Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 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 a number could reference a GPIO, you may use this predicate: 113 114 int gpio_is_valid(int number); 115 116 A number that's not valid will be rejected by calls which may request 117 or free GPIOs (see below). Other numbers may also be rejected; for 118 example, a number might be valid but unused on a given board. 119 120 Whether a platform supports multiple GPIO controllers is currently a 121 platform-specific implementation issue. 122 123 124 Using GPIOs 125 ----------- 126 The first thing a system should do with a GPIO is allocate it, using 127 the gpio_request() call; see later. 128 129 One of the next things to do with a GPIO, often in board setup code when 130 setting 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); 135 136 The return value is zero for success, else a negative errno. It should 137 be checked, since the get/set calls don't have error returns and since 138 misconfiguration is possible. You should normally issue these calls from 139 a task context. However, for spinlock-safe GPIOs it's OK to use them 140 before tasking is enabled, as part of early board setup. 141 142 For output GPIOs, the value provided becomes the initial output value. 143 This helps avoid signal glitching during system startup. 144 145 For compatibility with legacy interfaces to GPIOs, setting the direction 146 of a GPIO implicitly requests that GPIO (see below) if it has not been 147 requested already. That compatibility is being removed from the optional 148 gpiolib framework. 149 150 Setting the direction can fail if the GPIO number is invalid, or when 151 that particular GPIO can't be used in that mode. It's generally a bad 152 idea to rely on boot firmware to have set the direction correctly, since 153 it probably wasn't validated to do more than boot Linux. (Similarly, 154 that board setup code probably needs to multiplex that pin as a GPIO, 155 and configure pullups/pulldowns appropriately.) 156 157 158 Spinlock-Safe GPIO access 159 ------------------------- 160 Most GPIO controllers can be accessed with memory read/write instructions. 161 That doesn't need to sleep, and can safely be done from inside IRQ handlers. 162 (That includes hardirq contexts on RT kernels.) 163 164 Use 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); 171 172 The values are boolean, zero for low, nonzero for high. When reading the 173 value of an output pin, the value returned should be what's seen on the 174 pin ... that won't always match the specified output value, because of 175 issues including open-drain signaling and output latencies. 176 177 The get/set calls have no error returns because "invalid GPIO" should have 178 been reported earlier from gpio_direction_*(). However, note that not all 179 platforms can read the value of output pins; those that can't should always 180 return zero. Also, using these calls for GPIOs that can't safely be accessed 181 without sleeping (see below) is an error. 182 183 Platform-specific implementations are encouraged to optimize the two 184 calls to access the GPIO value in cases where the GPIO number (and for 185 output, value) are constant. It's normal for them to need only a couple 186 of instructions in such cases (reading or writing a hardware register), 187 and not to need spinlocks. Such optimized calls can make bitbanging 188 applications a lot more efficient (in both space and time) than spending 189 dozens of instructions on subroutine calls. 190 191 192 GPIO access that may sleep 193 -------------------------- 194 Some GPIO controllers must be accessed using message based busses like I2C 195 or SPI. Commands to read or write those GPIO values require waiting to 196 get to the head of a queue to transmit a command and get its response. 197 This requires sleeping, which can't be done from inside IRQ handlers. 198 199 Platforms that support this type of GPIO distinguish them from other GPIOs 200 by returning nonzero from this call (which requires a valid GPIO number, 201 which should have been previously allocated with gpio_request): 202 203 int gpio_cansleep(unsigned gpio); 204 205 To 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); 212 213 Other than the fact that these calls might sleep, and will not be ignored 214 for GPIOs that can't be accessed from IRQ handlers, these calls act the 215 same as the spinlock-safe calls. 216 217 218 Claiming and Releasing GPIOs 219 ---------------------------- 220 To 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); 229 230 Passing invalid GPIO numbers to gpio_request() will fail, as will requesting 231 GPIOs that have already been claimed with that call. The return value of 232 gpio_request() must be checked. You should normally issue these calls from 233 a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs 234 before tasking is enabled, as part of early board setup. 235 236 These calls serve two basic purposes. One is marking the signals which 237 are actually in use as GPIOs, for better diagnostics; systems may have 238 several hundred potential GPIOs, but often only a dozen are used on any 239 given board. Another is to catch conflicts, identifying errors when 240 (a) two or more drivers wrongly think they have exclusive use of that 241 signal, or (b) something wrongly believes it's safe to remove drivers 242 needed to manage a signal that's in active use. That is, requesting a 243 GPIO can serve as a kind of lock. 244 245 Some platforms may also use knowledge about what GPIOs are active for 246 power management, such as by powering down unused chip sectors and, more 247 easily, gating off unused clocks. 248 249 Note that requesting a GPIO does NOT cause it to be configured in any 250 way; it just marks that GPIO as in use. Separate code must handle any 251 pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown). 252 253 Also note that it's your responsibility to have stopped using a GPIO 254 before you free it. 255 256 257 GPIOs mapped to IRQs 258 -------------------- 259 GPIO numbers are unsigned integers; so are IRQ numbers. These make up 260 two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can 261 map 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); 268 269 Those return either the corresponding number in the other namespace, or 270 else a negative errno code if the mapping can't be done. (For example, 271 some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO 272 number that wasn't set up as an input using gpio_direction_input(), or 273 to use an IRQ number that didn't originally come from gpio_to_irq(). 274 275 These two mapping calls are expected to cost on the order of a single 276 addition or subtraction. They're not allowed to sleep. 277 278 Non-error values returned from gpio_to_irq() can be passed to request_irq() 279 or free_irq(). They will often be stored into IRQ resources for platform 280 devices, by the board-specific initialization code. Note that IRQ trigger 281 options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are 282 system wakeup capabilities. 283 284 Non-error values returned from irq_to_gpio() would most commonly be used 285 with gpio_get_value(), for example to initialize or update driver state 286 when the IRQ is edge-triggered. Note that some platforms don't support 287 this reverse mapping, so you should avoid using it. 288 289 290 Emulating Open Drain Signals 291 ---------------------------- 292 Sometimes shared signals need to use "open drain" signaling, where only the 293 low 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 295 level. This is sometimes called a "wire-AND"; or more practically, from the 296 negative logic (low=true) perspective this is a "wire-OR". 297 298 One common example of an open drain signal is a shared active-low IRQ line. 299 Also, bidirectional data bus signals sometimes use open drain signals. 300 301 Some GPIO controllers directly support open drain outputs; many don't. When 302 you need open drain signaling but your hardware doesn't directly support it, 303 there's a common idiom you can use to emulate it with any GPIO pin that can 304 be 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. 311 312 If you are "driving" the signal high but gpio_get_value(gpio) reports a low 313 value (after the appropriate rise time passes), you know some other component 314 is driving the shared signal low. That's not necessarily an error. As one 315 common example, that's how I2C clocks are stretched: a slave that needs a 316 slower clock delays the rising edge of SCK, and the I2C master adjusts its 317 signaling rate accordingly. 318 319 320 What do these conventions omit? 321 =============================== 322 One of the biggest things these conventions omit is pin multiplexing, since 323 this is highly chip-specific and nonportable. One platform might not need 324 explicit multiplexing; another might have just two options for use of any 325 given pin; another might have eight options per pin; another might be able 326 to route a given GPIO to any one of several pins. (Yes, those examples all 327 come from systems that run Linux today.) 328 329 Related to multiplexing is configuration and enabling of the pullups or 330 pulldowns integrated on some platforms. Not all platforms support them, 331 or support them in the same way; and any given board might use external 332 pullups (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.) 334 Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a 335 platform-specific issue, as are models like (not) having a one-to-one 336 correspondence between configurable pins and GPIOs. 337 338 There are other system-specific mechanisms that are not specified here, 339 like the aforementioned options for input de-glitching and wire-OR output. 340 Hardware may support reading or writing GPIOs in gangs, but that's usually 341 configuration dependent: for GPIOs sharing the same bank. (GPIOs are 342 commonly grouped in banks of 16 or 32, with a given SOC having several such 343 banks.) Some systems can trigger IRQs from output GPIOs, or read values 344 from pins not managed as GPIOs. Code relying on such mechanisms will 345 necessarily be nonportable. 346 347 Dynamic definition of GPIOs is not currently standard; for example, as 348 a side effect of configuring an add-on board with some GPIO expanders. 349 350 351 GPIO implementor's framework (OPTIONAL) 352 ======================================= 353 As noted earlier, there is an optional implementation framework making it 354 easier for platforms to support different kinds of GPIO controller using 355 the same programming interface. This framework is called "gpiolib". 356 357 As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file 358 will be found there. That will list all the controllers registered through 359 this framework, and the state of the GPIOs currently in use. 360 361 362 Controller Drivers: gpio_chip 363 ----------------------------- 364 In this framework each GPIO controller is packaged as a "struct gpio_chip" 365 with 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 372 373 There is also per-instance data, which may come from device.platform_data: 374 the number of its first GPIO, and how many GPIOs it exposes. 375 376 The code implementing a gpio_chip should support multiple instances of the 377 controller, possibly using the driver model. That code will configure each 378 gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be 379 rare; use gpiochip_remove() when it is unavoidable. 380 381 Most often a gpio_chip is part of an instance-specific structure with state 382 not exposed by the GPIO interfaces, such as addressing, power management, 383 and more. Chips such as codecs will have complex non-GPIO state. 384 385 Any debugfs dump method should normally ignore signals which haven't been 386 requested as GPIOs. They can use gpiochip_is_requested(), which returns 387 either NULL or the label associated with that GPIO when it was requested. 388 389 390 Platform Support 391 ---------------- 392 To support this framework, a platform's Kconfig will "select" either 393 ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB 394 and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines 395 three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep(). 396 They may also want to provide a custom value for ARCH_NR_GPIOS. 397 398 ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled 399 into the kernel on that architecture. 400 401 ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user 402 can enable it and build it into the kernel optionally. 403 404 If neither of these options are selected, the platform does not support 405 GPIOs through GPIO-lib and the code cannot be enabled by the user. 406 407 Trivial implementations of those functions can directly use framework 408 code, 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 413 414 Fancier implementations could instead define those as inline functions with 415 logic optimizing access to specific SOC-based GPIOs. For example, if the 416 referenced GPIO is the constant "12", getting or setting its value could 417 cost as little as two or three instructions, never sleeping. When such an 418 optimization is not possible those calls must delegate to the framework 419 code, costing at least a few dozen instructions. For bitbanged I/O, such 420 instruction savings can be significant. 421 422 For SOCs, platform-specific code defines and registers gpio_chip instances 423 for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to 424 match chip vendor documentation, and directly match board schematics. They 425 may well start at zero and go up to a platform-specific limit. Such GPIOs 426 are normally integrated into platform initialization to make them always be 427 available, from arch_initcall() or earlier; they can often serve as IRQs. 428 429 430 Board Support 431 ------------- 432 For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi 433 function devices, FPGAs or CPLDs -- most often board-specific code handles 434 registering controller devices and ensures that their drivers know what GPIO 435 numbers to use with gpiochip_add(). Their numbers often start right after 436 platform-specific GPIOs. 437 438 For example, board setup code could create structures identifying the range 439 of GPIOs that chip will expose, and passes them to each GPIO expander chip 440 using platform_data. Then the chip driver's probe() routine could pass that 441 data to gpiochip_add(). 442 443 Initialization order can be important. For example, when a device relies on 444 an I2C-based GPIO, its probe() routine should only be called after that GPIO 445 becomes available. That may mean the device should not be registered until 446 calls for that GPIO can work. One way to address such dependencies is for 447 such gpio_chip controllers to provide setup() and teardown() callbacks to 448 board specific code; those board specific callbacks would register devices 449 once all the necessary resources are available, and remove them later when 450 the GPIO controller device becomes unavailable. 451 452 453 Sysfs Interface for Userspace (OPTIONAL) 454 ======================================== 455 Platforms which use the "gpiolib" implementors framework may choose to 456 configure a sysfs user interface to GPIOs. This is different from the 457 debugfs interface, since it provides control over GPIO direction and 458 value instead of just showing a gpio state summary. Plus, it could be 459 present on production systems without debugging support. 460 461 Given appropriate hardware documentation for the system, userspace could 462 know for example that GPIO #23 controls the write protect line used to 463 protect boot loader segments in flash memory. System upgrade procedures 464 may need to temporarily remove that protection, first importing a GPIO, 465 then changing its output state, then updating the code before re-enabling 466 the write protection. In normal use, GPIO #23 would never be touched, 467 and the kernel would have no need to know about it. 468 469 Again depending on appropriate hardware documentation, on some systems 470 userspace GPIO can be used to determine system configuration data that 471 standard kernels won't know about. And for some tasks, simple userspace 472 GPIO drivers could be all that the system really needs. 473 474 Note that standard kernel drivers exist for common "LEDs and Buttons" 475 GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those 476 instead of talking directly to the GPIOs; they integrate with kernel 477 frameworks better than your userspace code could. 478 479 480 Paths in Sysfs 481 -------------- 482 There 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). 489 490 That's in addition to standard files including the "device" symlink. 491 492 The 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. 506 507 GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) 508 and 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. 533 534 "active_low" ... reads as either 0 (false) or 1 (true). Write 535 any nonzero value to invert the value attribute both 536 for reading and writing. Existing and subsequent 537 poll(2) support configuration via the edge attribute 538 for "rising" and "falling" edges will follow this 539 setting. 540 541 GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the 542 controller implementing GPIOs starting at #42) and have the following 543 read-only attributes: 544 545 /sys/class/gpio/gpiochipN/ 546 547 "base" ... same as N, the first GPIO managed by this chip 548 549 "label" ... provided for diagnostics (not always unique) 550 551 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) 552 553 Board documentation should in most cases cover what GPIOs are used for 554 what purposes. However, those numbers are not always stable; GPIOs on 555 a daughtercard might be different depending on the base board being used, 556 or other cards in the stack. In such cases, you may need to use the 557 gpiochip nodes (possibly in conjunction with schematics) to determine 558 the correct GPIO number to use for a given signal. 559 560 561 Exporting from Kernel code 562 -------------------------- 563 Kernel code can explicitly manage exports of GPIOs which have already been 564 requested using gpio_request(): 565 566 /* export the GPIO to userspace */ 567 int gpio_export(unsigned gpio, bool direction_may_change); 568 569 /* reverse gpio_export() */ 570 void gpio_unexport(); 571 572 /* create a sysfs link to an exported GPIO node */ 573 int gpio_export_link(struct device *dev, const char *name, 574 unsigned gpio) 575 576 /* change the polarity of a GPIO node in sysfs */ 577 int gpio_sysfs_set_active_low(unsigned gpio, int value); 578 579 After a kernel driver requests a GPIO, it may only be made available in 580 the sysfs interface by gpio_export(). The driver can control whether the 581 signal direction may change. This helps drivers prevent userspace code 582 from accidentally clobbering important system state. 583 584 This explicit exporting can help with debugging (by making some kinds 585 of experiments easier), or can provide an always-there interface that's 586 suitable for documenting as part of a board support package. 587 588 After the GPIO has been exported, gpio_export_link() allows creating 589 symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can 590 use this to provide the interface under their own device in sysfs with 591 a descriptive name. 592 593 Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity 594 differences between boards from user space. This only affects the 595 sysfs interface. Polarity change can be done both before and after 596 gpio_export(), and previously enabled poll(2) support for either 597 rising or falling edge will be reconfigured to follow this setting.