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David Brownell4c203862007-02-12 00:53:11 -08001GPIO Interfaces
2
3This provides an overview of GPIO access conventions on Linux.
4
David Brownell7560fa62008-03-04 14:28:27 -08005These calls use the gpio_* naming prefix. No other calls should use that
6prefix, or the related __gpio_* prefix.
7
David Brownell4c203862007-02-12 00:53:11 -08008
9What is a GPIO?
10===============
11A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
12digital signal. They are provided from many kinds of chip, and are familiar
13to Linux developers working with embedded and custom hardware. Each GPIO
14represents a bit connected to a particular pin, or "ball" on Ball Grid Array
15(BGA) packages. Board schematics show which external hardware connects to
16which GPIOs. Drivers can be written generically, so that board setup code
17passes such pin configuration data to drivers.
18
19System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
20non-dedicated pin can be configured as a GPIO; and most chips have at least
21several dozen of them. Programmable logic devices (like FPGAs) can easily
22provide GPIOs; multifunction chips like power managers, and audio codecs
23often have a few such pins to help with pin scarcity on SOCs; and there are
24also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
25Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
26firmware knowing how they're used).
27
28The exact capabilities of GPIOs vary between systems. Common options:
29
30 - Output values are writable (high=1, low=0). Some chips also have
31 options about how that value is driven, so that for example only one
32 value might be driven ... supporting "wire-OR" and similar schemes
David Brownell1668be72007-04-11 23:28:42 -070033 for the other value (notably, "open drain" signaling).
David Brownell4c203862007-02-12 00:53:11 -080034
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
David Brownell7c2db752008-02-04 22:28:21 -080038 input de-glitch/debounce logic, sometimes with software controls.
David Brownell4c203862007-02-12 00:53:11 -080039
40 - Inputs can often be used as IRQ signals, often edge triggered but
41 sometimes level triggered. Such IRQs may be configurable as system
42 wakeup events, to wake the system from a low power state.
43
44 - Usually a GPIO will be configurable as either input or output, as needed
45 by different product boards; single direction ones exist too.
46
47 - Most GPIOs can be accessed while holding spinlocks, but those accessed
48 through a serial bus normally can't. Some systems support both types.
49
50On a given board each GPIO is used for one specific purpose like monitoring
51MMC/SD card insertion/removal, detecting card writeprotect status, driving
52a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
53watchdog, sensing a switch, and so on.
54
55
56GPIO conventions
57================
58Note that this is called a "convention" because you don't need to do it this
59way, and it's no crime if you don't. There **are** cases where portability
60is not the main issue; GPIOs are often used for the kind of board-specific
61glue logic that may even change between board revisions, and can't ever be
62used on a board that's wired differently. Only least-common-denominator
63functionality can be very portable. Other features are platform-specific,
64and that can be critical for glue logic.
65
David Brownell7c2db752008-02-04 22:28:21 -080066Plus, this doesn't require any implementation framework, just an interface.
David Brownell4c203862007-02-12 00:53:11 -080067One platform might implement it as simple inline functions accessing chip
68registers; another might implement it by delegating through abstractions
David Brownell7c2db752008-02-04 22:28:21 -080069used for several very different kinds of GPIO controller. (There is some
70optional code supporting such an implementation strategy, described later
71in this document, but drivers acting as clients to the GPIO interface must
72not care how it's implemented.)
David Brownell4c203862007-02-12 00:53:11 -080073
74That said, if the convention is supported on their platform, drivers should
David Brownell7560fa62008-03-04 14:28:27 -080075use it when possible. Platforms must declare GENERIC_GPIO support in their
76Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't
77work without standard GPIO calls should have Kconfig entries which depend
78on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as
79optimized-away stubs, when drivers use the include file:
David Brownell4c203862007-02-12 00:53:11 -080080
David Brownell7560fa62008-03-04 14:28:27 -080081 #include <linux/gpio.h>
David Brownell4c203862007-02-12 00:53:11 -080082
83If you stick to this convention then it'll be easier for other developers to
84see what your code is doing, and help maintain it.
85
David Brownella0a99832007-07-19 01:47:52 -070086Note that these operations include I/O barriers on platforms which need to
87use them; drivers don't need to add them explicitly.
88
David Brownell4c203862007-02-12 00:53:11 -080089
90Identifying GPIOs
91-----------------
92GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
93reserves "negative" numbers for other purposes like marking signals as
David Brownellf5de6112007-02-16 01:27:14 -080094"not available on this board", or indicating faults. Code that doesn't
95touch the underlying hardware treats these integers as opaque cookies.
David Brownell4c203862007-02-12 00:53:11 -080096
97Platforms define how they use those integers, and usually #define symbols
98for the GPIO lines so that board-specific setup code directly corresponds
99to the relevant schematics. In contrast, drivers should only use GPIO
100numbers passed to them from that setup code, using platform_data to hold
101board-specific pin configuration data (along with other board specific
102data they need). That avoids portability problems.
103
104So for example one platform uses numbers 32-159 for GPIOs; while another
105uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
106type of GPIO controller, and on one particular board 80-95 with an FPGA.
107The numbers need not be contiguous; either of those platforms could also
108use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
109
Guennadi Liakhovetskie6de1802008-04-28 02:14:46 -0700110If you want to initialize a structure with an invalid GPIO number, use
111some negative number (perhaps "-EINVAL"); that will never be valid. To
112test if a number could reference a GPIO, you may use this predicate:
113
114 int gpio_is_valid(int number);
115
116A number that's not valid will be rejected by calls which may request
117or free GPIOs (see below). Other numbers may also be rejected; for
118example, a number might be valid but unused on a given board.
119
David Brownell4c203862007-02-12 00:53:11 -0800120Whether a platform supports multiple GPIO controllers is currently a
121platform-specific implementation issue.
122
123
124Using GPIOs
125-----------
David Brownell8a0cecf2009-04-02 16:57:06 -0700126The first thing a system should do with a GPIO is allocate it, using
127the gpio_request() call; see later.
128
129One of the next things to do with a GPIO, often in board setup code when
David Brownell4c203862007-02-12 00:53:11 -0800130setting 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);
David Brownell28735a72007-03-16 13:38:14 -0800134 int gpio_direction_output(unsigned gpio, int value);
David Brownell4c203862007-02-12 00:53:11 -0800135
136The return value is zero for success, else a negative errno. It should
137be checked, since the get/set calls don't have error returns and since
David Brownell83c65902007-05-16 22:11:13 -0700138misconfiguration is possible. You should normally issue these calls from
139a task context. However, for spinlock-safe GPIOs it's OK to use them
140before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800141
David Brownell28735a72007-03-16 13:38:14 -0800142For output GPIOs, the value provided becomes the initial output value.
143This helps avoid signal glitching during system startup.
144
David Brownell7c2db752008-02-04 22:28:21 -0800145For compatibility with legacy interfaces to GPIOs, setting the direction
146of a GPIO implicitly requests that GPIO (see below) if it has not been
David Brownell8a0cecf2009-04-02 16:57:06 -0700147requested already. That compatibility is being removed from the optional
148gpiolib framework.
David Brownell7c2db752008-02-04 22:28:21 -0800149
David Brownell4c203862007-02-12 00:53:11 -0800150Setting the direction can fail if the GPIO number is invalid, or when
151that particular GPIO can't be used in that mode. It's generally a bad
152idea to rely on boot firmware to have set the direction correctly, since
153it probably wasn't validated to do more than boot Linux. (Similarly,
154that board setup code probably needs to multiplex that pin as a GPIO,
155and configure pullups/pulldowns appropriately.)
156
157
158Spinlock-Safe GPIO access
159-------------------------
160Most GPIO controllers can be accessed with memory read/write instructions.
David Brownell9c4ba942010-08-10 18:02:24 -0700161Those don't need to sleep, and can safely be done from inside hard
162(nonthreaded) IRQ handlers and similar contexts.
David Brownell4c203862007-02-12 00:53:11 -0800163
David Brownell9c4ba942010-08-10 18:02:24 -0700164Use the following calls to access such GPIOs,
165for which gpio_cansleep() will always return false (see below):
David Brownell4c203862007-02-12 00:53:11 -0800166
167 /* GPIO INPUT: return zero or nonzero */
168 int gpio_get_value(unsigned gpio);
169
170 /* GPIO OUTPUT */
171 void gpio_set_value(unsigned gpio, int value);
172
173The values are boolean, zero for low, nonzero for high. When reading the
174value of an output pin, the value returned should be what's seen on the
175pin ... that won't always match the specified output value, because of
David Brownell7c2db752008-02-04 22:28:21 -0800176issues including open-drain signaling and output latencies.
David Brownell4c203862007-02-12 00:53:11 -0800177
178The get/set calls have no error returns because "invalid GPIO" should have
David Brownellbe1ff382007-07-23 18:43:57 -0700179been reported earlier from gpio_direction_*(). However, note that not all
David Brownell4c203862007-02-12 00:53:11 -0800180platforms can read the value of output pins; those that can't should always
David Brownellf5de6112007-02-16 01:27:14 -0800181return zero. Also, using these calls for GPIOs that can't safely be accessed
182without sleeping (see below) is an error.
David Brownell4c203862007-02-12 00:53:11 -0800183
David Brownellf5de6112007-02-16 01:27:14 -0800184Platform-specific implementations are encouraged to optimize the two
David Brownell4c203862007-02-12 00:53:11 -0800185calls to access the GPIO value in cases where the GPIO number (and for
186output, value) are constant. It's normal for them to need only a couple
187of instructions in such cases (reading or writing a hardware register),
188and not to need spinlocks. Such optimized calls can make bitbanging
189applications a lot more efficient (in both space and time) than spending
190dozens of instructions on subroutine calls.
191
192
193GPIO access that may sleep
194--------------------------
195Some GPIO controllers must be accessed using message based busses like I2C
196or SPI. Commands to read or write those GPIO values require waiting to
197get to the head of a queue to transmit a command and get its response.
198This requires sleeping, which can't be done from inside IRQ handlers.
199
200Platforms that support this type of GPIO distinguish them from other GPIOs
David Brownell7c2db752008-02-04 22:28:21 -0800201by returning nonzero from this call (which requires a valid GPIO number,
David Brownell8a0cecf2009-04-02 16:57:06 -0700202which should have been previously allocated with gpio_request):
David Brownell4c203862007-02-12 00:53:11 -0800203
204 int gpio_cansleep(unsigned gpio);
205
206To access such GPIOs, a different set of accessors is defined:
207
208 /* GPIO INPUT: return zero or nonzero, might sleep */
209 int gpio_get_value_cansleep(unsigned gpio);
210
211 /* GPIO OUTPUT, might sleep */
212 void gpio_set_value_cansleep(unsigned gpio, int value);
213
David Brownell9c4ba942010-08-10 18:02:24 -0700214
215Accessing such GPIOs requires a context which may sleep, for example
216a threaded IRQ handler, and those accessors must be used instead of
217spinlock-safe accessors without the cansleep() name suffix.
218
219Other than the fact that these accessors might sleep, and will work
220on GPIOs that can't be accessed from hardIRQ handlers, these calls act
221the same as the spinlock-safe calls.
222
223 ** IN ADDITION ** calls to setup and configure such GPIOs must be made
224from contexts which may sleep, since they may need to access the GPIO
225controller chip too: (These setup calls are usually made from board
226setup or driver probe/teardown code, so this is an easy constraint.)
227
228 gpio_direction_input()
229 gpio_direction_output()
230 gpio_request()
231
232## gpio_request_one()
233## gpio_request_array()
234## gpio_free_array()
235
236 gpio_free()
237 gpio_set_debounce()
238
David Brownell4c203862007-02-12 00:53:11 -0800239
240
David Brownell8a0cecf2009-04-02 16:57:06 -0700241Claiming and Releasing GPIOs
242----------------------------
David Brownell4c203862007-02-12 00:53:11 -0800243To help catch system configuration errors, two calls are defined.
David Brownell4c203862007-02-12 00:53:11 -0800244
245 /* request GPIO, returning 0 or negative errno.
246 * non-null labels may be useful for diagnostics.
247 */
248 int gpio_request(unsigned gpio, const char *label);
249
250 /* release previously-claimed GPIO */
251 void gpio_free(unsigned gpio);
252
253Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
254GPIOs that have already been claimed with that call. The return value of
David Brownell83c65902007-05-16 22:11:13 -0700255gpio_request() must be checked. You should normally issue these calls from
256a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
257before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800258
259These calls serve two basic purposes. One is marking the signals which
260are actually in use as GPIOs, for better diagnostics; systems may have
261several hundred potential GPIOs, but often only a dozen are used on any
David Brownell7c2db752008-02-04 22:28:21 -0800262given board. Another is to catch conflicts, identifying errors when
263(a) two or more drivers wrongly think they have exclusive use of that
264signal, or (b) something wrongly believes it's safe to remove drivers
265needed to manage a signal that's in active use. That is, requesting a
266GPIO can serve as a kind of lock.
David Brownell4c203862007-02-12 00:53:11 -0800267
David Brownell35e8bb52008-10-15 22:03:16 -0700268Some platforms may also use knowledge about what GPIOs are active for
269power management, such as by powering down unused chip sectors and, more
270easily, gating off unused clocks.
271
David Brownell4c203862007-02-12 00:53:11 -0800272Note that requesting a GPIO does NOT cause it to be configured in any
273way; it just marks that GPIO as in use. Separate code must handle any
274pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
275
David Brownell7c2db752008-02-04 22:28:21 -0800276Also note that it's your responsibility to have stopped using a GPIO
277before you free it.
278
Eric Miao3e45f1d2010-03-05 13:44:35 -0800279Considering in most cases GPIOs are actually configured right after they
280are claimed, three additional calls are defined:
281
282 /* request a single GPIO, with initial configuration specified by
283 * 'flags', identical to gpio_request() wrt other arguments and
284 * return value
285 */
286 int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
287
288 /* request multiple GPIOs in a single call
289 */
290 int gpio_request_array(struct gpio *array, size_t num);
291
292 /* release multiple GPIOs in a single call
293 */
294 void gpio_free_array(struct gpio *array, size_t num);
295
296where 'flags' is currently defined to specify the following properties:
297
298 * GPIOF_DIR_IN - to configure direction as input
299 * GPIOF_DIR_OUT - to configure direction as output
300
301 * GPIOF_INIT_LOW - as output, set initial level to LOW
302 * GPIOF_INIT_HIGH - as output, set initial level to HIGH
303
304since GPIOF_INIT_* are only valid when configured as output, so group valid
305combinations as:
306
307 * GPIOF_IN - configure as input
308 * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
309 * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
310
311In the future, these flags can be extended to support more properties such
312as open-drain status.
313
314Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
315introduced to encapsulate all three fields as:
316
317 struct gpio {
318 unsigned gpio;
319 unsigned long flags;
320 const char *label;
321 };
322
323A typical example of usage:
324
325 static struct gpio leds_gpios[] = {
326 { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
327 { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
328 { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
329 { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
330 { ... },
331 };
332
333 err = gpio_request_one(31, GPIOF_IN, "Reset Button");
334 if (err)
335 ...
336
337 err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
338 if (err)
339 ...
340
341 gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
342
David Brownell4c203862007-02-12 00:53:11 -0800343
344GPIOs mapped to IRQs
345--------------------
346GPIO numbers are unsigned integers; so are IRQ numbers. These make up
347two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
348map between them using calls like:
349
350 /* map GPIO numbers to IRQ numbers */
351 int gpio_to_irq(unsigned gpio);
352
David Brownell0f6d5042008-10-15 22:03:14 -0700353 /* map IRQ numbers to GPIO numbers (avoid using this) */
David Brownell4c203862007-02-12 00:53:11 -0800354 int irq_to_gpio(unsigned irq);
355
356Those return either the corresponding number in the other namespace, or
357else a negative errno code if the mapping can't be done. (For example,
David Brownell7c2db752008-02-04 22:28:21 -0800358some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
David Brownellbe1ff382007-07-23 18:43:57 -0700359number that wasn't set up as an input using gpio_direction_input(), or
David Brownell4c203862007-02-12 00:53:11 -0800360to use an IRQ number that didn't originally come from gpio_to_irq().
361
362These two mapping calls are expected to cost on the order of a single
363addition or subtraction. They're not allowed to sleep.
364
365Non-error values returned from gpio_to_irq() can be passed to request_irq()
366or free_irq(). They will often be stored into IRQ resources for platform
367devices, by the board-specific initialization code. Note that IRQ trigger
368options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
369system wakeup capabilities.
370
371Non-error values returned from irq_to_gpio() would most commonly be used
David Brownellf5de6112007-02-16 01:27:14 -0800372with gpio_get_value(), for example to initialize or update driver state
David Brownell0f6d5042008-10-15 22:03:14 -0700373when the IRQ is edge-triggered. Note that some platforms don't support
374this reverse mapping, so you should avoid using it.
David Brownell4c203862007-02-12 00:53:11 -0800375
376
David Brownell1668be72007-04-11 23:28:42 -0700377Emulating Open Drain Signals
378----------------------------
379Sometimes shared signals need to use "open drain" signaling, where only the
380low signal level is actually driven. (That term applies to CMOS transistors;
381"open collector" is used for TTL.) A pullup resistor causes the high signal
382level. This is sometimes called a "wire-AND"; or more practically, from the
383negative logic (low=true) perspective this is a "wire-OR".
384
385One common example of an open drain signal is a shared active-low IRQ line.
386Also, bidirectional data bus signals sometimes use open drain signals.
387
388Some GPIO controllers directly support open drain outputs; many don't. When
389you need open drain signaling but your hardware doesn't directly support it,
390there's a common idiom you can use to emulate it with any GPIO pin that can
391be used as either an input or an output:
392
393 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
394 and overrides the pullup.
395
396 HIGH: gpio_direction_input(gpio) ... this turns off the output,
397 so the pullup (or some other device) controls the signal.
398
399If you are "driving" the signal high but gpio_get_value(gpio) reports a low
400value (after the appropriate rise time passes), you know some other component
401is driving the shared signal low. That's not necessarily an error. As one
402common example, that's how I2C clocks are stretched: a slave that needs a
403slower clock delays the rising edge of SCK, and the I2C master adjusts its
404signaling rate accordingly.
405
David Brownell4c203862007-02-12 00:53:11 -0800406
407What do these conventions omit?
408===============================
409One of the biggest things these conventions omit is pin multiplexing, since
410this is highly chip-specific and nonportable. One platform might not need
411explicit multiplexing; another might have just two options for use of any
412given pin; another might have eight options per pin; another might be able
413to route a given GPIO to any one of several pins. (Yes, those examples all
414come from systems that run Linux today.)
415
416Related to multiplexing is configuration and enabling of the pullups or
417pulldowns integrated on some platforms. Not all platforms support them,
418or support them in the same way; and any given board might use external
419pullups (or pulldowns) so that the on-chip ones should not be used.
David Brownell7c2db752008-02-04 22:28:21 -0800420(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
David Brownell7560fa62008-03-04 14:28:27 -0800421Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
422platform-specific issue, as are models like (not) having a one-to-one
423correspondence between configurable pins and GPIOs.
David Brownell4c203862007-02-12 00:53:11 -0800424
425There are other system-specific mechanisms that are not specified here,
426like the aforementioned options for input de-glitching and wire-OR output.
427Hardware may support reading or writing GPIOs in gangs, but that's usually
David Brownellf5de6112007-02-16 01:27:14 -0800428configuration dependent: for GPIOs sharing the same bank. (GPIOs are
David Brownell4c203862007-02-12 00:53:11 -0800429commonly grouped in banks of 16 or 32, with a given SOC having several such
David Brownell7c2db752008-02-04 22:28:21 -0800430banks.) Some systems can trigger IRQs from output GPIOs, or read values
431from pins not managed as GPIOs. Code relying on such mechanisms will
432necessarily be nonportable.
David Brownell4c203862007-02-12 00:53:11 -0800433
David Brownell7c2db752008-02-04 22:28:21 -0800434Dynamic definition of GPIOs is not currently standard; for example, as
David Brownell4c203862007-02-12 00:53:11 -0800435a side effect of configuring an add-on board with some GPIO expanders.
436
David Brownell7c2db752008-02-04 22:28:21 -0800437
438GPIO implementor's framework (OPTIONAL)
439=======================================
440As noted earlier, there is an optional implementation framework making it
441easier for platforms to support different kinds of GPIO controller using
David Brownelld8f388d2008-07-25 01:46:07 -0700442the same programming interface. This framework is called "gpiolib".
David Brownell7c2db752008-02-04 22:28:21 -0800443
444As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
445will be found there. That will list all the controllers registered through
446this framework, and the state of the GPIOs currently in use.
447
448
449Controller Drivers: gpio_chip
450-----------------------------
451In this framework each GPIO controller is packaged as a "struct gpio_chip"
452with information common to each controller of that type:
453
454 - methods to establish GPIO direction
455 - methods used to access GPIO values
456 - flag saying whether calls to its methods may sleep
457 - optional debugfs dump method (showing extra state like pullup config)
458 - label for diagnostics
459
460There is also per-instance data, which may come from device.platform_data:
461the number of its first GPIO, and how many GPIOs it exposes.
462
463The code implementing a gpio_chip should support multiple instances of the
464controller, possibly using the driver model. That code will configure each
465gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
466rare; use gpiochip_remove() when it is unavoidable.
467
468Most often a gpio_chip is part of an instance-specific structure with state
469not exposed by the GPIO interfaces, such as addressing, power management,
Dominik D. Geyerbfc9dca2009-12-06 18:30:44 -0800470and more. Chips such as codecs will have complex non-GPIO state.
David Brownell7c2db752008-02-04 22:28:21 -0800471
472Any debugfs dump method should normally ignore signals which haven't been
473requested as GPIOs. They can use gpiochip_is_requested(), which returns
474either NULL or the label associated with that GPIO when it was requested.
475
476
477Platform Support
478----------------
Michael Buesch7444a722008-07-25 01:46:11 -0700479To support this framework, a platform's Kconfig will "select" either
480ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
David Brownell7c2db752008-02-04 22:28:21 -0800481and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
482three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
483They may also want to provide a custom value for ARCH_NR_GPIOS.
484
Michael Buesch7444a722008-07-25 01:46:11 -0700485ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled
486into the kernel on that architecture.
487
488ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user
489can enable it and build it into the kernel optionally.
490
491If neither of these options are selected, the platform does not support
492GPIOs through GPIO-lib and the code cannot be enabled by the user.
493
David Brownell7c2db752008-02-04 22:28:21 -0800494Trivial implementations of those functions can directly use framework
495code, which always dispatches through the gpio_chip:
496
497 #define gpio_get_value __gpio_get_value
498 #define gpio_set_value __gpio_set_value
499 #define gpio_cansleep __gpio_cansleep
500
501Fancier implementations could instead define those as inline functions with
502logic optimizing access to specific SOC-based GPIOs. For example, if the
503referenced GPIO is the constant "12", getting or setting its value could
504cost as little as two or three instructions, never sleeping. When such an
505optimization is not possible those calls must delegate to the framework
506code, costing at least a few dozen instructions. For bitbanged I/O, such
507instruction savings can be significant.
508
509For SOCs, platform-specific code defines and registers gpio_chip instances
510for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
511match chip vendor documentation, and directly match board schematics. They
512may well start at zero and go up to a platform-specific limit. Such GPIOs
513are normally integrated into platform initialization to make them always be
514available, from arch_initcall() or earlier; they can often serve as IRQs.
515
516
517Board Support
518-------------
519For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
520function devices, FPGAs or CPLDs -- most often board-specific code handles
521registering controller devices and ensures that their drivers know what GPIO
522numbers to use with gpiochip_add(). Their numbers often start right after
523platform-specific GPIOs.
524
525For example, board setup code could create structures identifying the range
526of GPIOs that chip will expose, and passes them to each GPIO expander chip
527using platform_data. Then the chip driver's probe() routine could pass that
528data to gpiochip_add().
529
530Initialization order can be important. For example, when a device relies on
531an I2C-based GPIO, its probe() routine should only be called after that GPIO
532becomes available. That may mean the device should not be registered until
533calls for that GPIO can work. One way to address such dependencies is for
534such gpio_chip controllers to provide setup() and teardown() callbacks to
535board specific code; those board specific callbacks would register devices
David Brownelld8f388d2008-07-25 01:46:07 -0700536once all the necessary resources are available, and remove them later when
537the GPIO controller device becomes unavailable.
538
539
540Sysfs Interface for Userspace (OPTIONAL)
541========================================
542Platforms which use the "gpiolib" implementors framework may choose to
543configure a sysfs user interface to GPIOs. This is different from the
544debugfs interface, since it provides control over GPIO direction and
545value instead of just showing a gpio state summary. Plus, it could be
546present on production systems without debugging support.
547
Matt LaPlante19f59462009-04-27 15:06:31 +0200548Given appropriate hardware documentation for the system, userspace could
David Brownelld8f388d2008-07-25 01:46:07 -0700549know for example that GPIO #23 controls the write protect line used to
550protect boot loader segments in flash memory. System upgrade procedures
551may need to temporarily remove that protection, first importing a GPIO,
552then changing its output state, then updating the code before re-enabling
553the write protection. In normal use, GPIO #23 would never be touched,
554and the kernel would have no need to know about it.
555
556Again depending on appropriate hardware documentation, on some systems
557userspace GPIO can be used to determine system configuration data that
558standard kernels won't know about. And for some tasks, simple userspace
559GPIO drivers could be all that the system really needs.
560
561Note that standard kernel drivers exist for common "LEDs and Buttons"
562GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
563instead of talking directly to the GPIOs; they integrate with kernel
564frameworks better than your userspace code could.
565
566
567Paths in Sysfs
568--------------
569There are three kinds of entry in /sys/class/gpio:
570
571 - Control interfaces used to get userspace control over GPIOs;
572
573 - GPIOs themselves; and
574
575 - GPIO controllers ("gpio_chip" instances).
576
577That's in addition to standard files including the "device" symlink.
578
579The control interfaces are write-only:
580
581 /sys/class/gpio/
582
583 "export" ... Userspace may ask the kernel to export control of
584 a GPIO to userspace by writing its number to this file.
585
586 Example: "echo 19 > export" will create a "gpio19" node
587 for GPIO #19, if that's not requested by kernel code.
588
589 "unexport" ... Reverses the effect of exporting to userspace.
590
591 Example: "echo 19 > unexport" will remove a "gpio19"
592 node exported using the "export" file.
593
594GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
595and have the following read/write attributes:
596
597 /sys/class/gpio/gpioN/
598
599 "direction" ... reads as either "in" or "out". This value may
600 normally be written. Writing as "out" defaults to
601 initializing the value as low. To ensure glitch free
602 operation, values "low" and "high" may be written to
603 configure the GPIO as an output with that initial value.
604
605 Note that this attribute *will not exist* if the kernel
606 doesn't support changing the direction of a GPIO, or
607 it was exported by kernel code that didn't explicitly
608 allow userspace to reconfigure this GPIO's direction.
609
610 "value" ... reads as either 0 (low) or 1 (high). If the GPIO
611 is configured as an output, this value may be written;
612 any nonzero value is treated as high.
613
Daniel Glöcknerff77c352009-09-22 16:46:38 -0700614 "edge" ... reads as either "none", "rising", "falling", or
615 "both". Write these strings to select the signal edge(s)
616 that will make poll(2) on the "value" file return.
617
618 This file exists only if the pin can be configured as an
619 interrupt generating input pin.
620
Jani Nikula07697462009-12-15 16:46:20 -0800621 "active_low" ... reads as either 0 (false) or 1 (true). Write
622 any nonzero value to invert the value attribute both
623 for reading and writing. Existing and subsequent
624 poll(2) support configuration via the edge attribute
625 for "rising" and "falling" edges will follow this
626 setting.
627
Dominik D. Geyerbfc9dca2009-12-06 18:30:44 -0800628GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
David Brownelld8f388d2008-07-25 01:46:07 -0700629controller implementing GPIOs starting at #42) and have the following
630read-only attributes:
631
632 /sys/class/gpio/gpiochipN/
633
634 "base" ... same as N, the first GPIO managed by this chip
635
636 "label" ... provided for diagnostics (not always unique)
637
638 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
639
640Board documentation should in most cases cover what GPIOs are used for
641what purposes. However, those numbers are not always stable; GPIOs on
642a daughtercard might be different depending on the base board being used,
643or other cards in the stack. In such cases, you may need to use the
644gpiochip nodes (possibly in conjunction with schematics) to determine
645the correct GPIO number to use for a given signal.
646
647
648Exporting from Kernel code
649--------------------------
650Kernel code can explicitly manage exports of GPIOs which have already been
651requested using gpio_request():
652
653 /* export the GPIO to userspace */
654 int gpio_export(unsigned gpio, bool direction_may_change);
655
656 /* reverse gpio_export() */
657 void gpio_unexport();
658
Jani Nikulaa4177ee2009-09-22 16:46:33 -0700659 /* create a sysfs link to an exported GPIO node */
660 int gpio_export_link(struct device *dev, const char *name,
661 unsigned gpio)
662
Jani Nikula07697462009-12-15 16:46:20 -0800663 /* change the polarity of a GPIO node in sysfs */
664 int gpio_sysfs_set_active_low(unsigned gpio, int value);
Jani Nikulaa4177ee2009-09-22 16:46:33 -0700665
David Brownelld8f388d2008-07-25 01:46:07 -0700666After a kernel driver requests a GPIO, it may only be made available in
667the sysfs interface by gpio_export(). The driver can control whether the
668signal direction may change. This helps drivers prevent userspace code
669from accidentally clobbering important system state.
670
671This explicit exporting can help with debugging (by making some kinds
672of experiments easier), or can provide an always-there interface that's
673suitable for documenting as part of a board support package.
Jani Nikulaa4177ee2009-09-22 16:46:33 -0700674
675After the GPIO has been exported, gpio_export_link() allows creating
676symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
677use this to provide the interface under their own device in sysfs with
678a descriptive name.
Jani Nikula07697462009-12-15 16:46:20 -0800679
680Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
681differences between boards from user space. This only affects the
682sysfs interface. Polarity change can be done both before and after
683gpio_export(), and previously enabled poll(2) support for either
684rising or falling edge will be reconfigured to follow this setting.