1GPIO Interfaces 2 3This provides an overview of GPIO access conventions on Linux. 4 5These calls use the gpio_* naming prefix. No other calls should use that 6prefix, or the related __gpio_* prefix. 7 8 9What is a GPIO? 10=============== 11A "General Purpose Input/Output" (GPIO) is a flexible software-controlled 12digital signal. They are provided from many kinds of chip, and are familiar 13to Linux developers working with embedded and custom hardware. Each GPIO 14represents a bit connected to a particular pin, or "ball" on Ball Grid Array 15(BGA) packages. Board schematics show which external hardware connects to 16which GPIOs. Drivers can be written generically, so that board setup code 17passes such pin configuration data to drivers. 18 19System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every 20non-dedicated pin can be configured as a GPIO; and most chips have at least 21several dozen of them. Programmable logic devices (like FPGAs) can easily 22provide GPIOs; multifunction chips like power managers, and audio codecs 23often have a few such pins to help with pin scarcity on SOCs; and there are 24also "GPIO Expander" chips that connect using the I2C or SPI serial busses. 25Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS 26firmware knowing how they're used). 27 28The exact capabilities of GPIOs vary between systems. Common options: 29 30 - Output values are writable (high=1, low=0). Some chips also have 31 options about how that value is driven, so that for example only one 32 value might be driven ... supporting "wire-OR" and similar schemes 33 for the other value (notably, "open drain" signaling). 34 35 - Input values are likewise readable (1, 0). Some chips support readback 36 of pins configured as "output", which is very useful in such "wire-OR" 37 cases (to support bidirectional signaling). GPIO controllers may have 38 input de-glitch/debounce logic, sometimes with software controls. 39 40 - Inputs can often be used as IRQ signals, often edge triggered but 41 sometimes level triggered. Such IRQs may be configurable as system 42 wakeup events, to wake the system from a low power state. 43 44 - Usually a GPIO will be configurable as either input or output, as needed 45 by different product boards; single direction ones exist too. 46 47 - Most GPIOs can be accessed while holding spinlocks, but those accessed 48 through a serial bus normally can't. Some systems support both types. 49 50On a given board each GPIO is used for one specific purpose like monitoring 51MMC/SD card insertion/removal, detecting card writeprotect status, driving 52a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware 53watchdog, sensing a switch, and so on. 54 55 56GPIO conventions 57================ 58Note that this is called a "convention" because you don't need to do it this 59way, and it's no crime if you don't. There **are** cases where portability 60is not the main issue; GPIOs are often used for the kind of board-specific 61glue logic that may even change between board revisions, and can't ever be 62used on a board that's wired differently. Only least-common-denominator 63functionality can be very portable. Other features are platform-specific, 64and that can be critical for glue logic. 65 66Plus, this doesn't require any implementation framework, just an interface. 67One platform might implement it as simple inline functions accessing chip 68registers; another might implement it by delegating through abstractions 69used for several very different kinds of GPIO controller. (There is some 70optional code supporting such an implementation strategy, described later 71in this document, but drivers acting as clients to the GPIO interface must 72not care how it's implemented.) 73 74That said, if the convention is supported on their platform, drivers should 75use it when possible. Platforms must declare GENERIC_GPIO support in their 76Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't 77work without standard GPIO calls should have Kconfig entries which depend 78on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as 79optimized-away stubs, when drivers use the include file: 80 81 #include <linux/gpio.h> 82 83If you stick to this convention then it'll be easier for other developers to 84see what your code is doing, and help maintain it. 85 86Note that these operations include I/O barriers on platforms which need to 87use them; drivers don't need to add them explicitly. 88 89 90Identifying GPIOs 91----------------- 92GPIOs are identified by unsigned integers in the range 0..MAX_INT. That 93reserves "negative" numbers for other purposes like marking signals as 94"not available on this board", or indicating faults. Code that doesn't 95touch the underlying hardware treats these integers as opaque cookies. 96 97Platforms define how they use those integers, and usually #define symbols 98for the GPIO lines so that board-specific setup code directly corresponds 99to the relevant schematics. In contrast, drivers should only use GPIO 100numbers passed to them from that setup code, using platform_data to hold 101board-specific pin configuration data (along with other board specific 102data they need). That avoids portability problems. 103 104So for example one platform uses numbers 32-159 for GPIOs; while another 105uses numbers 0..63 with one set of GPIO controllers, 64-79 with another 106type of GPIO controller, and on one particular board 80-95 with an FPGA. 107The numbers need not be contiguous; either of those platforms could also 108use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders. 109 110If you want to initialize a structure with an invalid GPIO number, use 111some negative number (perhaps "-EINVAL"); that will never be valid. To 112test if a number could reference a GPIO, you may use this predicate: 113 114 int gpio_is_valid(int number); 115 116A number that's not valid will be rejected by calls which may request 117or free GPIOs (see below). Other numbers may also be rejected; for 118example, a number might be valid but unused on a given board. 119 120Whether a platform supports multiple GPIO controllers is currently a 121platform-specific implementation issue. 122 123 124Using GPIOs 125----------- 126One of the first things to do with a GPIO, often in board setup code when 127setting up a platform_device using the GPIO, is mark its direction: 128 129 /* set as input or output, returning 0 or negative errno */ 130 int gpio_direction_input(unsigned gpio); 131 int gpio_direction_output(unsigned gpio, int value); 132 133The return value is zero for success, else a negative errno. It should 134be checked, since the get/set calls don't have error returns and since 135misconfiguration is possible. You should normally issue these calls from 136a task context. However, for spinlock-safe GPIOs it's OK to use them 137before tasking is enabled, as part of early board setup. 138 139For output GPIOs, the value provided becomes the initial output value. 140This helps avoid signal glitching during system startup. 141 142For compatibility with legacy interfaces to GPIOs, setting the direction 143of a GPIO implicitly requests that GPIO (see below) if it has not been 144requested already. That compatibility may be removed in the future; 145explicitly requesting GPIOs is strongly preferred. 146 147Setting the direction can fail if the GPIO number is invalid, or when 148that particular GPIO can't be used in that mode. It's generally a bad 149idea to rely on boot firmware to have set the direction correctly, since 150it probably wasn't validated to do more than boot Linux. (Similarly, 151that board setup code probably needs to multiplex that pin as a GPIO, 152and configure pullups/pulldowns appropriately.) 153 154 155Spinlock-Safe GPIO access 156------------------------- 157Most GPIO controllers can be accessed with memory read/write instructions. 158That doesn't need to sleep, and can safely be done from inside IRQ handlers. 159(That includes hardirq contexts on RT kernels.) 160 161Use these calls to access such GPIOs: 162 163 /* GPIO INPUT: return zero or nonzero */ 164 int gpio_get_value(unsigned gpio); 165 166 /* GPIO OUTPUT */ 167 void gpio_set_value(unsigned gpio, int value); 168 169The values are boolean, zero for low, nonzero for high. When reading the 170value of an output pin, the value returned should be what's seen on the 171pin ... that won't always match the specified output value, because of 172issues including open-drain signaling and output latencies. 173 174The get/set calls have no error returns because "invalid GPIO" should have 175been reported earlier from gpio_direction_*(). However, note that not all 176platforms can read the value of output pins; those that can't should always 177return zero. Also, using these calls for GPIOs that can't safely be accessed 178without sleeping (see below) is an error. 179 180Platform-specific implementations are encouraged to optimize the two 181calls to access the GPIO value in cases where the GPIO number (and for 182output, value) are constant. It's normal for them to need only a couple 183of instructions in such cases (reading or writing a hardware register), 184and not to need spinlocks. Such optimized calls can make bitbanging 185applications a lot more efficient (in both space and time) than spending 186dozens of instructions on subroutine calls. 187 188 189GPIO access that may sleep 190-------------------------- 191Some GPIO controllers must be accessed using message based busses like I2C 192or SPI. Commands to read or write those GPIO values require waiting to 193get to the head of a queue to transmit a command and get its response. 194This requires sleeping, which can't be done from inside IRQ handlers. 195 196Platforms that support this type of GPIO distinguish them from other GPIOs 197by returning nonzero from this call (which requires a valid GPIO number, 198either explicitly or implicitly requested): 199 200 int gpio_cansleep(unsigned gpio); 201 202To access such GPIOs, a different set of accessors is defined: 203 204 /* GPIO INPUT: return zero or nonzero, might sleep */ 205 int gpio_get_value_cansleep(unsigned gpio); 206 207 /* GPIO OUTPUT, might sleep */ 208 void gpio_set_value_cansleep(unsigned gpio, int value); 209 210Other than the fact that these calls might sleep, and will not be ignored 211for GPIOs that can't be accessed from IRQ handlers, these calls act the 212same as the spinlock-safe calls. 213 214 215Claiming and Releasing GPIOs (OPTIONAL) 216--------------------------------------- 217To help catch system configuration errors, two calls are defined. 218However, many platforms don't currently support this mechanism. 219 220 /* request GPIO, returning 0 or negative errno. 221 * non-null labels may be useful for diagnostics. 222 */ 223 int gpio_request(unsigned gpio, const char *label); 224 225 /* release previously-claimed GPIO */ 226 void gpio_free(unsigned gpio); 227 228Passing invalid GPIO numbers to gpio_request() will fail, as will requesting 229GPIOs that have already been claimed with that call. The return value of 230gpio_request() must be checked. You should normally issue these calls from 231a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs 232before tasking is enabled, as part of early board setup. 233 234These calls serve two basic purposes. One is marking the signals which 235are actually in use as GPIOs, for better diagnostics; systems may have 236several hundred potential GPIOs, but often only a dozen are used on any 237given board. Another is to catch conflicts, identifying errors when 238(a) two or more drivers wrongly think they have exclusive use of that 239signal, or (b) something wrongly believes it's safe to remove drivers 240needed to manage a signal that's in active use. That is, requesting a 241GPIO can serve as a kind of lock. 242 243Some platforms may also use knowledge about what GPIOs are active for 244power management, such as by powering down unused chip sectors and, more 245easily, gating off unused clocks. 246 247These two calls are optional because not not all current Linux platforms 248offer such functionality in their GPIO support; a valid implementation 249could return success for all gpio_request() calls. Unlike the other calls, 250the state they represent doesn't normally match anything from a hardware 251register; it's just a software bitmap which clearly is not necessary for 252correct operation of hardware or (bug free) drivers. 253 254Note that requesting a GPIO does NOT cause it to be configured in any 255way; it just marks that GPIO as in use. Separate code must handle any 256pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown). 257 258Also note that it's your responsibility to have stopped using a GPIO 259before you free it. 260 261 262GPIOs mapped to IRQs 263-------------------- 264GPIO numbers are unsigned integers; so are IRQ numbers. These make up 265two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can 266map between them using calls like: 267 268 /* map GPIO numbers to IRQ numbers */ 269 int gpio_to_irq(unsigned gpio); 270 271 /* map IRQ numbers to GPIO numbers (avoid using this) */ 272 int irq_to_gpio(unsigned irq); 273 274Those return either the corresponding number in the other namespace, or 275else a negative errno code if the mapping can't be done. (For example, 276some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO 277number that wasn't set up as an input using gpio_direction_input(), or 278to use an IRQ number that didn't originally come from gpio_to_irq(). 279 280These two mapping calls are expected to cost on the order of a single 281addition or subtraction. They're not allowed to sleep. 282 283Non-error values returned from gpio_to_irq() can be passed to request_irq() 284or free_irq(). They will often be stored into IRQ resources for platform 285devices, by the board-specific initialization code. Note that IRQ trigger 286options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are 287system wakeup capabilities. 288 289Non-error values returned from irq_to_gpio() would most commonly be used 290with gpio_get_value(), for example to initialize or update driver state 291when the IRQ is edge-triggered. Note that some platforms don't support 292this reverse mapping, so you should avoid using it. 293 294 295Emulating Open Drain Signals 296---------------------------- 297Sometimes shared signals need to use "open drain" signaling, where only the 298low signal level is actually driven. (That term applies to CMOS transistors; 299"open collector" is used for TTL.) A pullup resistor causes the high signal 300level. This is sometimes called a "wire-AND"; or more practically, from the 301negative logic (low=true) perspective this is a "wire-OR". 302 303One common example of an open drain signal is a shared active-low IRQ line. 304Also, bidirectional data bus signals sometimes use open drain signals. 305 306Some GPIO controllers directly support open drain outputs; many don't. When 307you need open drain signaling but your hardware doesn't directly support it, 308there's a common idiom you can use to emulate it with any GPIO pin that can 309be used as either an input or an output: 310 311 LOW: gpio_direction_output(gpio, 0) ... this drives the signal 312 and overrides the pullup. 313 314 HIGH: gpio_direction_input(gpio) ... this turns off the output, 315 so the pullup (or some other device) controls the signal. 316 317If you are "driving" the signal high but gpio_get_value(gpio) reports a low 318value (after the appropriate rise time passes), you know some other component 319is driving the shared signal low. That's not necessarily an error. As one 320common example, that's how I2C clocks are stretched: a slave that needs a 321slower clock delays the rising edge of SCK, and the I2C master adjusts its 322signaling rate accordingly. 323 324 325What do these conventions omit? 326=============================== 327One of the biggest things these conventions omit is pin multiplexing, since 328this is highly chip-specific and nonportable. One platform might not need 329explicit multiplexing; another might have just two options for use of any 330given pin; another might have eight options per pin; another might be able 331to route a given GPIO to any one of several pins. (Yes, those examples all 332come from systems that run Linux today.) 333 334Related to multiplexing is configuration and enabling of the pullups or 335pulldowns integrated on some platforms. Not all platforms support them, 336or support them in the same way; and any given board might use external 337pullups (or pulldowns) so that the on-chip ones should not be used. 338(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.) 339Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a 340platform-specific issue, as are models like (not) having a one-to-one 341correspondence between configurable pins and GPIOs. 342 343There are other system-specific mechanisms that are not specified here, 344like the aforementioned options for input de-glitching and wire-OR output. 345Hardware may support reading or writing GPIOs in gangs, but that's usually 346configuration dependent: for GPIOs sharing the same bank. (GPIOs are 347commonly grouped in banks of 16 or 32, with a given SOC having several such 348banks.) Some systems can trigger IRQs from output GPIOs, or read values 349from pins not managed as GPIOs. Code relying on such mechanisms will 350necessarily be nonportable. 351 352Dynamic definition of GPIOs is not currently standard; for example, as 353a side effect of configuring an add-on board with some GPIO expanders. 354 355 356GPIO implementor's framework (OPTIONAL) 357======================================= 358As noted earlier, there is an optional implementation framework making it 359easier for platforms to support different kinds of GPIO controller using 360the same programming interface. This framework is called "gpiolib". 361 362As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file 363will be found there. That will list all the controllers registered through 364this framework, and the state of the GPIOs currently in use. 365 366 367Controller Drivers: gpio_chip 368----------------------------- 369In this framework each GPIO controller is packaged as a "struct gpio_chip" 370with information common to each controller of that type: 371 372 - methods to establish GPIO direction 373 - methods used to access GPIO values 374 - flag saying whether calls to its methods may sleep 375 - optional debugfs dump method (showing extra state like pullup config) 376 - label for diagnostics 377 378There is also per-instance data, which may come from device.platform_data: 379the number of its first GPIO, and how many GPIOs it exposes. 380 381The code implementing a gpio_chip should support multiple instances of the 382controller, possibly using the driver model. That code will configure each 383gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be 384rare; use gpiochip_remove() when it is unavoidable. 385 386Most often a gpio_chip is part of an instance-specific structure with state 387not exposed by the GPIO interfaces, such as addressing, power management, 388and more. Chips such as codecs will have complex non-GPIO state, 389 390Any debugfs dump method should normally ignore signals which haven't been 391requested as GPIOs. They can use gpiochip_is_requested(), which returns 392either NULL or the label associated with that GPIO when it was requested. 393 394 395Platform Support 396---------------- 397To support this framework, a platform's Kconfig will "select" either 398ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB 399and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines 400three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep(). 401They may also want to provide a custom value for ARCH_NR_GPIOS. 402 403ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled 404into the kernel on that architecture. 405 406ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user 407can enable it and build it into the kernel optionally. 408 409If neither of these options are selected, the platform does not support 410GPIOs through GPIO-lib and the code cannot be enabled by the user. 411 412Trivial implementations of those functions can directly use framework 413code, which always dispatches through the gpio_chip: 414 415 #define gpio_get_value __gpio_get_value 416 #define gpio_set_value __gpio_set_value 417 #define gpio_cansleep __gpio_cansleep 418 419Fancier implementations could instead define those as inline functions with 420logic optimizing access to specific SOC-based GPIOs. For example, if the 421referenced GPIO is the constant "12", getting or setting its value could 422cost as little as two or three instructions, never sleeping. When such an 423optimization is not possible those calls must delegate to the framework 424code, costing at least a few dozen instructions. For bitbanged I/O, such 425instruction savings can be significant. 426 427For SOCs, platform-specific code defines and registers gpio_chip instances 428for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to 429match chip vendor documentation, and directly match board schematics. They 430may well start at zero and go up to a platform-specific limit. Such GPIOs 431are normally integrated into platform initialization to make them always be 432available, from arch_initcall() or earlier; they can often serve as IRQs. 433 434 435Board Support 436------------- 437For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi 438function devices, FPGAs or CPLDs -- most often board-specific code handles 439registering controller devices and ensures that their drivers know what GPIO 440numbers to use with gpiochip_add(). Their numbers often start right after 441platform-specific GPIOs. 442 443For example, board setup code could create structures identifying the range 444of GPIOs that chip will expose, and passes them to each GPIO expander chip 445using platform_data. Then the chip driver's probe() routine could pass that 446data to gpiochip_add(). 447 448Initialization order can be important. For example, when a device relies on 449an I2C-based GPIO, its probe() routine should only be called after that GPIO 450becomes available. That may mean the device should not be registered until 451calls for that GPIO can work. One way to address such dependencies is for 452such gpio_chip controllers to provide setup() and teardown() callbacks to 453board specific code; those board specific callbacks would register devices 454once all the necessary resources are available, and remove them later when 455the GPIO controller device becomes unavailable. 456 457 458Sysfs Interface for Userspace (OPTIONAL) 459======================================== 460Platforms which use the "gpiolib" implementors framework may choose to 461configure a sysfs user interface to GPIOs. This is different from the 462debugfs interface, since it provides control over GPIO direction and 463value instead of just showing a gpio state summary. Plus, it could be 464present on production systems without debugging support. 465 466Given approprate hardware documentation for the system, userspace could 467know for example that GPIO #23 controls the write protect line used to 468protect boot loader segments in flash memory. System upgrade procedures 469may need to temporarily remove that protection, first importing a GPIO, 470then changing its output state, then updating the code before re-enabling 471the write protection. In normal use, GPIO #23 would never be touched, 472and the kernel would have no need to know about it. 473 474Again depending on appropriate hardware documentation, on some systems 475userspace GPIO can be used to determine system configuration data that 476standard kernels won't know about. And for some tasks, simple userspace 477GPIO drivers could be all that the system really needs. 478 479Note that standard kernel drivers exist for common "LEDs and Buttons" 480GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those 481instead of talking directly to the GPIOs; they integrate with kernel 482frameworks better than your userspace code could. 483 484 485Paths in Sysfs 486-------------- 487There are three kinds of entry in /sys/class/gpio: 488 489 - Control interfaces used to get userspace control over GPIOs; 490 491 - GPIOs themselves; and 492 493 - GPIO controllers ("gpio_chip" instances). 494 495That's in addition to standard files including the "device" symlink. 496 497The control interfaces are write-only: 498 499 /sys/class/gpio/ 500 501 "export" ... Userspace may ask the kernel to export control of 502 a GPIO to userspace by writing its number to this file. 503 504 Example: "echo 19 > export" will create a "gpio19" node 505 for GPIO #19, if that's not requested by kernel code. 506 507 "unexport" ... Reverses the effect of exporting to userspace. 508 509 Example: "echo 19 > unexport" will remove a "gpio19" 510 node exported using the "export" file. 511 512GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) 513and have the following read/write attributes: 514 515 /sys/class/gpio/gpioN/ 516 517 "direction" ... reads as either "in" or "out". This value may 518 normally be written. Writing as "out" defaults to 519 initializing the value as low. To ensure glitch free 520 operation, values "low" and "high" may be written to 521 configure the GPIO as an output with that initial value. 522 523 Note that this attribute *will not exist* if the kernel 524 doesn't support changing the direction of a GPIO, or 525 it was exported by kernel code that didn't explicitly 526 allow userspace to reconfigure this GPIO's direction. 527 528 "value" ... reads as either 0 (low) or 1 (high). If the GPIO 529 is configured as an output, this value may be written; 530 any nonzero value is treated as high. 531 532GPIO controllers have paths like /sys/class/gpio/chipchip42/ (for the 533controller implementing GPIOs starting at #42) and have the following 534read-only attributes: 535 536 /sys/class/gpio/gpiochipN/ 537 538 "base" ... same as N, the first GPIO managed by this chip 539 540 "label" ... provided for diagnostics (not always unique) 541 542 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) 543 544Board documentation should in most cases cover what GPIOs are used for 545what purposes. However, those numbers are not always stable; GPIOs on 546a daughtercard might be different depending on the base board being used, 547or other cards in the stack. In such cases, you may need to use the 548gpiochip nodes (possibly in conjunction with schematics) to determine 549the correct GPIO number to use for a given signal. 550 551 552Exporting from Kernel code 553-------------------------- 554Kernel code can explicitly manage exports of GPIOs which have already been 555requested using gpio_request(): 556 557 /* export the GPIO to userspace */ 558 int gpio_export(unsigned gpio, bool direction_may_change); 559 560 /* reverse gpio_export() */ 561 void gpio_unexport(); 562 563After a kernel driver requests a GPIO, it may only be made available in 564the sysfs interface by gpio_export(). The driver can control whether the 565signal direction may change. This helps drivers prevent userspace code 566from accidentally clobbering important system state. 567 568This explicit exporting can help with debugging (by making some kinds 569of experiments easier), or can provide an always-there interface that's 570suitable for documenting as part of a board support package. 571