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diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt deleted file mode 100644 index 6f83fa9..0000000 --- a/Documentation/gpio.txt +++ /dev/null @@ -1,775 +0,0 @@ -GPIO Interfaces - -This provides an overview of GPIO access conventions on Linux. - -These calls use the gpio_* naming prefix. No other calls should use that -prefix, or the related __gpio_* prefix. - - -What is a GPIO? -=============== -A "General Purpose Input/Output" (GPIO) is a flexible software-controlled -digital signal. They are provided from many kinds of chip, and are familiar -to Linux developers working with embedded and custom hardware. Each GPIO -represents a bit connected to a particular pin, or "ball" on Ball Grid Array -(BGA) packages. Board schematics show which external hardware connects to -which GPIOs. Drivers can be written generically, so that board setup code -passes such pin configuration data to drivers. - -System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every -non-dedicated pin can be configured as a GPIO; and most chips have at least -several dozen of them. Programmable logic devices (like FPGAs) can easily -provide GPIOs; multifunction chips like power managers, and audio codecs -often have a few such pins to help with pin scarcity on SOCs; and there are -also "GPIO Expander" chips that connect using the I2C or SPI serial busses. -Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS -firmware knowing how they're used). - -The exact capabilities of GPIOs vary between systems. Common options: - - - Output values are writable (high=1, low=0). Some chips also have - options about how that value is driven, so that for example only one - value might be driven ... supporting "wire-OR" and similar schemes - for the other value (notably, "open drain" signaling). - - - Input values are likewise readable (1, 0). Some chips support readback - of pins configured as "output", which is very useful in such "wire-OR" - cases (to support bidirectional signaling). GPIO controllers may have - input de-glitch/debounce logic, sometimes with software controls. - - - Inputs can often be used as IRQ signals, often edge triggered but - sometimes level triggered. Such IRQs may be configurable as system - wakeup events, to wake the system from a low power state. - - - Usually a GPIO will be configurable as either input or output, as needed - by different product boards; single direction ones exist too. - - - Most GPIOs can be accessed while holding spinlocks, but those accessed - through a serial bus normally can't. Some systems support both types. - -On a given board each GPIO is used for one specific purpose like monitoring -MMC/SD card insertion/removal, detecting card writeprotect status, driving -a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware -watchdog, sensing a switch, and so on. - - -GPIO conventions -================ -Note that this is called a "convention" because you don't need to do it this -way, and it's no crime if you don't. There **are** cases where portability -is not the main issue; GPIOs are often used for the kind of board-specific -glue logic that may even change between board revisions, and can't ever be -used on a board that's wired differently. Only least-common-denominator -functionality can be very portable. Other features are platform-specific, -and that can be critical for glue logic. - -Plus, this doesn't require any implementation framework, just an interface. -One platform might implement it as simple inline functions accessing chip -registers; another might implement it by delegating through abstractions -used for several very different kinds of GPIO controller. (There is some -optional code supporting such an implementation strategy, described later -in this document, but drivers acting as clients to the GPIO interface must -not care how it's implemented.) - -That said, if the convention is supported on their platform, drivers should -use it when possible. Platforms must select ARCH_REQUIRE_GPIOLIB or -ARCH_WANT_OPTIONAL_GPIOLIB in their Kconfig. Drivers that can't work without -standard GPIO calls should have Kconfig entries which depend on GPIOLIB. The -GPIO calls are available, either as "real code" or as optimized-away stubs, -when drivers use the include file: - - #include <linux/gpio.h> - -If you stick to this convention then it'll be easier for other developers to -see what your code is doing, and help maintain it. - -Note that these operations include I/O barriers on platforms which need to -use them; drivers don't need to add them explicitly. - - -Identifying GPIOs ------------------ -GPIOs are identified by unsigned integers in the range 0..MAX_INT. That -reserves "negative" numbers for other purposes like marking signals as -"not available on this board", or indicating faults. Code that doesn't -touch the underlying hardware treats these integers as opaque cookies. - -Platforms define how they use those integers, and usually #define symbols -for the GPIO lines so that board-specific setup code directly corresponds -to the relevant schematics. In contrast, drivers should only use GPIO -numbers passed to them from that setup code, using platform_data to hold -board-specific pin configuration data (along with other board specific -data they need). That avoids portability problems. - -So for example one platform uses numbers 32-159 for GPIOs; while another -uses numbers 0..63 with one set of GPIO controllers, 64-79 with another -type of GPIO controller, and on one particular board 80-95 with an FPGA. -The numbers need not be contiguous; either of those platforms could also -use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders. - -If you want to initialize a structure with an invalid GPIO number, use -some negative number (perhaps "-EINVAL"); that will never be valid. To -test if such number from such a structure could reference a GPIO, you -may use this predicate: - - int gpio_is_valid(int number); - -A number that's not valid will be rejected by calls which may request -or free GPIOs (see below). Other numbers may also be rejected; for -example, a number might be valid but temporarily unused on a given board. - -Whether a platform supports multiple GPIO controllers is a platform-specific -implementation issue, as are whether that support can leave "holes" in the space -of GPIO numbers, and whether new controllers can be added at runtime. Such issues -can affect things including whether adjacent GPIO numbers are both valid. - -Using GPIOs ------------ -The first thing a system should do with a GPIO is allocate it, using -the gpio_request() call; see later. - -One of the next things to do with a GPIO, often in board setup code when -setting up a platform_device using the GPIO, is mark its direction: - - /* set as input or output, returning 0 or negative errno */ - int gpio_direction_input(unsigned gpio); - int gpio_direction_output(unsigned gpio, int value); - -The return value is zero for success, else a negative errno. It should -be checked, since the get/set calls don't have error returns and since -misconfiguration is possible. You should normally issue these calls from -a task context. However, for spinlock-safe GPIOs it's OK to use them -before tasking is enabled, as part of early board setup. - -For output GPIOs, the value provided becomes the initial output value. -This helps avoid signal glitching during system startup. - -For compatibility with legacy interfaces to GPIOs, setting the direction -of a GPIO implicitly requests that GPIO (see below) if it has not been -requested already. That compatibility is being removed from the optional -gpiolib framework. - -Setting the direction can fail if the GPIO number is invalid, or when -that particular GPIO can't be used in that mode. It's generally a bad -idea to rely on boot firmware to have set the direction correctly, since -it probably wasn't validated to do more than boot Linux. (Similarly, -that board setup code probably needs to multiplex that pin as a GPIO, -and configure pullups/pulldowns appropriately.) - - -Spinlock-Safe GPIO access -------------------------- -Most GPIO controllers can be accessed with memory read/write instructions. -Those don't need to sleep, and can safely be done from inside hard -(nonthreaded) IRQ handlers and similar contexts. - -Use the following calls to access such GPIOs, -for which gpio_cansleep() will always return false (see below): - - /* GPIO INPUT: return zero or nonzero */ - int gpio_get_value(unsigned gpio); - - /* GPIO OUTPUT */ - void gpio_set_value(unsigned gpio, int value); - -The values are boolean, zero for low, nonzero for high. When reading the -value of an output pin, the value returned should be what's seen on the -pin ... that won't always match the specified output value, because of -issues including open-drain signaling and output latencies. - -The get/set calls have no error returns because "invalid GPIO" should have -been reported earlier from gpio_direction_*(). However, note that not all -platforms can read the value of output pins; those that can't should always -return zero. Also, using these calls for GPIOs that can't safely be accessed -without sleeping (see below) is an error. - -Platform-specific implementations are encouraged to optimize the two -calls to access the GPIO value in cases where the GPIO number (and for -output, value) are constant. It's normal for them to need only a couple -of instructions in such cases (reading or writing a hardware register), -and not to need spinlocks. Such optimized calls can make bitbanging -applications a lot more efficient (in both space and time) than spending -dozens of instructions on subroutine calls. - - -GPIO access that may sleep --------------------------- -Some GPIO controllers must be accessed using message based busses like I2C -or SPI. Commands to read or write those GPIO values require waiting to -get to the head of a queue to transmit a command and get its response. -This requires sleeping, which can't be done from inside IRQ handlers. - -Platforms that support this type of GPIO distinguish them from other GPIOs -by returning nonzero from this call (which requires a valid GPIO number, -which should have been previously allocated with gpio_request): - - int gpio_cansleep(unsigned gpio); - -To access such GPIOs, a different set of accessors is defined: - - /* GPIO INPUT: return zero or nonzero, might sleep */ - int gpio_get_value_cansleep(unsigned gpio); - - /* GPIO OUTPUT, might sleep */ - void gpio_set_value_cansleep(unsigned gpio, int value); - - -Accessing such GPIOs requires a context which may sleep, for example -a threaded IRQ handler, and those accessors must be used instead of -spinlock-safe accessors without the cansleep() name suffix. - -Other than the fact that these accessors might sleep, and will work -on GPIOs that can't be accessed from hardIRQ handlers, these calls act -the same as the spinlock-safe calls. - - ** IN ADDITION ** calls to setup and configure such GPIOs must be made -from contexts which may sleep, since they may need to access the GPIO -controller chip too: (These setup calls are usually made from board -setup or driver probe/teardown code, so this is an easy constraint.) - - gpio_direction_input() - gpio_direction_output() - gpio_request() - -## gpio_request_one() -## gpio_request_array() -## gpio_free_array() - - gpio_free() - gpio_set_debounce() - - - -Claiming and Releasing GPIOs ----------------------------- -To help catch system configuration errors, two calls are defined. - - /* request GPIO, returning 0 or negative errno. - * non-null labels may be useful for diagnostics. - */ - int gpio_request(unsigned gpio, const char *label); - - /* release previously-claimed GPIO */ - void gpio_free(unsigned gpio); - -Passing invalid GPIO numbers to gpio_request() will fail, as will requesting -GPIOs that have already been claimed with that call. The return value of -gpio_request() must be checked. You should normally issue these calls from -a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs -before tasking is enabled, as part of early board setup. - -These calls serve two basic purposes. One is marking the signals which -are actually in use as GPIOs, for better diagnostics; systems may have -several hundred potential GPIOs, but often only a dozen are used on any -given board. Another is to catch conflicts, identifying errors when -(a) two or more drivers wrongly think they have exclusive use of that -signal, or (b) something wrongly believes it's safe to remove drivers -needed to manage a signal that's in active use. That is, requesting a -GPIO can serve as a kind of lock. - -Some platforms may also use knowledge about what GPIOs are active for -power management, such as by powering down unused chip sectors and, more -easily, gating off unused clocks. - -For GPIOs that use pins known to the pinctrl subsystem, that subsystem should -be informed of their use; a gpiolib driver's .request() operation may call -pinctrl_request_gpio(), and a gpiolib driver's .free() operation may call -pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio() -to succeed concurrently with a pin or pingroup being "owned" by a device for -pin multiplexing. - -Any programming of pin multiplexing hardware that is needed to route the -GPIO signal to the appropriate pin should occur within a GPIO driver's -.direction_input() or .direction_output() operations, and occur after any -setup of an output GPIO's value. This allows a glitch-free migration from a -pin's special function to GPIO. This is sometimes required when using a GPIO -to implement a workaround on signals typically driven by a non-GPIO HW block. - -Some platforms allow some or all GPIO signals to be routed to different pins. -Similarly, other aspects of the GPIO or pin may need to be configured, such as -pullup/pulldown. Platform software should arrange that any such details are -configured prior to gpio_request() being called for those GPIOs, e.g. using -the pinctrl subsystem's mapping table, so that GPIO users need not be aware -of these details. - -Also note that it's your responsibility to have stopped using a GPIO -before you free it. - -Considering in most cases GPIOs are actually configured right after they -are claimed, three additional calls are defined: - - /* request a single GPIO, with initial configuration specified by - * 'flags', identical to gpio_request() wrt other arguments and - * return value - */ - int gpio_request_one(unsigned gpio, unsigned long flags, const char *label); - - /* request multiple GPIOs in a single call - */ - int gpio_request_array(struct gpio *array, size_t num); - - /* release multiple GPIOs in a single call - */ - void gpio_free_array(struct gpio *array, size_t num); - -where 'flags' is currently defined to specify the following properties: - - * GPIOF_DIR_IN - to configure direction as input - * GPIOF_DIR_OUT - to configure direction as output - - * GPIOF_INIT_LOW - as output, set initial level to LOW - * GPIOF_INIT_HIGH - as output, set initial level to HIGH - * GPIOF_OPEN_DRAIN - gpio pin is open drain type. - * GPIOF_OPEN_SOURCE - gpio pin is open source type. - - * GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction - * GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction - -since GPIOF_INIT_* are only valid when configured as output, so group valid -combinations as: - - * GPIOF_IN - configure as input - * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW - * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH - -When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is -open drain type. Such pins will not be driven to 1 in output mode. It is -require to connect pull-up on such pins. By enabling this flag, gpio lib will -make the direction to input when it is asked to set value of 1 in output mode -to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode. - -When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is -open source type. Such pins will not be driven to 0 in output mode. It is -require to connect pull-down on such pin. By enabling this flag, gpio lib will -make the direction to input when it is asked to set value of 0 in output mode -to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode. - -In the future, these flags can be extended to support more properties. - -Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is -introduced to encapsulate all three fields as: - - struct gpio { - unsigned gpio; - unsigned long flags; - const char *label; - }; - -A typical example of usage: - - static struct gpio leds_gpios[] = { - { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */ - { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */ - { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */ - { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */ - { ... }, - }; - - err = gpio_request_one(31, GPIOF_IN, "Reset Button"); - if (err) - ... - - err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios)); - if (err) - ... - - gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios)); - - -GPIOs mapped to IRQs --------------------- -GPIO numbers are unsigned integers; so are IRQ numbers. These make up -two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can -map between them using calls like: - - /* map GPIO numbers to IRQ numbers */ - int gpio_to_irq(unsigned gpio); - - /* map IRQ numbers to GPIO numbers (avoid using this) */ - int irq_to_gpio(unsigned irq); - -Those return either the corresponding number in the other namespace, or -else a negative errno code if the mapping can't be done. (For example, -some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO -number that wasn't set up as an input using gpio_direction_input(), or -to use an IRQ number that didn't originally come from gpio_to_irq(). - -These two mapping calls are expected to cost on the order of a single -addition or subtraction. They're not allowed to sleep. - -Non-error values returned from gpio_to_irq() can be passed to request_irq() -or free_irq(). They will often be stored into IRQ resources for platform -devices, by the board-specific initialization code. Note that IRQ trigger -options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are -system wakeup capabilities. - -Non-error values returned from irq_to_gpio() would most commonly be used -with gpio_get_value(), for example to initialize or update driver state -when the IRQ is edge-triggered. Note that some platforms don't support -this reverse mapping, so you should avoid using it. - - -Emulating Open Drain Signals ----------------------------- -Sometimes shared signals need to use "open drain" signaling, where only the -low signal level is actually driven. (That term applies to CMOS transistors; -"open collector" is used for TTL.) A pullup resistor causes the high signal -level. This is sometimes called a "wire-AND"; or more practically, from the -negative logic (low=true) perspective this is a "wire-OR". - -One common example of an open drain signal is a shared active-low IRQ line. -Also, bidirectional data bus signals sometimes use open drain signals. - -Some GPIO controllers directly support open drain outputs; many don't. When -you need open drain signaling but your hardware doesn't directly support it, -there's a common idiom you can use to emulate it with any GPIO pin that can -be used as either an input or an output: - - LOW: gpio_direction_output(gpio, 0) ... this drives the signal - and overrides the pullup. - - HIGH: gpio_direction_input(gpio) ... this turns off the output, - so the pullup (or some other device) controls the signal. - -If you are "driving" the signal high but gpio_get_value(gpio) reports a low -value (after the appropriate rise time passes), you know some other component -is driving the shared signal low. That's not necessarily an error. As one -common example, that's how I2C clocks are stretched: a slave that needs a -slower clock delays the rising edge of SCK, and the I2C master adjusts its -signaling rate accordingly. - - -GPIO controllers and the pinctrl subsystem ------------------------------------------- - -A GPIO controller on a SOC might be tightly coupled with the pinctrl -subsystem, in the sense that the pins can be used by other functions -together with an optional gpio feature. We have already covered the -case where e.g. a GPIO controller need to reserve a pin or set the -direction of a pin by calling any of: - -pinctrl_request_gpio() -pinctrl_free_gpio() -pinctrl_gpio_direction_input() -pinctrl_gpio_direction_output() - -But how does the pin control subsystem cross-correlate the GPIO -numbers (which are a global business) to a certain pin on a certain -pin controller? - -This is done by registering "ranges" of pins, which are essentially -cross-reference tables. These are described in -Documentation/pinctrl.txt - -While the pin allocation is totally managed by the pinctrl subsystem, -gpio (under gpiolib) is still maintained by gpio drivers. It may happen -that different pin ranges in a SoC is managed by different gpio drivers. - -This makes it logical to let gpio drivers announce their pin ranges to -the pin ctrl subsystem before it will call 'pinctrl_request_gpio' in order -to request the corresponding pin to be prepared by the pinctrl subsystem -before any gpio usage. - -For this, the gpio controller can register its pin range with pinctrl -subsystem. There are two ways of doing it currently: with or without DT. - -For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt. - -For non-DT support, user can call gpiochip_add_pin_range() with appropriate -parameters to register a range of gpio pins with a pinctrl driver. For this -exact name string of pinctrl device has to be passed as one of the -argument to this routine. - - -What do these conventions omit? -=============================== -One of the biggest things these conventions omit is pin multiplexing, since -this is highly chip-specific and nonportable. One platform might not need -explicit multiplexing; another might have just two options for use of any -given pin; another might have eight options per pin; another might be able -to route a given GPIO to any one of several pins. (Yes, those examples all -come from systems that run Linux today.) - -Related to multiplexing is configuration and enabling of the pullups or -pulldowns integrated on some platforms. Not all platforms support them, -or support them in the same way; and any given board might use external -pullups (or pulldowns) so that the on-chip ones should not be used. -(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.) -Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a -platform-specific issue, as are models like (not) having a one-to-one -correspondence between configurable pins and GPIOs. - -There are other system-specific mechanisms that are not specified here, -like the aforementioned options for input de-glitching and wire-OR output. -Hardware may support reading or writing GPIOs in gangs, but that's usually -configuration dependent: for GPIOs sharing the same bank. (GPIOs are -commonly grouped in banks of 16 or 32, with a given SOC having several such -banks.) Some systems can trigger IRQs from output GPIOs, or read values -from pins not managed as GPIOs. Code relying on such mechanisms will -necessarily be nonportable. - -Dynamic definition of GPIOs is not currently standard; for example, as -a side effect of configuring an add-on board with some GPIO expanders. - - -GPIO implementor's framework (OPTIONAL) -======================================= -As noted earlier, there is an optional implementation framework making it -easier for platforms to support different kinds of GPIO controller using -the same programming interface. This framework is called "gpiolib". - -As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file -will be found there. That will list all the controllers registered through -this framework, and the state of the GPIOs currently in use. - - -Controller Drivers: gpio_chip ------------------------------ -In this framework each GPIO controller is packaged as a "struct gpio_chip" -with information common to each controller of that type: - - - methods to establish GPIO direction - - methods used to access GPIO values - - flag saying whether calls to its methods may sleep - - optional debugfs dump method (showing extra state like pullup config) - - label for diagnostics - -There is also per-instance data, which may come from device.platform_data: -the number of its first GPIO, and how many GPIOs it exposes. - -The code implementing a gpio_chip should support multiple instances of the -controller, possibly using the driver model. That code will configure each -gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be -rare; use gpiochip_remove() when it is unavoidable. - -Most often a gpio_chip is part of an instance-specific structure with state -not exposed by the GPIO interfaces, such as addressing, power management, -and more. Chips such as codecs will have complex non-GPIO state. - -Any debugfs dump method should normally ignore signals which haven't been -requested as GPIOs. They can use gpiochip_is_requested(), which returns -either NULL or the label associated with that GPIO when it was requested. - - -Platform Support ----------------- -To support this framework, a platform's Kconfig will "select" either -ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB -and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines -three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep(). - -It may also provide a custom value for ARCH_NR_GPIOS, so that it better -reflects the number of GPIOs in actual use on that platform, without -wasting static table space. (It should count both built-in/SoC GPIOs and -also ones on GPIO expanders. - -ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled -into the kernel on that architecture. - -ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user -can enable it and build it into the kernel optionally. - -If neither of these options are selected, the platform does not support -GPIOs through GPIO-lib and the code cannot be enabled by the user. - -Trivial implementations of those functions can directly use framework -code, which always dispatches through the gpio_chip: - - #define gpio_get_value __gpio_get_value - #define gpio_set_value __gpio_set_value - #define gpio_cansleep __gpio_cansleep - -Fancier implementations could instead define those as inline functions with -logic optimizing access to specific SOC-based GPIOs. For example, if the -referenced GPIO is the constant "12", getting or setting its value could -cost as little as two or three instructions, never sleeping. When such an -optimization is not possible those calls must delegate to the framework -code, costing at least a few dozen instructions. For bitbanged I/O, such -instruction savings can be significant. - -For SOCs, platform-specific code defines and registers gpio_chip instances -for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to -match chip vendor documentation, and directly match board schematics. They -may well start at zero and go up to a platform-specific limit. Such GPIOs -are normally integrated into platform initialization to make them always be -available, from arch_initcall() or earlier; they can often serve as IRQs. - - -Board Support -------------- -For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi -function devices, FPGAs or CPLDs -- most often board-specific code handles -registering controller devices and ensures that their drivers know what GPIO -numbers to use with gpiochip_add(). Their numbers often start right after -platform-specific GPIOs. - -For example, board setup code could create structures identifying the range -of GPIOs that chip will expose, and passes them to each GPIO expander chip -using platform_data. Then the chip driver's probe() routine could pass that -data to gpiochip_add(). - -Initialization order can be important. For example, when a device relies on -an I2C-based GPIO, its probe() routine should only be called after that GPIO -becomes available. That may mean the device should not be registered until -calls for that GPIO can work. One way to address such dependencies is for -such gpio_chip controllers to provide setup() and teardown() callbacks to -board specific code; those board specific callbacks would register devices -once all the necessary resources are available, and remove them later when -the GPIO controller device becomes unavailable. - - -Sysfs Interface for Userspace (OPTIONAL) -======================================== -Platforms which use the "gpiolib" implementors framework may choose to -configure a sysfs user interface to GPIOs. This is different from the -debugfs interface, since it provides control over GPIO direction and -value instead of just showing a gpio state summary. Plus, it could be -present on production systems without debugging support. - -Given appropriate hardware documentation for the system, userspace could -know for example that GPIO #23 controls the write protect line used to -protect boot loader segments in flash memory. System upgrade procedures -may need to temporarily remove that protection, first importing a GPIO, -then changing its output state, then updating the code before re-enabling -the write protection. In normal use, GPIO #23 would never be touched, -and the kernel would have no need to know about it. - -Again depending on appropriate hardware documentation, on some systems -userspace GPIO can be used to determine system configuration data that -standard kernels won't know about. And for some tasks, simple userspace -GPIO drivers could be all that the system really needs. - -Note that standard kernel drivers exist for common "LEDs and Buttons" -GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those -instead of talking directly to the GPIOs; they integrate with kernel -frameworks better than your userspace code could. - - -Paths in Sysfs --------------- -There are three kinds of entry in /sys/class/gpio: - - - Control interfaces used to get userspace control over GPIOs; - - - GPIOs themselves; and - - - GPIO controllers ("gpio_chip" instances). - -That's in addition to standard files including the "device" symlink. - -The control interfaces are write-only: - - /sys/class/gpio/ - - "export" ... Userspace may ask the kernel to export control of - a GPIO to userspace by writing its number to this file. - - Example: "echo 19 > export" will create a "gpio19" node - for GPIO #19, if that's not requested by kernel code. - - "unexport" ... Reverses the effect of exporting to userspace. - - Example: "echo 19 > unexport" will remove a "gpio19" - node exported using the "export" file. - -GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) -and have the following read/write attributes: - - /sys/class/gpio/gpioN/ - - "direction" ... reads as either "in" or "out". This value may - normally be written. Writing as "out" defaults to - initializing the value as low. To ensure glitch free - operation, values "low" and "high" may be written to - configure the GPIO as an output with that initial value. - - Note that this attribute *will not exist* if the kernel - doesn't support changing the direction of a GPIO, or - it was exported by kernel code that didn't explicitly - allow userspace to reconfigure this GPIO's direction. - - "value" ... reads as either 0 (low) or 1 (high). If the GPIO - is configured as an output, this value may be written; - any nonzero value is treated as high. - - If the pin can be configured as interrupt-generating interrupt - and if it has been configured to generate interrupts (see the - description of "edge"), you can poll(2) on that file and - poll(2) will return whenever the interrupt was triggered. If - you use poll(2), set the events POLLPRI and POLLERR. If you - use select(2), set the file descriptor in exceptfds. After - poll(2) returns, either lseek(2) to the beginning of the sysfs - file and read the new value or close the file and re-open it - to read the value. - - "edge" ... reads as either "none", "rising", "falling", or - "both". Write these strings to select the signal edge(s) - that will make poll(2) on the "value" file return. - - This file exists only if the pin can be configured as an - interrupt generating input pin. - - "active_low" ... reads as either 0 (false) or 1 (true). Write - any nonzero value to invert the value attribute both - for reading and writing. Existing and subsequent - poll(2) support configuration via the edge attribute - for "rising" and "falling" edges will follow this - setting. - -GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the -controller implementing GPIOs starting at #42) and have the following -read-only attributes: - - /sys/class/gpio/gpiochipN/ - - "base" ... same as N, the first GPIO managed by this chip - - "label" ... provided for diagnostics (not always unique) - - "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) - -Board documentation should in most cases cover what GPIOs are used for -what purposes. However, those numbers are not always stable; GPIOs on -a daughtercard might be different depending on the base board being used, -or other cards in the stack. In such cases, you may need to use the -gpiochip nodes (possibly in conjunction with schematics) to determine -the correct GPIO number to use for a given signal. - - -Exporting from Kernel code --------------------------- -Kernel code can explicitly manage exports of GPIOs which have already been -requested using gpio_request(): - - /* export the GPIO to userspace */ - int gpio_export(unsigned gpio, bool direction_may_change); - - /* reverse gpio_export() */ - void gpio_unexport(); - - /* create a sysfs link to an exported GPIO node */ - int gpio_export_link(struct device *dev, const char *name, - unsigned gpio) - - /* change the polarity of a GPIO node in sysfs */ - int gpio_sysfs_set_active_low(unsigned gpio, int value); - -After a kernel driver requests a GPIO, it may only be made available in -the sysfs interface by gpio_export(). The driver can control whether the -signal direction may change. This helps drivers prevent userspace code -from accidentally clobbering important system state. - -This explicit exporting can help with debugging (by making some kinds -of experiments easier), or can provide an always-there interface that's -suitable for documenting as part of a board support package. - -After the GPIO has been exported, gpio_export_link() allows creating -symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can -use this to provide the interface under their own device in sysfs with -a descriptive name. - -Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity -differences between boards from user space. This only affects the -sysfs interface. Polarity change can be done both before and after -gpio_export(), and previously enabled poll(2) support for either -rising or falling edge will be reconfigured to follow this setting. |
