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----------- 126The first thing a system should do with a GPIO is allocate it, using 127the gpio_request() call; see later. 128 129One of the next things to do with a GPIO, often in board setup code when 130setting up a platform_device using the GPIO, is mark its direction: 131 132 /* set as input or output, returning 0 or negative errno */ 133 int gpio_direction_input(unsigned gpio); 134 int gpio_direction_output(unsigned gpio, int value); 135 136The return value is zero for success, else a negative errno. It should 137be checked, since the get/set calls don't have error returns and since 138misconfiguration is possible. You should normally issue these calls from 139a task context. However, for spinlock-safe GPIOs it's OK to use them 140before tasking is enabled, as part of early board setup. 141 142For output GPIOs, the value provided becomes the initial output value. 143This helps avoid signal glitching during system startup. 144 145For compatibility with legacy interfaces to GPIOs, setting the direction 146of a GPIO implicitly requests that GPIO (see below) if it has not been 147requested already. That compatibility is being removed from the optional 148gpiolib framework. 149 150Setting the direction can fail if the GPIO number is invalid, or when 151that particular GPIO can't be used in that mode. It's generally a bad 152idea to rely on boot firmware to have set the direction correctly, since 153it probably wasn't validated to do more than boot Linux. (Similarly, 154that board setup code probably needs to multiplex that pin as a GPIO, 155and configure pullups/pulldowns appropriately.) 156 157 158Spinlock-Safe GPIO access 159------------------------- 160Most GPIO controllers can be accessed with memory read/write instructions. 161That doesn't need to sleep, and can safely be done from inside IRQ handlers. 162(That includes hardirq contexts on RT kernels.) 163 164Use these calls to access such GPIOs: 165 166 /* GPIO INPUT: return zero or nonzero */ 167 int gpio_get_value(unsigned gpio); 168 169 /* GPIO OUTPUT */ 170 void gpio_set_value(unsigned gpio, int value); 171 172The values are boolean, zero for low, nonzero for high. When reading the 173value of an output pin, the value returned should be what's seen on the 174pin ... that won't always match the specified output value, because of 175issues including open-drain signaling and output latencies. 176 177The get/set calls have no error returns because "invalid GPIO" should have 178been reported earlier from gpio_direction_*(). However, note that not all 179platforms can read the value of output pins; those that can't should always 180return zero. Also, using these calls for GPIOs that can't safely be accessed 181without sleeping (see below) is an error. 182 183Platform-specific implementations are encouraged to optimize the two 184calls to access the GPIO value in cases where the GPIO number (and for 185output, value) are constant. It's normal for them to need only a couple 186of instructions in such cases (reading or writing a hardware register), 187and not to need spinlocks. Such optimized calls can make bitbanging 188applications a lot more efficient (in both space and time) than spending 189dozens of instructions on subroutine calls. 190 191 192GPIO access that may sleep 193-------------------------- 194Some GPIO controllers must be accessed using message based busses like I2C 195or SPI. Commands to read or write those GPIO values require waiting to 196get to the head of a queue to transmit a command and get its response. 197This requires sleeping, which can't be done from inside IRQ handlers. 198 199Platforms that support this type of GPIO distinguish them from other GPIOs 200by returning nonzero from this call (which requires a valid GPIO number, 201which should have been previously allocated with gpio_request): 202 203 int gpio_cansleep(unsigned gpio); 204 205To access such GPIOs, a different set of accessors is defined: 206 207 /* GPIO INPUT: return zero or nonzero, might sleep */ 208 int gpio_get_value_cansleep(unsigned gpio); 209 210 /* GPIO OUTPUT, might sleep */ 211 void gpio_set_value_cansleep(unsigned gpio, int value); 212 213Other than the fact that these calls might sleep, and will not be ignored 214for GPIOs that can't be accessed from IRQ handlers, these calls act the 215same as the spinlock-safe calls. 216 217 218Claiming and Releasing GPIOs 219---------------------------- 220To help catch system configuration errors, two calls are defined. 221 222 /* request GPIO, returning 0 or negative errno. 223 * non-null labels may be useful for diagnostics. 224 */ 225 int gpio_request(unsigned gpio, const char *label); 226 227 /* release previously-claimed GPIO */ 228 void gpio_free(unsigned gpio); 229 230Passing invalid GPIO numbers to gpio_request() will fail, as will requesting 231GPIOs that have already been claimed with that call. The return value of 232gpio_request() must be checked. You should normally issue these calls from 233a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs 234before tasking is enabled, as part of early board setup. 235 236These calls serve two basic purposes. One is marking the signals which 237are actually in use as GPIOs, for better diagnostics; systems may have 238several hundred potential GPIOs, but often only a dozen are used on any 239given board. Another is to catch conflicts, identifying errors when 240(a) two or more drivers wrongly think they have exclusive use of that 241signal, or (b) something wrongly believes it's safe to remove drivers 242needed to manage a signal that's in active use. That is, requesting a 243GPIO can serve as a kind of lock. 244 245Some platforms may also use knowledge about what GPIOs are active for 246power management, such as by powering down unused chip sectors and, more 247easily, gating off unused clocks. 248 249Note that requesting a GPIO does NOT cause it to be configured in any 250way; it just marks that GPIO as in use. Separate code must handle any 251pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown). 252 253Also note that it's your responsibility to have stopped using a GPIO 254before you free it. 255 256 257GPIOs mapped to IRQs 258-------------------- 259GPIO numbers are unsigned integers; so are IRQ numbers. These make up 260two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can 261map between them using calls like: 262 263 /* map GPIO numbers to IRQ numbers */ 264 int gpio_to_irq(unsigned gpio); 265 266 /* map IRQ numbers to GPIO numbers (avoid using this) */ 267 int irq_to_gpio(unsigned irq); 268 269Those return either the corresponding number in the other namespace, or 270else a negative errno code if the mapping can't be done. (For example, 271some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO 272number that wasn't set up as an input using gpio_direction_input(), or 273to use an IRQ number that didn't originally come from gpio_to_irq(). 274 275These two mapping calls are expected to cost on the order of a single 276addition or subtraction. They're not allowed to sleep. 277 278Non-error values returned from gpio_to_irq() can be passed to request_irq() 279or free_irq(). They will often be stored into IRQ resources for platform 280devices, by the board-specific initialization code. Note that IRQ trigger 281options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are 282system wakeup capabilities. 283 284Non-error values returned from irq_to_gpio() would most commonly be used 285with gpio_get_value(), for example to initialize or update driver state 286when the IRQ is edge-triggered. Note that some platforms don't support 287this reverse mapping, so you should avoid using it. 288 289 290Emulating Open Drain Signals 291---------------------------- 292Sometimes shared signals need to use "open drain" signaling, where only the 293low signal level is actually driven. (That term applies to CMOS transistors; 294"open collector" is used for TTL.) A pullup resistor causes the high signal 295level. This is sometimes called a "wire-AND"; or more practically, from the 296negative logic (low=true) perspective this is a "wire-OR". 297 298One common example of an open drain signal is a shared active-low IRQ line. 299Also, bidirectional data bus signals sometimes use open drain signals. 300 301Some GPIO controllers directly support open drain outputs; many don't. When 302you need open drain signaling but your hardware doesn't directly support it, 303there's a common idiom you can use to emulate it with any GPIO pin that can 304be used as either an input or an output: 305 306 LOW: gpio_direction_output(gpio, 0) ... this drives the signal 307 and overrides the pullup. 308 309 HIGH: gpio_direction_input(gpio) ... this turns off the output, 310 so the pullup (or some other device) controls the signal. 311 312If you are "driving" the signal high but gpio_get_value(gpio) reports a low 313value (after the appropriate rise time passes), you know some other component 314is driving the shared signal low. That's not necessarily an error. As one 315common example, that's how I2C clocks are stretched: a slave that needs a 316slower clock delays the rising edge of SCK, and the I2C master adjusts its 317signaling rate accordingly. 318 319 320What do these conventions omit? 321=============================== 322One of the biggest things these conventions omit is pin multiplexing, since 323this is highly chip-specific and nonportable. One platform might not need 324explicit multiplexing; another might have just two options for use of any 325given pin; another might have eight options per pin; another might be able 326to route a given GPIO to any one of several pins. (Yes, those examples all 327come from systems that run Linux today.) 328 329Related to multiplexing is configuration and enabling of the pullups or 330pulldowns integrated on some platforms. Not all platforms support them, 331or support them in the same way; and any given board might use external 332pullups (or pulldowns) so that the on-chip ones should not be used. 333(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.) 334Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a 335platform-specific issue, as are models like (not) having a one-to-one 336correspondence between configurable pins and GPIOs. 337 338There are other system-specific mechanisms that are not specified here, 339like the aforementioned options for input de-glitching and wire-OR output. 340Hardware may support reading or writing GPIOs in gangs, but that's usually 341configuration dependent: for GPIOs sharing the same bank. (GPIOs are 342commonly grouped in banks of 16 or 32, with a given SOC having several such 343banks.) Some systems can trigger IRQs from output GPIOs, or read values 344from pins not managed as GPIOs. Code relying on such mechanisms will 345necessarily be nonportable. 346 347Dynamic definition of GPIOs is not currently standard; for example, as 348a side effect of configuring an add-on board with some GPIO expanders. 349 350 351GPIO implementor's framework (OPTIONAL) 352======================================= 353As noted earlier, there is an optional implementation framework making it 354easier for platforms to support different kinds of GPIO controller using 355the same programming interface. This framework is called "gpiolib". 356 357As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file 358will be found there. That will list all the controllers registered through 359this framework, and the state of the GPIOs currently in use. 360 361 362Controller Drivers: gpio_chip 363----------------------------- 364In this framework each GPIO controller is packaged as a "struct gpio_chip" 365with information common to each controller of that type: 366 367 - methods to establish GPIO direction 368 - methods used to access GPIO values 369 - flag saying whether calls to its methods may sleep 370 - optional debugfs dump method (showing extra state like pullup config) 371 - label for diagnostics 372 373There is also per-instance data, which may come from device.platform_data: 374the number of its first GPIO, and how many GPIOs it exposes. 375 376The code implementing a gpio_chip should support multiple instances of the 377controller, possibly using the driver model. That code will configure each 378gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be 379rare; use gpiochip_remove() when it is unavoidable. 380 381Most often a gpio_chip is part of an instance-specific structure with state 382not exposed by the GPIO interfaces, such as addressing, power management, 383and more. Chips such as codecs will have complex non-GPIO state, 384 385Any debugfs dump method should normally ignore signals which haven't been 386requested as GPIOs. They can use gpiochip_is_requested(), which returns 387either NULL or the label associated with that GPIO when it was requested. 388 389 390Platform Support 391---------------- 392To support this framework, a platform's Kconfig will "select" either 393ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB 394and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines 395three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep(). 396They may also want to provide a custom value for ARCH_NR_GPIOS. 397 398ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled 399into the kernel on that architecture. 400 401ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user 402can enable it and build it into the kernel optionally. 403 404If neither of these options are selected, the platform does not support 405GPIOs through GPIO-lib and the code cannot be enabled by the user. 406 407Trivial implementations of those functions can directly use framework 408code, which always dispatches through the gpio_chip: 409 410 #define gpio_get_value __gpio_get_value 411 #define gpio_set_value __gpio_set_value 412 #define gpio_cansleep __gpio_cansleep 413 414Fancier implementations could instead define those as inline functions with 415logic optimizing access to specific SOC-based GPIOs. For example, if the 416referenced GPIO is the constant "12", getting or setting its value could 417cost as little as two or three instructions, never sleeping. When such an 418optimization is not possible those calls must delegate to the framework 419code, costing at least a few dozen instructions. For bitbanged I/O, such 420instruction savings can be significant. 421 422For SOCs, platform-specific code defines and registers gpio_chip instances 423for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to 424match chip vendor documentation, and directly match board schematics. They 425may well start at zero and go up to a platform-specific limit. Such GPIOs 426are normally integrated into platform initialization to make them always be 427available, from arch_initcall() or earlier; they can often serve as IRQs. 428 429 430Board Support 431------------- 432For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi 433function devices, FPGAs or CPLDs -- most often board-specific code handles 434registering controller devices and ensures that their drivers know what GPIO 435numbers to use with gpiochip_add(). Their numbers often start right after 436platform-specific GPIOs. 437 438For example, board setup code could create structures identifying the range 439of GPIOs that chip will expose, and passes them to each GPIO expander chip 440using platform_data. Then the chip driver's probe() routine could pass that 441data to gpiochip_add(). 442 443Initialization order can be important. For example, when a device relies on 444an I2C-based GPIO, its probe() routine should only be called after that GPIO 445becomes available. That may mean the device should not be registered until 446calls for that GPIO can work. One way to address such dependencies is for 447such gpio_chip controllers to provide setup() and teardown() callbacks to 448board specific code; those board specific callbacks would register devices 449once all the necessary resources are available, and remove them later when 450the GPIO controller device becomes unavailable. 451 452 453Sysfs Interface for Userspace (OPTIONAL) 454======================================== 455Platforms which use the "gpiolib" implementors framework may choose to 456configure a sysfs user interface to GPIOs. This is different from the 457debugfs interface, since it provides control over GPIO direction and 458value instead of just showing a gpio state summary. Plus, it could be 459present on production systems without debugging support. 460 461Given appropriate hardware documentation for the system, userspace could 462know for example that GPIO #23 controls the write protect line used to 463protect boot loader segments in flash memory. System upgrade procedures 464may need to temporarily remove that protection, first importing a GPIO, 465then changing its output state, then updating the code before re-enabling 466the write protection. In normal use, GPIO #23 would never be touched, 467and the kernel would have no need to know about it. 468 469Again depending on appropriate hardware documentation, on some systems 470userspace GPIO can be used to determine system configuration data that 471standard kernels won't know about. And for some tasks, simple userspace 472GPIO drivers could be all that the system really needs. 473 474Note that standard kernel drivers exist for common "LEDs and Buttons" 475GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those 476instead of talking directly to the GPIOs; they integrate with kernel 477frameworks better than your userspace code could. 478 479 480Paths in Sysfs 481-------------- 482There are three kinds of entry in /sys/class/gpio: 483 484 - Control interfaces used to get userspace control over GPIOs; 485 486 - GPIOs themselves; and 487 488 - GPIO controllers ("gpio_chip" instances). 489 490That's in addition to standard files including the "device" symlink. 491 492The control interfaces are write-only: 493 494 /sys/class/gpio/ 495 496 "export" ... Userspace may ask the kernel to export control of 497 a GPIO to userspace by writing its number to this file. 498 499 Example: "echo 19 > export" will create a "gpio19" node 500 for GPIO #19, if that's not requested by kernel code. 501 502 "unexport" ... Reverses the effect of exporting to userspace. 503 504 Example: "echo 19 > unexport" will remove a "gpio19" 505 node exported using the "export" file. 506 507GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) 508and have the following read/write attributes: 509 510 /sys/class/gpio/gpioN/ 511 512 "direction" ... reads as either "in" or "out". This value may 513 normally be written. Writing as "out" defaults to 514 initializing the value as low. To ensure glitch free 515 operation, values "low" and "high" may be written to 516 configure the GPIO as an output with that initial value. 517 518 Note that this attribute *will not exist* if the kernel 519 doesn't support changing the direction of a GPIO, or 520 it was exported by kernel code that didn't explicitly 521 allow userspace to reconfigure this GPIO's direction. 522 523 "value" ... reads as either 0 (low) or 1 (high). If the GPIO 524 is configured as an output, this value may be written; 525 any nonzero value is treated as high. 526 527 "edge" ... reads as either "none", "rising", "falling", or 528 "both". Write these strings to select the signal edge(s) 529 that will make poll(2) on the "value" file return. 530 531 This file exists only if the pin can be configured as an 532 interrupt generating input pin. 533 534GPIO controllers have paths like /sys/class/gpio/chipchip42/ (for the 535controller implementing GPIOs starting at #42) and have the following 536read-only attributes: 537 538 /sys/class/gpio/gpiochipN/ 539 540 "base" ... same as N, the first GPIO managed by this chip 541 542 "label" ... provided for diagnostics (not always unique) 543 544 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) 545 546Board documentation should in most cases cover what GPIOs are used for 547what purposes. However, those numbers are not always stable; GPIOs on 548a daughtercard might be different depending on the base board being used, 549or other cards in the stack. In such cases, you may need to use the 550gpiochip nodes (possibly in conjunction with schematics) to determine 551the correct GPIO number to use for a given signal. 552 553 554Exporting from Kernel code 555-------------------------- 556Kernel code can explicitly manage exports of GPIOs which have already been 557requested using gpio_request(): 558 559 /* export the GPIO to userspace */ 560 int gpio_export(unsigned gpio, bool direction_may_change); 561 562 /* reverse gpio_export() */ 563 void gpio_unexport(); 564 565 /* create a sysfs link to an exported GPIO node */ 566 int gpio_export_link(struct device *dev, const char *name, 567 unsigned gpio) 568 569 570After a kernel driver requests a GPIO, it may only be made available in 571the sysfs interface by gpio_export(). The driver can control whether the 572signal direction may change. This helps drivers prevent userspace code 573from accidentally clobbering important system state. 574 575This explicit exporting can help with debugging (by making some kinds 576of experiments easier), or can provide an always-there interface that's 577suitable for documenting as part of a board support package. 578 579After the GPIO has been exported, gpio_export_link() allows creating 580symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can 581use this to provide the interface under their own device in sysfs with 582a descriptive name. 583