driver-api/gpio/legacy.rst

13 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
21 System-on-Chip (SOC) processors heavily rely on GPIOs.  In some cases, every
22 non-dedicated pin can be configured as a GPIO; and most chips have at least
27 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
32   - Output values are writable (high=1, low=0).  Some chips also have
34     value might be driven ... supporting "wire-OR" and similar schemes
37   - Input values are likewise readable (1, 0).  Some chips support readback
38     of pins configured as "output", which is very useful in such "wire-OR"
40     input de-glitch/debounce logic, sometimes with software controls.
42   - Inputs can often be used as IRQ signals, often edge triggered but
44     wakeup events, to wake the system from a low power state.
46   - Usually a GPIO will be configurable as either input or output, as needed
49   - Most GPIOs can be accessed while holding spinlocks, but those accessed
62 is not the main issue; GPIOs are often used for the kind of board-specific
64 used on a board that's wired differently.  Only least-common-denominator
65 functionality can be very portable.  Other features are platform-specific,
80 GPIO calls are available, either as "real code" or as optimized-away stubs,
93 -----------------
100 for the GPIO lines so that board-specific setup code directly corresponds
103 board-specific pin configuration data (along with other board specific
106 So for example one platform uses numbers 32-159 for GPIOs; while another
107 uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
108 type of GPIO controller, and on one particular board 80-95 with an FPGA.
110 use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
113 some negative number (perhaps "-EINVAL"); that will never be valid.  To
123 Whether a platform supports multiple GPIO controllers is a platform-specific
129 -----------
136 	/* set as input or output, returning 0 or negative errno */
143 a task context.  However, for spinlock-safe GPIOs it's OK to use them
146 For output GPIOs, the value provided becomes the initial output value.
162 Spinlock-Safe GPIO access
163 -------------------------
174 	/* GPIO OUTPUT */
177 The values are boolean, zero for low, nonzero for high.  When reading the
178 value of an output pin, the value returned should be what's seen on the
179 pin ... that won't always match the specified output value, because of
180 issues including open-drain signaling and output latencies.
184 platforms can read the value of output pins; those that can't should always
188 Platform-specific implementations are encouraged to optimize the two
190 output, value) are constant.  It's normal for them to need only a couple
198 --------------------------
215 	/* GPIO OUTPUT, might sleep */
221 spinlock-safe accessors without the cansleep() name suffix.
225 the same as the spinlock-safe calls.
246 ----------------------------
250 	 * non-null labels may be useful for diagnostics.
254 	/* release previously-claimed GPIO */
260 a task context.  However, for spinlock-safe GPIOs it's OK to request GPIOs
286 setup of an output GPIO's value. This allows a glitch-free migration from a
288 to implement a workaround on signals typically driven by a non-GPIO HW block.
319 	* GPIOF_DIR_IN		- to configure direction as input
320 	* GPIOF_DIR_OUT		- to configure direction as output
322 	* GPIOF_INIT_LOW	- as output, set initial level to LOW
323 	* GPIOF_INIT_HIGH	- as output, set initial level to HIGH
324 	* GPIOF_OPEN_DRAIN	- gpio pin is open drain type.
325 	* GPIOF_OPEN_SOURCE	- gpio pin is open source type.
327 	* GPIOF_EXPORT_DIR_FIXED	- export gpio to sysfs, keep direction
328 	* GPIOF_EXPORT_DIR_CHANGEABLE	- also export, allow changing direction
330 since GPIOF_INIT_* are only valid when configured as output, so group valid
333 	* GPIOF_IN		- configure as input
334 	* GPIOF_OUT_INIT_LOW	- configured as output, initial level LOW
335 	* GPIOF_OUT_INIT_HIGH	- configured as output, initial level HIGH
338 open drain type. Such pins will not be driven to 1 in output mode. It is
339 require to connect pull-up on such pins. By enabling this flag, gpio lib will
340 make the direction to input when it is asked to set value of 1 in output mode
341 to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
344 open source type. Such pins will not be driven to 0 in output mode. It is
345 require to connect pull-down on such pin. By enabling this flag, gpio lib will
346 make the direction to input when it is asked to set value of 0 in output mode
347 to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
382 --------------------
402 Non-error values returned from gpio_to_irq() can be passed to request_irq()
404 devices, by the board-specific initialization code.  Note that IRQ trigger
408 Non-error values returned from irq_to_gpio() would most commonly be used
410 when the IRQ is edge-triggered.  Note that some platforms don't support
415 ----------------------------
417 low signal level is actually driven.  (That term applies to CMOS transistors;
419 level.  This is sometimes called a "wire-AND"; or more practically, from the
420 negative logic (low=true) perspective this is a "wire-OR".
422 One common example of an open drain signal is a shared active-low IRQ line.
428 be used as either an input or an output:
430  LOW:	gpio_direction_output(gpio, 0) ... this drives the signal
433  HIGH:	gpio_direction_input(gpio) ... this turns off the output,
436 If you are "driving" the signal high but gpio_get_value(gpio) reports a low
438 is driving the shared signal low.  That's not necessarily an error.  As one
445 ------------------------------------------
458 But how does the pin control subsystem cross-correlate the GPIO
463 cross-reference tables. These are described in
464 Documentation/driver-api/pinctl.rst
480 For non-DT support, user can call gpiochip_add_pin_range() with appropriate
489 this is highly chip-specific and nonportable.  One platform might not need
498 pullups (or pulldowns) so that the on-chip ones should not be used.
499 (When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
501 platform-specific issue, as are models like (not) having a one-to-one
504 There are other system-specific mechanisms that are not specified here,
505 like the aforementioned options for input de-glitching and wire-OR output.
509 banks.)  Some systems can trigger IRQs from output GPIOs, or read values
514 a side effect of configuring an add-on board with some GPIO expanders.
529 -----------------------------
533  - methods to establish GPIO direction
534  - methods used to access GPIO values
535  - flag saying whether calls to its methods may sleep
536  - optional debugfs dump method (showing extra state like pullup config)
537  - label for diagnostics
539 There is also per-instance data, which may come from device.platform_data:
547 Most often a gpio_chip is part of an instance-specific structure with state
549 and more.  Chips such as codecs will have complex non-GPIO state.
557 ----------------
558 To force-enable this framework, a platform's Kconfig will "select" GPIOLIB,
563 wasting static table space.  (It should count both built-in/SoC GPIOs and
567 GPIOs through GPIO-lib and the code cannot be enabled by the user.
577 logic optimizing access to specific SOC-based GPIOs.  For example, if the
584 For SOCs, platform-specific code defines and registers gpio_chip instances
585 for each bank of on-chip GPIOs.  Those GPIOs should be numbered/labeled to
587 may well start at zero and go up to a platform-specific limit.  Such GPIOs
593 -------------
594 For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
595 function devices, FPGAs or CPLDs -- most often board-specific code handles
598 platform-specific GPIOs.
606 an I2C-based GPIO, its probe() routine should only be called after that GPIO
627 then changing its output state, then updating the code before re-enabling
637 GPIO tasks:  "leds-gpio" and "gpio_keys", respectively.  Use those
643 --------------
646    -	Control interfaces used to get userspace control over GPIOs;
648    -	GPIOs themselves; and
650    -	GPIO controllers ("gpio_chip" instances).
654 The control interfaces are write-only:
676 		initializing the value as low.  To ensure glitch free
677 		operation, values "low" and "high" may be written to
678 		configure the GPIO as an output with that initial value.
685 	"value" ... reads as either 0 (low) or 1 (high).  If the GPIO
686 		is configured as an output, this value may be written;
689 		If the pin can be configured as interrupt-generating interrupt
696 		new value or close the file and re-open it to read the value.
714 read-only attributes:
722     	"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
733 --------------------------
753 of experiments easier), or can provide an always-there interface that's
768 .. kernel-doc:: drivers/gpio/gpiolib-legacy.c