);
}

If you want to grab a specific control mapping and not just the first one
found for this device you can specify a specific mapping name, for example in
the above example the second i2c0 setting: pinctrl_get(&device, "spi0-pos-B");

This get/enable/disable/put sequence can just as well be handled by bus drivers
if you don't want each and every driver to handle it and you know the
arrangement on your bus.

The semantics of the get/enable respective disable/put is as follows:

- pinctrl_get() is called in process context to reserve the pins affected with
  a certain mapping and set up the pinmux core and the driver. It will allocate
  a struct from the kernel memory to hold the pinmux state.

- pinctrl_enable()/pinctrl_disable() is quick and can be called from fastpath
  (irq context) when you quickly want to set up/tear down the hardware muxing
  when running a device driver. Usually it will just poke some values into a
  register.

- pinctrl_disable() is called in process context to tear down the pin requests
  and release the state holder struct for the mux setting etc.

Usually the pin control core handled the get/put pair and call out to the
device drivers bookkeeping operations, like checking available functions and
the associated pins, whereas the enable/disable pass on to the pin controller
driver which takes care of activating and/or deactivating the mux setting by
quickly poking some registers.

The pins are allocated for your device when you issue the pinctrl_get() call,
after this you should be able to see this in the debugfs listing of all pins.


System pin control hogging
==========================

A system pin control map entry, i.e. a pin control setting that does not have
a device associated with it, can be hogged by the core when the pin controller
is registered. This means that the core will attempt to call pinctrl_get() and
pinctrl_enable() on it immediately after the pin control device has been
registered.

This is enabled by simply setting the .dev_name field in the map to the name
of the pin controller itself, like this:

{
	.name = "POWERMAP"
	.ctrl_dev_name = "pinctrl-foo",
	.function = "power_func",
	.dev_name = "pinctrl-foo",
},

Since it may be common to request the core to hog a few always-applicable
mux settings on the primary pin controller, there is a convenience macro for
this:

PIN_MAP_PRIMARY_SYS_HOG("POWERMAP", "pinctrl-foo", "power_func")

This gives the exact same result as the above construction.


Runtime pinmuxing
=================

It is possible to mux a certain function in and out at runtime, say to move
an SPI port from one set of pins to another set of pins. Say for example for
spi0 in the example above, we expose two different groups of pins for the same
function, but with different named in the mapping as described under
"Advanced mapping" above. So we have two mappings named "spi0-pos-A" and
"spi0-pos-B".

This snippet first muxes the function in the pins defined by group A, enables
it, disables and releases it, and muxes it in on the pins defined by group B:

#include <linux/pinctrl/consumer.h>

foo_switch()
{
	struct pinctrl *p;

	/* Enable on position A */
	p = pinctrl_get(&device, "spi0-pos-A");
	if IS_ERR(p)
		return PTR_ERR(p);
	pinctrl_enable(p);

	/* This releases the pins again */
	pinctrl_disable(p);
	pinctrl_put(p);

	/* Enable on position B */
	p = pinctrl_get(&device, "spi0-pos-B");
	if IS_ERR(p)
		return PTR_ERR(p);
	pinctrl_enable(p);
	...
}

The above has to be done from process context.