Based on kernel version 3.2. Page generated on 2012-01-05 23:28 EST.
1 GPIO Interfaces 2 3 This provides an overview of GPIO access conventions on Linux. 4 5 These calls use the gpio_* naming prefix. No other calls should use that 6 prefix, or the related __gpio_* prefix. 7 8 9 What is a GPIO? 10 =============== 11 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled 12 digital signal. They are provided from many kinds of chip, and are familiar 13 to Linux developers working with embedded and custom hardware. Each GPIO 14 represents a bit connected to a particular pin, or "ball" on Ball Grid Array 15 (BGA) packages. Board schematics show which external hardware connects to 16 which GPIOs. Drivers can be written generically, so that board setup code 17 passes such pin configuration data to drivers. 18 19 System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every 20 non-dedicated pin can be configured as a GPIO; and most chips have at least 21 several dozen of them. Programmable logic devices (like FPGAs) can easily 22 provide GPIOs; multifunction chips like power managers, and audio codecs 23 often have a few such pins to help with pin scarcity on SOCs; and there are 24 also "GPIO Expander" chips that connect using the I2C or SPI serial busses. 25 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS 26 firmware knowing how they're used). 27 28 The 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 50 On a given board each GPIO is used for one specific purpose like monitoring 51 MMC/SD card insertion/removal, detecting card writeprotect status, driving 52 a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware 53 watchdog, sensing a switch, and so on. 54 55 56 GPIO conventions 57 ================ 58 Note that this is called a "convention" because you don't need to do it this 59 way, and it's no crime if you don't. There **are** cases where portability 60 is not the main issue; GPIOs are often used for the kind of board-specific 61 glue logic that may even change between board revisions, and can't ever be 62 used on a board that's wired differently. Only least-common-denominator 63 functionality can be very portable. Other features are platform-specific, 64 and that can be critical for glue logic. 65 66 Plus, this doesn't require any implementation framework, just an interface. 67 One platform might implement it as simple inline functions accessing chip 68 registers; another might implement it by delegating through abstractions 69 used for several very different kinds of GPIO controller. (There is some 70 optional code supporting such an implementation strategy, described later 71 in this document, but drivers acting as clients to the GPIO interface must 72 not care how it's implemented.) 73 74 That said, if the convention is supported on their platform, drivers should 75 use it when possible. Platforms must declare GENERIC_GPIO support in their 76 Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't 77 work without standard GPIO calls should have Kconfig entries which depend 78 on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as 79 optimized-away stubs, when drivers use the include file: 80 81 #include <linux/gpio.h> 82 83 If you stick to this convention then it'll be easier for other developers to 84 see what your code is doing, and help maintain it. 85 86 Note that these operations include I/O barriers on platforms which need to 87 use them; drivers don't need to add them explicitly. 88 89 90 Identifying GPIOs 91 ----------------- 92 GPIOs are identified by unsigned integers in the range 0..MAX_INT. That 93 reserves "negative" numbers for other purposes like marking signals as 94 "not available on this board", or indicating faults. Code that doesn't 95 touch the underlying hardware treats these integers as opaque cookies. 96 97 Platforms define how they use those integers, and usually #define symbols 98 for the GPIO lines so that board-specific setup code directly corresponds 99 to the relevant schematics. In contrast, drivers should only use GPIO 100 numbers passed to them from that setup code, using platform_data to hold 101 board-specific pin configuration data (along with other board specific 102 data they need). That avoids portability problems. 103 104 So for example one platform uses numbers 32-159 for GPIOs; while another 105 uses numbers 0..63 with one set of GPIO controllers, 64-79 with another 106 type of GPIO controller, and on one particular board 80-95 with an FPGA. 107 The numbers need not be contiguous; either of those platforms could also 108 use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders. 109 110 If you want to initialize a structure with an invalid GPIO number, use 111 some negative number (perhaps "-EINVAL"); that will never be valid. To 112 test if such number from such a structure could reference a GPIO, you 113 may use this predicate: 114 115 int gpio_is_valid(int number); 116 117 A number that's not valid will be rejected by calls which may request 118 or free GPIOs (see below). Other numbers may also be rejected; for 119 example, a number might be valid but temporarily unused on a given board. 120 121 Whether a platform supports multiple GPIO controllers is a platform-specific 122 implementation issue, as are whether that support can leave "holes" in the space 123 of GPIO numbers, and whether new controllers can be added at runtime. Such issues 124 can affect things including whether adjacent GPIO numbers are both valid. 125 126 Using GPIOs 127 ----------- 128 The first thing a system should do with a GPIO is allocate it, using 129 the gpio_request() call; see later. 130 131 One of the next things to do with a GPIO, often in board setup code when 132 setting up a platform_device using the GPIO, is mark its direction: 133 134 /* set as input or output, returning 0 or negative errno */ 135 int gpio_direction_input(unsigned gpio); 136 int gpio_direction_output(unsigned gpio, int value); 137 138 The return value is zero for success, else a negative errno. It should 139 be checked, since the get/set calls don't have error returns and since 140 misconfiguration is possible. You should normally issue these calls from 141 a task context. However, for spinlock-safe GPIOs it's OK to use them 142 before tasking is enabled, as part of early board setup. 143 144 For output GPIOs, the value provided becomes the initial output value. 145 This helps avoid signal glitching during system startup. 146 147 For compatibility with legacy interfaces to GPIOs, setting the direction 148 of a GPIO implicitly requests that GPIO (see below) if it has not been 149 requested already. That compatibility is being removed from the optional 150 gpiolib framework. 151 152 Setting the direction can fail if the GPIO number is invalid, or when 153 that particular GPIO can't be used in that mode. It's generally a bad 154 idea to rely on boot firmware to have set the direction correctly, since 155 it probably wasn't validated to do more than boot Linux. (Similarly, 156 that board setup code probably needs to multiplex that pin as a GPIO, 157 and configure pullups/pulldowns appropriately.) 158 159 160 Spinlock-Safe GPIO access 161 ------------------------- 162 Most GPIO controllers can be accessed with memory read/write instructions. 163 Those don't need to sleep, and can safely be done from inside hard 164 (nonthreaded) IRQ handlers and similar contexts. 165 166 Use the following calls to access such GPIOs, 167 for which gpio_cansleep() will always return false (see below): 168 169 /* GPIO INPUT: return zero or nonzero */ 170 int gpio_get_value(unsigned gpio); 171 172 /* GPIO OUTPUT */ 173 void gpio_set_value(unsigned gpio, int value); 174 175 The values are boolean, zero for low, nonzero for high. When reading the 176 value of an output pin, the value returned should be what's seen on the 177 pin ... that won't always match the specified output value, because of 178 issues including open-drain signaling and output latencies. 179 180 The get/set calls have no error returns because "invalid GPIO" should have 181 been reported earlier from gpio_direction_*(). However, note that not all 182 platforms can read the value of output pins; those that can't should always 183 return zero. Also, using these calls for GPIOs that can't safely be accessed 184 without sleeping (see below) is an error. 185 186 Platform-specific implementations are encouraged to optimize the two 187 calls to access the GPIO value in cases where the GPIO number (and for 188 output, value) are constant. It's normal for them to need only a couple 189 of instructions in such cases (reading or writing a hardware register), 190 and not to need spinlocks. Such optimized calls can make bitbanging 191 applications a lot more efficient (in both space and time) than spending 192 dozens of instructions on subroutine calls. 193 194 195 GPIO access that may sleep 196 -------------------------- 197 Some GPIO controllers must be accessed using message based busses like I2C 198 or SPI. Commands to read or write those GPIO values require waiting to 199 get to the head of a queue to transmit a command and get its response. 200 This requires sleeping, which can't be done from inside IRQ handlers. 201 202 Platforms that support this type of GPIO distinguish them from other GPIOs 203 by returning nonzero from this call (which requires a valid GPIO number, 204 which should have been previously allocated with gpio_request): 205 206 int gpio_cansleep(unsigned gpio); 207 208 To access such GPIOs, a different set of accessors is defined: 209 210 /* GPIO INPUT: return zero or nonzero, might sleep */ 211 int gpio_get_value_cansleep(unsigned gpio); 212 213 /* GPIO OUTPUT, might sleep */ 214 void gpio_set_value_cansleep(unsigned gpio, int value); 215 216 217 Accessing such GPIOs requires a context which may sleep, for example 218 a threaded IRQ handler, and those accessors must be used instead of 219 spinlock-safe accessors without the cansleep() name suffix. 220 221 Other than the fact that these accessors might sleep, and will work 222 on GPIOs that can't be accessed from hardIRQ handlers, these calls act 223 the same as the spinlock-safe calls. 224 225 ** IN ADDITION ** calls to setup and configure such GPIOs must be made 226 from contexts which may sleep, since they may need to access the GPIO 227 controller chip too: (These setup calls are usually made from board 228 setup or driver probe/teardown code, so this is an easy constraint.) 229 230 gpio_direction_input() 231 gpio_direction_output() 232 gpio_request() 233 234 ## gpio_request_one() 235 ## gpio_request_array() 236 ## gpio_free_array() 237 238 gpio_free() 239 gpio_set_debounce() 240 241 242 243 Claiming and Releasing GPIOs 244 ---------------------------- 245 To help catch system configuration errors, two calls are defined. 246 247 /* request GPIO, returning 0 or negative errno. 248 * non-null labels may be useful for diagnostics. 249 */ 250 int gpio_request(unsigned gpio, const char *label); 251 252 /* release previously-claimed GPIO */ 253 void gpio_free(unsigned gpio); 254 255 Passing invalid GPIO numbers to gpio_request() will fail, as will requesting 256 GPIOs that have already been claimed with that call. The return value of 257 gpio_request() must be checked. You should normally issue these calls from 258 a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs 259 before tasking is enabled, as part of early board setup. 260 261 These calls serve two basic purposes. One is marking the signals which 262 are actually in use as GPIOs, for better diagnostics; systems may have 263 several hundred potential GPIOs, but often only a dozen are used on any 264 given board. Another is to catch conflicts, identifying errors when 265 (a) two or more drivers wrongly think they have exclusive use of that 266 signal, or (b) something wrongly believes it's safe to remove drivers 267 needed to manage a signal that's in active use. That is, requesting a 268 GPIO can serve as a kind of lock. 269 270 Some platforms may also use knowledge about what GPIOs are active for 271 power management, such as by powering down unused chip sectors and, more 272 easily, gating off unused clocks. 273 274 Note that requesting a GPIO does NOT cause it to be configured in any 275 way; it just marks that GPIO as in use. Separate code must handle any 276 pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown). 277 278 Also note that it's your responsibility to have stopped using a GPIO 279 before you free it. 280 281 Considering in most cases GPIOs are actually configured right after they 282 are claimed, three additional calls are defined: 283 284 /* request a single GPIO, with initial configuration specified by 285 * 'flags', identical to gpio_request() wrt other arguments and 286 * return value 287 */ 288 int gpio_request_one(unsigned gpio, unsigned long flags, const char *label); 289 290 /* request multiple GPIOs in a single call 291 */ 292 int gpio_request_array(struct gpio *array, size_t num); 293 294 /* release multiple GPIOs in a single call 295 */ 296 void gpio_free_array(struct gpio *array, size_t num); 297 298 where 'flags' is currently defined to specify the following properties: 299 300 * GPIOF_DIR_IN - to configure direction as input 301 * GPIOF_DIR_OUT - to configure direction as output 302 303 * GPIOF_INIT_LOW - as output, set initial level to LOW 304 * GPIOF_INIT_HIGH - as output, set initial level to HIGH 305 306 since GPIOF_INIT_* are only valid when configured as output, so group valid 307 combinations as: 308 309 * GPIOF_IN - configure as input 310 * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW 311 * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH 312 313 In the future, these flags can be extended to support more properties such 314 as open-drain status. 315 316 Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is 317 introduced to encapsulate all three fields as: 318 319 struct gpio { 320 unsigned gpio; 321 unsigned long flags; 322 const char *label; 323 }; 324 325 A typical example of usage: 326 327 static struct gpio leds_gpios[] = { 328 { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */ 329 { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */ 330 { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */ 331 { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */ 332 { ... }, 333 }; 334 335 err = gpio_request_one(31, GPIOF_IN, "Reset Button"); 336 if (err) 337 ... 338 339 err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios)); 340 if (err) 341 ... 342 343 gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios)); 344 345 346 GPIOs mapped to IRQs 347 -------------------- 348 GPIO numbers are unsigned integers; so are IRQ numbers. These make up 349 two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can 350 map between them using calls like: 351 352 /* map GPIO numbers to IRQ numbers */ 353 int gpio_to_irq(unsigned gpio); 354 355 /* map IRQ numbers to GPIO numbers (avoid using this) */ 356 int irq_to_gpio(unsigned irq); 357 358 Those return either the corresponding number in the other namespace, or 359 else a negative errno code if the mapping can't be done. (For example, 360 some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO 361 number that wasn't set up as an input using gpio_direction_input(), or 362 to use an IRQ number that didn't originally come from gpio_to_irq(). 363 364 These two mapping calls are expected to cost on the order of a single 365 addition or subtraction. They're not allowed to sleep. 366 367 Non-error values returned from gpio_to_irq() can be passed to request_irq() 368 or free_irq(). They will often be stored into IRQ resources for platform 369 devices, by the board-specific initialization code. Note that IRQ trigger 370 options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are 371 system wakeup capabilities. 372 373 Non-error values returned from irq_to_gpio() would most commonly be used 374 with gpio_get_value(), for example to initialize or update driver state 375 when the IRQ is edge-triggered. Note that some platforms don't support 376 this reverse mapping, so you should avoid using it. 377 378 379 Emulating Open Drain Signals 380 ---------------------------- 381 Sometimes shared signals need to use "open drain" signaling, where only the 382 low signal level is actually driven. (That term applies to CMOS transistors; 383 "open collector" is used for TTL.) A pullup resistor causes the high signal 384 level. This is sometimes called a "wire-AND"; or more practically, from the 385 negative logic (low=true) perspective this is a "wire-OR". 386 387 One common example of an open drain signal is a shared active-low IRQ line. 388 Also, bidirectional data bus signals sometimes use open drain signals. 389 390 Some GPIO controllers directly support open drain outputs; many don't. When 391 you need open drain signaling but your hardware doesn't directly support it, 392 there's a common idiom you can use to emulate it with any GPIO pin that can 393 be used as either an input or an output: 394 395 LOW: gpio_direction_output(gpio, 0) ... this drives the signal 396 and overrides the pullup. 397 398 HIGH: gpio_direction_input(gpio) ... this turns off the output, 399 so the pullup (or some other device) controls the signal. 400 401 If you are "driving" the signal high but gpio_get_value(gpio) reports a low 402 value (after the appropriate rise time passes), you know some other component 403 is driving the shared signal low. That's not necessarily an error. As one 404 common example, that's how I2C clocks are stretched: a slave that needs a 405 slower clock delays the rising edge of SCK, and the I2C master adjusts its 406 signaling rate accordingly. 407 408 409 What do these conventions omit? 410 =============================== 411 One of the biggest things these conventions omit is pin multiplexing, since 412 this is highly chip-specific and nonportable. One platform might not need 413 explicit multiplexing; another might have just two options for use of any 414 given pin; another might have eight options per pin; another might be able 415 to route a given GPIO to any one of several pins. (Yes, those examples all 416 come from systems that run Linux today.) 417 418 Related to multiplexing is configuration and enabling of the pullups or 419 pulldowns integrated on some platforms. Not all platforms support them, 420 or support them in the same way; and any given board might use external 421 pullups (or pulldowns) so that the on-chip ones should not be used. 422 (When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.) 423 Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a 424 platform-specific issue, as are models like (not) having a one-to-one 425 correspondence between configurable pins and GPIOs. 426 427 There are other system-specific mechanisms that are not specified here, 428 like the aforementioned options for input de-glitching and wire-OR output. 429 Hardware may support reading or writing GPIOs in gangs, but that's usually 430 configuration dependent: for GPIOs sharing the same bank. (GPIOs are 431 commonly grouped in banks of 16 or 32, with a given SOC having several such 432 banks.) Some systems can trigger IRQs from output GPIOs, or read values 433 from pins not managed as GPIOs. Code relying on such mechanisms will 434 necessarily be nonportable. 435 436 Dynamic definition of GPIOs is not currently standard; for example, as 437 a side effect of configuring an add-on board with some GPIO expanders. 438 439 440 GPIO implementor's framework (OPTIONAL) 441 ======================================= 442 As noted earlier, there is an optional implementation framework making it 443 easier for platforms to support different kinds of GPIO controller using 444 the same programming interface. This framework is called "gpiolib". 445 446 As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file 447 will be found there. That will list all the controllers registered through 448 this framework, and the state of the GPIOs currently in use. 449 450 451 Controller Drivers: gpio_chip 452 ----------------------------- 453 In this framework each GPIO controller is packaged as a "struct gpio_chip" 454 with information common to each controller of that type: 455 456 - methods to establish GPIO direction 457 - methods used to access GPIO values 458 - flag saying whether calls to its methods may sleep 459 - optional debugfs dump method (showing extra state like pullup config) 460 - label for diagnostics 461 462 There is also per-instance data, which may come from device.platform_data: 463 the number of its first GPIO, and how many GPIOs it exposes. 464 465 The code implementing a gpio_chip should support multiple instances of the 466 controller, possibly using the driver model. That code will configure each 467 gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be 468 rare; use gpiochip_remove() when it is unavoidable. 469 470 Most often a gpio_chip is part of an instance-specific structure with state 471 not exposed by the GPIO interfaces, such as addressing, power management, 472 and more. Chips such as codecs will have complex non-GPIO state. 473 474 Any debugfs dump method should normally ignore signals which haven't been 475 requested as GPIOs. They can use gpiochip_is_requested(), which returns 476 either NULL or the label associated with that GPIO when it was requested. 477 478 479 Platform Support 480 ---------------- 481 To support this framework, a platform's Kconfig will "select" either 482 ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB 483 and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines 484 three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep(). 485 486 It may also provide a custom value for ARCH_NR_GPIOS, so that it better 487 reflects the number of GPIOs in actual use on that platform, without 488 wasting static table space. (It should count both built-in/SoC GPIOs and 489 also ones on GPIO expanders. 490 491 ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled 492 into the kernel on that architecture. 493 494 ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user 495 can enable it and build it into the kernel optionally. 496 497 If neither of these options are selected, the platform does not support 498 GPIOs through GPIO-lib and the code cannot be enabled by the user. 499 500 Trivial implementations of those functions can directly use framework 501 code, which always dispatches through the gpio_chip: 502 503 #define gpio_get_value __gpio_get_value 504 #define gpio_set_value __gpio_set_value 505 #define gpio_cansleep __gpio_cansleep 506 507 Fancier implementations could instead define those as inline functions with 508 logic optimizing access to specific SOC-based GPIOs. For example, if the 509 referenced GPIO is the constant "12", getting or setting its value could 510 cost as little as two or three instructions, never sleeping. When such an 511 optimization is not possible those calls must delegate to the framework 512 code, costing at least a few dozen instructions. For bitbanged I/O, such 513 instruction savings can be significant. 514 515 For SOCs, platform-specific code defines and registers gpio_chip instances 516 for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to 517 match chip vendor documentation, and directly match board schematics. They 518 may well start at zero and go up to a platform-specific limit. Such GPIOs 519 are normally integrated into platform initialization to make them always be 520 available, from arch_initcall() or earlier; they can often serve as IRQs. 521 522 523 Board Support 524 ------------- 525 For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi 526 function devices, FPGAs or CPLDs -- most often board-specific code handles 527 registering controller devices and ensures that their drivers know what GPIO 528 numbers to use with gpiochip_add(). Their numbers often start right after 529 platform-specific GPIOs. 530 531 For example, board setup code could create structures identifying the range 532 of GPIOs that chip will expose, and passes them to each GPIO expander chip 533 using platform_data. Then the chip driver's probe() routine could pass that 534 data to gpiochip_add(). 535 536 Initialization order can be important. For example, when a device relies on 537 an I2C-based GPIO, its probe() routine should only be called after that GPIO 538 becomes available. That may mean the device should not be registered until 539 calls for that GPIO can work. One way to address such dependencies is for 540 such gpio_chip controllers to provide setup() and teardown() callbacks to 541 board specific code; those board specific callbacks would register devices 542 once all the necessary resources are available, and remove them later when 543 the GPIO controller device becomes unavailable. 544 545 546 Sysfs Interface for Userspace (OPTIONAL) 547 ======================================== 548 Platforms which use the "gpiolib" implementors framework may choose to 549 configure a sysfs user interface to GPIOs. This is different from the 550 debugfs interface, since it provides control over GPIO direction and 551 value instead of just showing a gpio state summary. Plus, it could be 552 present on production systems without debugging support. 553 554 Given appropriate hardware documentation for the system, userspace could 555 know for example that GPIO #23 controls the write protect line used to 556 protect boot loader segments in flash memory. System upgrade procedures 557 may need to temporarily remove that protection, first importing a GPIO, 558 then changing its output state, then updating the code before re-enabling 559 the write protection. In normal use, GPIO #23 would never be touched, 560 and the kernel would have no need to know about it. 561 562 Again depending on appropriate hardware documentation, on some systems 563 userspace GPIO can be used to determine system configuration data that 564 standard kernels won't know about. And for some tasks, simple userspace 565 GPIO drivers could be all that the system really needs. 566 567 Note that standard kernel drivers exist for common "LEDs and Buttons" 568 GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those 569 instead of talking directly to the GPIOs; they integrate with kernel 570 frameworks better than your userspace code could. 571 572 573 Paths in Sysfs 574 -------------- 575 There are three kinds of entry in /sys/class/gpio: 576 577 - Control interfaces used to get userspace control over GPIOs; 578 579 - GPIOs themselves; and 580 581 - GPIO controllers ("gpio_chip" instances). 582 583 That's in addition to standard files including the "device" symlink. 584 585 The control interfaces are write-only: 586 587 /sys/class/gpio/ 588 589 "export" ... Userspace may ask the kernel to export control of 590 a GPIO to userspace by writing its number to this file. 591 592 Example: "echo 19 > export" will create a "gpio19" node 593 for GPIO #19, if that's not requested by kernel code. 594 595 "unexport" ... Reverses the effect of exporting to userspace. 596 597 Example: "echo 19 > unexport" will remove a "gpio19" 598 node exported using the "export" file. 599 600 GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) 601 and have the following read/write attributes: 602 603 /sys/class/gpio/gpioN/ 604 605 "direction" ... reads as either "in" or "out". This value may 606 normally be written. Writing as "out" defaults to 607 initializing the value as low. To ensure glitch free 608 operation, values "low" and "high" may be written to 609 configure the GPIO as an output with that initial value. 610 611 Note that this attribute *will not exist* if the kernel 612 doesn't support changing the direction of a GPIO, or 613 it was exported by kernel code that didn't explicitly 614 allow userspace to reconfigure this GPIO's direction. 615 616 "value" ... reads as either 0 (low) or 1 (high). If the GPIO 617 is configured as an output, this value may be written; 618 any nonzero value is treated as high. 619 620 If the pin can be configured as interrupt-generating interrupt 621 and if it has been configured to generate interrupts (see the 622 description of "edge"), you can poll(2) on that file and 623 poll(2) will return whenever the interrupt was triggered. If 624 you use poll(2), set the events POLLPRI and POLLERR. If you 625 use select(2), set the file descriptor in exceptfds. After 626 poll(2) returns, either lseek(2) to the beginning of the sysfs 627 file and read the new value or close the file and re-open it 628 to read the value. 629 630 "edge" ... reads as either "none", "rising", "falling", or 631 "both". Write these strings to select the signal edge(s) 632 that will make poll(2) on the "value" file return. 633 634 This file exists only if the pin can be configured as an 635 interrupt generating input pin. 636 637 "active_low" ... reads as either 0 (false) or 1 (true). Write 638 any nonzero value to invert the value attribute both 639 for reading and writing. Existing and subsequent 640 poll(2) support configuration via the edge attribute 641 for "rising" and "falling" edges will follow this 642 setting. 643 644 GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the 645 controller implementing GPIOs starting at #42) and have the following 646 read-only attributes: 647 648 /sys/class/gpio/gpiochipN/ 649 650 "base" ... same as N, the first GPIO managed by this chip 651 652 "label" ... provided for diagnostics (not always unique) 653 654 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) 655 656 Board documentation should in most cases cover what GPIOs are used for 657 what purposes. However, those numbers are not always stable; GPIOs on 658 a daughtercard might be different depending on the base board being used, 659 or other cards in the stack. In such cases, you may need to use the 660 gpiochip nodes (possibly in conjunction with schematics) to determine 661 the correct GPIO number to use for a given signal. 662 663 664 Exporting from Kernel code 665 -------------------------- 666 Kernel code can explicitly manage exports of GPIOs which have already been 667 requested using gpio_request(): 668 669 /* export the GPIO to userspace */ 670 int gpio_export(unsigned gpio, bool direction_may_change); 671 672 /* reverse gpio_export() */ 673 void gpio_unexport(); 674 675 /* create a sysfs link to an exported GPIO node */ 676 int gpio_export_link(struct device *dev, const char *name, 677 unsigned gpio) 678 679 /* change the polarity of a GPIO node in sysfs */ 680 int gpio_sysfs_set_active_low(unsigned gpio, int value); 681 682 After a kernel driver requests a GPIO, it may only be made available in 683 the sysfs interface by gpio_export(). The driver can control whether the 684 signal direction may change. This helps drivers prevent userspace code 685 from accidentally clobbering important system state. 686 687 This explicit exporting can help with debugging (by making some kinds 688 of experiments easier), or can provide an always-there interface that's 689 suitable for documenting as part of a board support package. 690 691 After the GPIO has been exported, gpio_export_link() allows creating 692 symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can 693 use this to provide the interface under their own device in sysfs with 694 a descriptive name. 695 696 Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity 697 differences between boards from user space. This only affects the 698 sysfs interface. Polarity change can be done both before and after 699 gpio_export(), and previously enabled poll(2) support for either 700 rising or falling edge will be reconfigured to follow this setting.