Documentation: gpio: Move driver documentation to driver-api

Move gpio/driver.txt to driver-api/gpio/driver.rst and make sure it
builds cleanly as ReST.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

1 files changed, 0 insertions, 427 deletions

diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
deleted file mode 100644
index 3392a0fd4c23..000000000000
--- a/Documentation/gpio/driver.txt
+++ /dev/null
@@ -1,427 +0,0 @@
-GPIO Descriptor Driver Interface
-================================
-
-This document serves as a guide for GPIO chip drivers writers. Note that it
-describes the new descriptor-based interface. For a description of the
-deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
-
-Each GPIO controller driver needs to include the following header, which defines
-the structures used to define a GPIO driver:
-
-        #include <linux/gpio/driver.h>
-
-
-Internal Representation of GPIOs
-================================
-
-Inside a GPIO driver, individual GPIOs are identified by their hardware number,
-which is a unique number between 0 and n, n being the number of GPIOs managed by
-the chip. This number is purely internal: the hardware number of a particular
-GPIO descriptor is never made visible outside of the driver.
-
-On top of this internal number, each GPIO also need to have a global number in
-the integer GPIO namespace so that it can be used with the legacy GPIO
-interface. Each chip must thus have a "base" number (which can be automatically
-assigned), and for each GPIO the global number will be (base + hardware number).
-Although the integer representation is considered deprecated, it still has many
-users and thus needs to be maintained.
-
-So for example one platform could use numbers 32-159 for GPIOs, with a
-controller defining 128 GPIOs at a "base" of 32 ; while another platform uses
-numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO
-controller, and on one particular board 80-95 with an FPGA. The numbers need not
-be contiguous; either of those platforms could also use numbers 2000-2063 to
-identify GPIOs in a bank of I2C GPIO expanders.
-
-
-Controller Drivers: gpio_chip
-=============================
-
-In the gpiolib framework each GPIO controller is packaged as a "struct
-gpio_chip" (see linux/gpio/driver.h for its complete definition) with members
-common to each controller of that type:
-
- - methods to establish GPIO line direction
- - methods used to access GPIO line values
- - method to set electrical configuration to a a given GPIO line
- - method to return the IRQ number associated to a given GPIO line
- - flag saying whether calls to its methods may sleep
- - optional line names array to identify lines
- - optional debugfs dump method (showing extra state like pullup config)
- - optional base number (will be automatically assigned if omitted)
- - optional label for diagnostics and GPIO chip mapping using platform data
-
-The code implementing a gpio_chip should support multiple instances of the
-controller, possibly using the driver model. That code will configure each
-gpio_chip and issue gpiochip_add[_data]() or devm_gpiochip_add_data().
-Removing a GPIO controller should be rare; use [devm_]gpiochip_remove() when
-it is unavoidable.
-
-Often a gpio_chip is part of an instance-specific structure with states not
-exposed by the GPIO interfaces, such as addressing, power management, and more.
-Chips such as audio codecs will have complex non-GPIO states.
-
-Any debugfs dump method should normally ignore signals which haven't been
-requested as GPIOs. They can use gpiochip_is_requested(), which returns either
-NULL or the label associated with that GPIO when it was requested.
-
-RT_FULL: the GPIO driver should not use spinlock_t or any sleepable APIs
-(like PM runtime) in its gpio_chip implementation (.get/.set and direction
-control callbacks) if it is expected to call GPIO APIs from atomic context
-on -RT (inside hard IRQ handlers and similar contexts). Normally this should
-not be required.
-
-
-GPIO electrical configuration
------------------------------
-
-GPIOs can be configured for several electrical modes of operation by using the
-.set_config() callback. Currently this API supports setting debouncing and
-single-ended modes (open drain/open source). These settings are described
-below.
-
-The .set_config() callback uses the same enumerators and configuration
-semantics as the generic pin control drivers. This is not a coincidence: it is
-possible to assign the .set_config() to the function gpiochip_generic_config()
-which will result in pinctrl_gpio_set_config() being called and eventually
-ending up in the pin control back-end "behind" the GPIO controller, usually
-closer to the actual pins. This way the pin controller can manage the below
-listed GPIO configurations.
-
-If a pin controller back-end is used, the GPIO controller or hardware
-description needs to provide "GPIO ranges" mapping the GPIO line offsets to pin
-numbers on the pin controller so they can properly cross-reference each other.
-
-
-GPIOs with debounce support
----------------------------
-
-Debouncing is a configuration set to a pin indicating that it is connected to
-a mechanical switch or button, or similar that may bounce. Bouncing means the
-line is pulled high/low quickly at very short intervals for mechanical
-reasons. This can result in the value being unstable or irqs fireing repeatedly
-unless the line is debounced.
-
-Debouncing in practice involves setting up a timer when something happens on
-the line, wait a little while and then sample the line again, so see if it
-still has the same value (low or high). This could also be repeated by a clever
-state machine, waiting for a line to become stable. In either case, it sets
-a certain number of milliseconds for debouncing, or just "on/off" if that time
-is not configurable.
-
-
-GPIOs with open drain/source support
-------------------------------------
-
-Open drain (CMOS) or open collector (TTL) means the line is not actively driven
-high: instead you provide the drain/collector as output, so when the transistor
-is not open, it will present a high-impedance (tristate) to the external rail.
-
-
-   CMOS CONFIGURATION      TTL CONFIGURATION
-
-            ||--- out              +--- out
-     in ----||                   |/
-            ||--+         in ----|
-                |                |\
-               GND                 GND
-
-This configuration is normally used as a way to achieve one of two things:
-
-- Level-shifting: to reach a logical level higher than that of the silicon
-  where the output resides.
-
-- inverse wire-OR on an I/O line, for example a GPIO line, making it possible
-  for any driving stage on the line to drive it low even if any other output
-  to the same line is simultaneously driving it high. A special case of this
-  is driving the SCL and SCA lines of an I2C bus, which is by definition a
-  wire-OR bus.
-
-Both usecases require that the line be equipped with a pull-up resistor. This
-resistor will make the line tend to high level unless one of the transistors on
-the rail actively pulls it down.
-
-The level on the line will go as high as the VDD on the pull-up resistor, which
-may be higher than the level supported by the transistor, achieveing a
-level-shift to the higher VDD.
-
-Integrated electronics often have an output driver stage in the form of a CMOS
-"totem-pole" with one N-MOS and one P-MOS transistor where one of them drives
-the line high and one of them drives the line low. This is called a push-pull
-output. The "totem-pole" looks like so:
-
-                 VDD
-                  |
-        OD    ||--+
-     +--/ ---o||     P-MOS-FET
-     |        ||--+
-IN --+            +----- out
-     |        ||--+
-     +--/ ----||     N-MOS-FET
-        OS    ||--+
-                  |
-                 GND
-
-The desired output signal (e.g. coming directly from some GPIO output register)
-arrives at IN. The switches named "OD" and "OS" are normally closed, creating
-a push-pull circuit.
-
-Consider the little "switches" named "OD" and "OS" that enable/disable the
-P-MOS or N-MOS transistor right after the split of the input. As you can see,
-either transistor will go totally numb if this switch is open. The totem-pole
-is then halved and give high impedance instead of actively driving the line
-high or low respectively. That is usually how software-controlled open
-drain/source works.
-
-Some GPIO hardware come in open drain / open source configuration. Some are
-hard-wired lines that will only support open drain or open source no matter
-what: there is only one transistor there. Some are software-configurable:
-by flipping a bit in a register the output can be configured as open drain
-or open source, in practice by flicking open the switches labeled "OD" and "OS"
-in the drawing above.
-
-By disabling the P-MOS transistor, the output can be driven between GND and
-high impedance (open drain), and by disabling the N-MOS transistor, the output
-can be driven between VDD and high impedance (open source). In the first case,
-a pull-up resistor is needed on the outgoing rail to complete the circuit, and
-in the second case, a pull-down resistor is needed on the rail.
-
-Hardware that supports open drain or open source or both, can implement a
-special callback in the gpio_chip: .set_config() that takes a generic
-pinconf packed value telling whether to configure the line as open drain,
-open source or push-pull. This will happen in response to the
-GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag set in the machine file, or coming
-from other hardware descriptions.
-
-If this state can not be configured in hardware, i.e. if the GPIO hardware does
-not support open drain/open source in hardware, the GPIO library will instead
-use a trick: when a line is set as output, if the line is flagged as open
-drain, and the IN output value is low, it will be driven low as usual. But
-if the IN output value is set to high, it will instead *NOT* be driven high,
-instead it will be switched to input, as input mode is high impedance, thus
-achieveing an "open drain emulation" of sorts: electrically the behaviour will
-be identical, with the exception of possible hardware glitches when switching
-the mode of the line.
-
-For open source configuration the same principle is used, just that instead
-of actively driving the line low, it is set to input.
-
-
-GPIO drivers providing IRQs
----------------------------
-It is custom that GPIO drivers (GPIO chips) are also providing interrupts,
-most often cascaded off a parent interrupt controller, and in some special
-cases the GPIO logic is melded with a SoC's primary interrupt controller.
-
-The IRQ portions of the GPIO block are implemented using an irqchip, using
-the header <linux/irq.h>. So basically such a driver is utilizing two sub-
-systems simultaneously: gpio and irq.
-
-RT_FULL: a realtime compliant GPIO driver should not use spinlock_t or any
-sleepable APIs (like PM runtime) as part of its irq_chip implementation.
-- spinlock_t should be replaced with raw_spinlock_t [1].
-- If sleepable APIs have to be used, these can be done from the .irq_bus_lock()
-  and .irq_bus_unlock() callbacks, as these are the only slowpath callbacks
-  on an irqchip. Create the callbacks if needed [2].
-
-GPIO irqchips usually fall in one of two categories:
-
-* CHAINED GPIO irqchips: these are usually the type that is embedded on
-  an SoC. This means that there is a fast IRQ flow handler for the GPIOs that
-  gets called in a chain from the parent IRQ handler, most typically the
-  system interrupt controller. This means that the GPIO irqchip handler will
-  be called immediately from the parent irqchip, while holding the IRQs
-  disabled. The GPIO irqchip will then end up calling something like this
-  sequence in its interrupt handler:
-
-  static irqreturn_t foo_gpio_irq(int irq, void *data)
-      chained_irq_enter(...);
-      generic_handle_irq(...);
-      chained_irq_exit(...);
-
-  Chained GPIO irqchips typically can NOT set the .can_sleep flag on
-  struct gpio_chip, as everything happens directly in the callbacks: no
-  slow bus traffic like I2C can be used.
-
-  RT_FULL: Note, chained IRQ handlers will not be forced threaded on -RT.
-  As result, spinlock_t or any sleepable APIs (like PM runtime) can't be used
-  in chained IRQ handler.
-  If required (and if it can't be converted to the nested threaded GPIO irqchip)
-  a chained IRQ handler can be converted to generic irq handler and this way
-  it will be a threaded IRQ handler on -RT and a hard IRQ handler on non-RT
-  (for example, see [3]).
-  Know W/A: The generic_handle_irq() is expected to be called with IRQ disabled,
-  so the IRQ core will complain if it is called from an IRQ handler which is
-  forced to a thread. The "fake?" raw lock can be used to W/A this problem:
-
-        raw_spinlock_t wa_lock;
-        static irqreturn_t omap_gpio_irq_handler(int irq, void *gpiobank)
-                unsigned long wa_lock_flags;
-                raw_spin_lock_irqsave(&bank->wa_lock, wa_lock_flags);
-                generic_handle_irq(irq_find_mapping(bank->chip.irq.domain, bit));
-                raw_spin_unlock_irqrestore(&bank->wa_lock, wa_lock_flags);
-
-* GENERIC CHAINED GPIO irqchips: these are the same as "CHAINED GPIO irqchips",
-  but chained IRQ handlers are not used. Instead GPIO IRQs dispatching is
-  performed by generic IRQ handler which is configured using request_irq().
-  The GPIO irqchip will then end up calling something like this sequence in
-  its interrupt handler:
-
-  static irqreturn_t gpio_rcar_irq_handler(int irq, void *dev_id)
-        for each detected GPIO IRQ
-                generic_handle_irq(...);
-
-  RT_FULL: Such kind of handlers will be forced threaded on -RT, as result IRQ
-  core will complain that generic_handle_irq() is called with IRQ enabled and
-  the same W/A as for "CHAINED GPIO irqchips" can be applied.
-
-* NESTED THREADED GPIO irqchips: these are off-chip GPIO expanders and any
-  other GPIO irqchip residing on the other side of a sleeping bus. Of course
-  such drivers that need slow bus traffic to read out IRQ status and similar,
-  traffic which may in turn incur other IRQs to happen, cannot be handled
-  in a quick IRQ handler with IRQs disabled. Instead they need to spawn a
-  thread and then mask the parent IRQ line until the interrupt is handled
-  by the driver. The hallmark of this driver is to call something like
-  this in its interrupt handler:
-
-  static irqreturn_t foo_gpio_irq(int irq, void *data)
-      ...
-      handle_nested_irq(irq);
-
-  The hallmark of threaded GPIO irqchips is that they set the .can_sleep
-  flag on struct gpio_chip to true, indicating that this chip may sleep
-  when accessing the GPIOs.
-
-To help out in handling the set-up and management of GPIO irqchips and the
-associated irqdomain and resource allocation callbacks, the gpiolib has
-some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig
-symbol:
-
-* gpiochip_irqchip_add(): adds a chained irqchip to a gpiochip. It will pass
-  the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks
-  need to embed the gpio_chip in its state container and obtain a pointer
-  to the container using container_of().
-  (See Documentation/driver-model/design-patterns.txt)
-
-* gpiochip_irqchip_add_nested(): adds a nested irqchip to a gpiochip.
-  Apart from that it works exactly like the chained irqchip.
-
-* gpiochip_set_chained_irqchip(): sets up a chained irq handler for a
-  gpio_chip from a parent IRQ and passes the struct gpio_chip* as handler
-  data. (Notice handler data, since the irqchip data is likely used by the
-  parent irqchip!).
-
-* gpiochip_set_nested_irqchip(): sets up a nested irq handler for a
-  gpio_chip from a parent IRQ. As the parent IRQ has usually been
-  explicitly requested by the driver, this does very little more than
-  mark all the child IRQs as having the other IRQ as parent.
-
-If there is a need to exclude certain GPIOs from the IRQ domain, you can
-set .irq.need_valid_mask of the gpiochip before gpiochip_add_data() is
-called. This allocates an .irq.valid_mask with as many bits set as there
-are GPIOs in the chip. Drivers can exclude GPIOs by clearing bits from this
-mask. The mask must be filled in before gpiochip_irqchip_add() or
-gpiochip_irqchip_add_nested() is called.
-
-To use the helpers please keep the following in mind:
-
-- Make sure to assign all relevant members of the struct gpio_chip so that
-  the irqchip can initialize. E.g. .dev and .can_sleep shall be set up
-  properly.
-
-- Nominally set all handlers to handle_bad_irq() in the setup call and pass
-  handle_bad_irq() as flow handler parameter in gpiochip_irqchip_add() if it is
-  expected for GPIO driver that irqchip .set_type() callback have to be called
-  before using/enabling GPIO IRQ. Then set the handler to handle_level_irq()
-  and/or handle_edge_irq() in the irqchip .set_type() callback depending on
-  what your controller supports.
-
-It is legal for any IRQ consumer to request an IRQ from any irqchip no matter
-if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and
-irq_chip are orthogonal, and offering their services independent of each
-other.
-
-gpiod_to_irq() is just a convenience function to figure out the IRQ for a
-certain GPIO line and should not be relied upon to have been called before
-the IRQ is used.
-
-So always prepare the hardware and make it ready for action in respective
-callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having
-been called first.
-
-This orthogonality leads to ambiguities that we need to solve: if there is
-competition inside the subsystem which side is using the resource (a certain
-GPIO line and register for example) it needs to deny certain operations and
-keep track of usage inside of the gpiolib subsystem. This is why the API
-below exists.
-
-
-Locking IRQ usage
------------------
-Input GPIOs can be used as IRQ signals. When this happens, a driver is requested
-to mark the GPIO as being used as an IRQ:
-
-        int gpiochip_lock_as_irq(struct gpio_chip *chip, unsigned int offset)
-
-This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
-is released:
-
-        void gpiochip_unlock_as_irq(struct gpio_chip *chip, unsigned int offset)
-
-When implementing an irqchip inside a GPIO driver, these two functions should
-typically be called in the .startup() and .shutdown() callbacks from the
-irqchip.
-
-When using the gpiolib irqchip helpers, these callback are automatically
-assigned.
-
-Real-Time compliance for GPIO IRQ chips
----------------------------------------
-
-Any provider of irqchips needs to be carefully tailored to support Real Time
-preemption. It is desirable that all irqchips in the GPIO subsystem keep this
-in mind and does the proper testing to assure they are real time-enabled.
-So, pay attention on above " RT_FULL:" notes, please.
-The following is a checklist to follow when preparing a driver for real
-time-compliance:
-
-- ensure spinlock_t is not used as part irq_chip implementation;
-- ensure that sleepable APIs are not used as part irq_chip implementation.
-  If sleepable APIs have to be used, these can be done from the .irq_bus_lock()
-  and .irq_bus_unlock() callbacks;
-- Chained GPIO irqchips: ensure spinlock_t or any sleepable APIs are not used
-  from chained IRQ handler;
-- Generic chained GPIO irqchips: take care about generic_handle_irq() calls and
-  apply corresponding W/A;
-- Chained GPIO irqchips: get rid of chained IRQ handler and use generic irq
-  handler if possible :)
-- regmap_mmio: Sry, but you are in trouble :( if MMIO regmap is used as for
-  GPIO IRQ chip implementation;
-- Test your driver with the appropriate in-kernel real time test cases for both
-  level and edge IRQs.
-
-
-Requesting self-owned GPIO pins
--------------------------------
-
-Sometimes it is useful to allow a GPIO chip driver to request its own GPIO
-descriptors through the gpiolib API. Using gpio_request() for this purpose
-does not help since it pins the module to the kernel forever (it calls
-try_module_get()). A GPIO driver can use the following functions instead
-to request and free descriptors without being pinned to the kernel forever.
-
-        struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc,
-                                                    const char *label)
-
-        void gpiochip_free_own_desc(struct gpio_desc *desc)
-
-Descriptors requested with gpiochip_request_own_desc() must be released with
-gpiochip_free_own_desc().
-
-These functions must be used with care since they do not affect module use
-count. Do not use the functions to request gpio descriptors not owned by the
-calling driver.
-
-[1] http://www.spinics.net/lists/linux-omap/msg120425.html
-[2] https://lkml.org/lkml/2015/9/25/494
-[3] https://lkml.org/lkml/2015/9/25/495