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diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt new file mode 100644 index 0000000..09dd510 --- /dev/null +++ b/Documentation/gpio.txt @@ -0,0 +1,271 @@ +GPIO Interfaces + +This provides an overview of GPIO access conventions on Linux. + + +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. + + - 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 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 define an 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. + +That said, if the convention is supported on their platform, drivers should +use it when possible: + + #include <asm/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. + + +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. + +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. + +Whether a platform supports multiple GPIO controllers is currently a +platform-specific implementation issue. + + +Using GPIOs +----------- +One of the first 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); + +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. (These calls could sleep.) + +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. +That doesn't need to sleep, and can safely be done from inside IRQ handlers. + +Use these calls to access such GPIOs: + + /* 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 wire-OR and output latencies. + +The get/set calls have no error returns because "invalid GPIO" should have +been reported earlier in gpio_set_direction(). However, note that not all +platforms can read the value of output pins; those that can't should always +return zero. Also, these calls will be ignored for GPIOs that can't safely +be accessed wihtout sleeping (see below). + +Platform-specific implementations are encouraged to optimise 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: + + 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); + +Other than the fact that these calls might sleep, and will not be ignored +for GPIOs that can't be accessed from IRQ handlers, these calls act the +same as the spinlock-safe calls. + + +Claiming and Releasing GPIOs (OPTIONAL) +--------------------------------------- +To help catch system configuration errors, two calls are defined. +However, many platforms don't currently support this mechanism. + + /* 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. (These calls could sleep.) + +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 between drivers, reporting +errors when drivers wrongly think they have exclusive use of that signal. + +These two calls are optional because not not all current Linux platforms +offer such functionality in their GPIO support; a valid implementation +could return success for all gpio_request() calls. Unlike the other calls, +the state they represent doesn't normally match anything from a hardware +register; it's just a software bitmap which clearly is not necessary for +correct operation of hardware or (bug free) drivers. + +Note that requesting a GPIO does NOT cause it to be configured in any +way; it just marks that GPIO as in use. Separate code must handle any +pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown). + + +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 */ + 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 used as IRQs.) It is an unchecked error to use a GPIO +number that hasn't been marked as an input using gpio_set_direction(), 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(). + + + +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. + +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 dependednt: for GPIOs sharing the same bank. (GPIOs are +commonly grouped in banks of 16 or 32, with a given SOC having several such +banks.) Code relying on such mechanisms will necessarily be nonportable. + +Dynamic definition of GPIOs is not currently supported; for example, as +a side effect of configuring an add-on board with some GPIO expanders. + +These calls are purely for kernel space, but a userspace API could be built +on top of it. |
