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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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 select ARCH_REQUIRE_GPIOLIB or
76	ARCH_WANT_OPTIONAL_GPIOLIB in their Kconfig.  Drivers that can't work without
77	standard GPIO calls should have Kconfig entries which depend on GPIOLIB.  The
78	GPIO calls are available, either as "real code" or as optimized-away stubs,
79	when drivers use the include file:
80	
81		#include <linux/gpio.h>
82	
83	If you stick to this convention then it'll be easier for other developers to
84	see what your code is doing, and help maintain it.
85	
86	Note that these operations include I/O barriers on platforms which need to
87	use them; drivers don't need to add them explicitly.
88	
89	
90	Identifying GPIOs
91	-----------------
92	GPIOs are identified by unsigned integers in the range 0..MAX_INT.  That
93	reserves "negative" numbers for other purposes like marking signals as
94	"not available on this board", or indicating faults.  Code that doesn't
95	touch the underlying hardware treats these integers as opaque cookies.
96	
97	Platforms define how they use those integers, and usually #define symbols
98	for the GPIO lines so that board-specific setup code directly corresponds
99	to the relevant schematics.  In contrast, drivers should only use GPIO
100	numbers passed to them from that setup code, using platform_data to hold
101	board-specific pin configuration data (along with other board specific
102	data they need).  That avoids portability problems.
103	
104	So for example one platform uses numbers 32-159 for GPIOs; while another
105	uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
106	type of GPIO controller, and on one particular board 80-95 with an FPGA.
107	The numbers need not be contiguous; either of those platforms could also
108	use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
109	
110	If you want to initialize a structure with an invalid GPIO number, use
111	some negative number (perhaps "-EINVAL"); that will never be valid.  To
112	test if such number from such a structure could reference a GPIO, you
113	may use this predicate:
114	
115		int gpio_is_valid(int number);
116	
117	A number that's not valid will be rejected by calls which may request
118	or free GPIOs (see below).  Other numbers may also be rejected; for
119	example, a number might be valid but temporarily unused on a given board.
120	
121	Whether a platform supports multiple GPIO controllers is a platform-specific
122	implementation issue, as are whether that support can leave "holes" in the space
123	of GPIO numbers, and whether new controllers can be added at runtime.  Such issues
124	can affect things including whether adjacent GPIO numbers are both valid.
125	
126	Using GPIOs
127	-----------
128	The first thing a system should do with a GPIO is allocate it, using
129	the gpio_request() call; see later.
130	
131	One of the next things to do with a GPIO, often in board setup code when
132	setting up a platform_device using the GPIO, is mark its direction:
133	
134		/* set as input or output, returning 0 or negative errno */
135		int gpio_direction_input(unsigned gpio);
136		int gpio_direction_output(unsigned gpio, int value);
137	
138	The return value is zero for success, else a negative errno.  It should
139	be checked, since the get/set calls don't have error returns and since
140	misconfiguration is possible.  You should normally issue these calls from
141	a task context.  However, for spinlock-safe GPIOs it's OK to use them
142	before tasking is enabled, as part of early board setup.
143	
144	For output GPIOs, the value provided becomes the initial output value.
145	This helps avoid signal glitching during system startup.
146	
147	For compatibility with legacy interfaces to GPIOs, setting the direction
148	of a GPIO implicitly requests that GPIO (see below) if it has not been
149	requested already.  That compatibility is being removed from the optional
150	gpiolib framework.
151	
152	Setting the direction can fail if the GPIO number is invalid, or when
153	that particular GPIO can't be used in that mode.  It's generally a bad
154	idea to rely on boot firmware to have set the direction correctly, since
155	it probably wasn't validated to do more than boot Linux.  (Similarly,
156	that board setup code probably needs to multiplex that pin as a GPIO,
157	and configure pullups/pulldowns appropriately.)
158	
159	
160	Spinlock-Safe GPIO access
161	-------------------------
162	Most GPIO controllers can be accessed with memory read/write instructions.
163	Those don't need to sleep, and can safely be done from inside hard
164	(nonthreaded) IRQ handlers and similar contexts.
165	
166	Use the following calls to access such GPIOs,
167	for which gpio_cansleep() will always return false (see below):
168	
169		/* GPIO INPUT:  return zero or nonzero */
170		int gpio_get_value(unsigned gpio);
171	
172		/* GPIO OUTPUT */
173		void gpio_set_value(unsigned gpio, int value);
174	
175	The values are boolean, zero for low, nonzero for high.  When reading the
176	value of an output pin, the value returned should be what's seen on the
177	pin ... that won't always match the specified output value, because of
178	issues including open-drain signaling and output latencies.
179	
180	The get/set calls have no error returns because "invalid GPIO" should have
181	been reported earlier from gpio_direction_*().  However, note that not all
182	platforms can read the value of output pins; those that can't should always
183	return zero.  Also, using these calls for GPIOs that can't safely be accessed
184	without sleeping (see below) is an error.
185	
186	Platform-specific implementations are encouraged to optimize the two
187	calls to access the GPIO value in cases where the GPIO number (and for
188	output, value) are constant.  It's normal for them to need only a couple
189	of instructions in such cases (reading or writing a hardware register),
190	and not to need spinlocks.  Such optimized calls can make bitbanging
191	applications a lot more efficient (in both space and time) than spending
192	dozens of instructions on subroutine calls.
193	
194	
195	GPIO access that may sleep
196	--------------------------
197	Some GPIO controllers must be accessed using message based busses like I2C
198	or SPI.  Commands to read or write those GPIO values require waiting to
199	get to the head of a queue to transmit a command and get its response.
200	This requires sleeping, which can't be done from inside IRQ handlers.
201	
202	Platforms that support this type of GPIO distinguish them from other GPIOs
203	by returning nonzero from this call (which requires a valid GPIO number,
204	which should have been previously allocated with gpio_request):
205	
206		int gpio_cansleep(unsigned gpio);
207	
208	To access such GPIOs, a different set of accessors is defined:
209	
210		/* GPIO INPUT:  return zero or nonzero, might sleep */
211		int gpio_get_value_cansleep(unsigned gpio);
212	
213		/* GPIO OUTPUT, might sleep */
214		void gpio_set_value_cansleep(unsigned gpio, int value);
215	
216	
217	Accessing such GPIOs requires a context which may sleep,  for example
218	a threaded IRQ handler, and those accessors must be used instead of
219	spinlock-safe accessors without the cansleep() name suffix.
220	
221	Other than the fact that these accessors might sleep, and will work
222	on GPIOs that can't be accessed from hardIRQ handlers, these calls act
223	the same as the spinlock-safe calls.
224	
225	  ** IN ADDITION ** calls to setup and configure such GPIOs must be made
226	from contexts which may sleep, since they may need to access the GPIO
227	controller chip too:  (These setup calls are usually made from board
228	setup or driver probe/teardown code, so this is an easy constraint.)
229	
230		gpio_direction_input()
231		gpio_direction_output()
232		gpio_request()
233	
234	## 	gpio_request_one()
235	##	gpio_request_array()
236	## 	gpio_free_array()
237	
238		gpio_free()
239		gpio_set_debounce()
240	
241	
242	
243	Claiming and Releasing GPIOs
244	----------------------------
245	To help catch system configuration errors, two calls are defined.
246	
247		/* request GPIO, returning 0 or negative errno.
248		 * non-null labels may be useful for diagnostics.
249		 */
250		int gpio_request(unsigned gpio, const char *label);
251	
252		/* release previously-claimed GPIO */
253		void gpio_free(unsigned gpio);
254	
255	Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
256	GPIOs that have already been claimed with that call.  The return value of
257	gpio_request() must be checked.  You should normally issue these calls from
258	a task context.  However, for spinlock-safe GPIOs it's OK to request GPIOs
259	before tasking is enabled, as part of early board setup.
260	
261	These calls serve two basic purposes.  One is marking the signals which
262	are actually in use as GPIOs, for better diagnostics; systems may have
263	several hundred potential GPIOs, but often only a dozen are used on any
264	given board.  Another is to catch conflicts, identifying errors when
265	(a) two or more drivers wrongly think they have exclusive use of that
266	signal, or (b) something wrongly believes it's safe to remove drivers
267	needed to manage a signal that's in active use.  That is, requesting a
268	GPIO can serve as a kind of lock.
269	
270	Some platforms may also use knowledge about what GPIOs are active for
271	power management, such as by powering down unused chip sectors and, more
272	easily, gating off unused clocks.
273	
274	For GPIOs that use pins known to the pinctrl subsystem, that subsystem should
275	be informed of their use; a gpiolib driver's .request() operation may call
276	pinctrl_request_gpio(), and a gpiolib driver's .free() operation may call
277	pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio()
278	to succeed concurrently with a pin or pingroup being "owned" by a device for
279	pin multiplexing.
280	
281	Any programming of pin multiplexing hardware that is needed to route the
282	GPIO signal to the appropriate pin should occur within a GPIO driver's
283	.direction_input() or .direction_output() operations, and occur after any
284	setup of an output GPIO's value. This allows a glitch-free migration from a
285	pin's special function to GPIO. This is sometimes required when using a GPIO
286	to implement a workaround on signals typically driven by a non-GPIO HW block.
287	
288	Some platforms allow some or all GPIO signals to be routed to different pins.
289	Similarly, other aspects of the GPIO or pin may need to be configured, such as
290	pullup/pulldown. Platform software should arrange that any such details are
291	configured prior to gpio_request() being called for those GPIOs, e.g. using
292	the pinctrl subsystem's mapping table, so that GPIO users need not be aware
293	of these details.
294	
295	Also note that it's your responsibility to have stopped using a GPIO
296	before you free it.
297	
298	Considering in most cases GPIOs are actually configured right after they
299	are claimed, three additional calls are defined:
300	
301		/* request a single GPIO, with initial configuration specified by
302		 * 'flags', identical to gpio_request() wrt other arguments and
303		 * return value
304		 */
305		int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
306	
307		/* request multiple GPIOs in a single call
308		 */
309		int gpio_request_array(struct gpio *array, size_t num);
310	
311		/* release multiple GPIOs in a single call
312		 */
313		void gpio_free_array(struct gpio *array, size_t num);
314	
315	where 'flags' is currently defined to specify the following properties:
316	
317		* GPIOF_DIR_IN		- to configure direction as input
318		* GPIOF_DIR_OUT		- to configure direction as output
319	
320		* GPIOF_INIT_LOW	- as output, set initial level to LOW
321		* GPIOF_INIT_HIGH	- as output, set initial level to HIGH
322		* GPIOF_OPEN_DRAIN	- gpio pin is open drain type.
323		* GPIOF_OPEN_SOURCE	- gpio pin is open source type.
324	
325		* GPIOF_EXPORT_DIR_FIXED	- export gpio to sysfs, keep direction
326		* GPIOF_EXPORT_DIR_CHANGEABLE	- also export, allow changing direction
327	
328	since GPIOF_INIT_* are only valid when configured as output, so group valid
329	combinations as:
330	
331		* GPIOF_IN		- configure as input
332		* GPIOF_OUT_INIT_LOW	- configured as output, initial level LOW
333		* GPIOF_OUT_INIT_HIGH	- configured as output, initial level HIGH
334	
335	When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is
336	open drain type. Such pins will not be driven to 1 in output mode. It is
337	require to connect pull-up on such pins. By enabling this flag, gpio lib will
338	make the direction to input when it is asked to set value of 1 in output mode
339	to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
340	
341	When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is
342	open source type. Such pins will not be driven to 0 in output mode. It is
343	require to connect pull-down on such pin. By enabling this flag, gpio lib will
344	make the direction to input when it is asked to set value of 0 in output mode
345	to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
346	
347	In the future, these flags can be extended to support more properties.
348	
349	Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
350	introduced to encapsulate all three fields as:
351	
352		struct gpio {
353			unsigned	gpio;
354			unsigned long	flags;
355			const char	*label;
356		};
357	
358	A typical example of usage:
359	
360		static struct gpio leds_gpios[] = {
361			{ 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
362			{ 33, GPIOF_OUT_INIT_LOW,  "Green LED" }, /* default to OFF */
363			{ 34, GPIOF_OUT_INIT_LOW,  "Red LED"   }, /* default to OFF */
364			{ 35, GPIOF_OUT_INIT_LOW,  "Blue LED"  }, /* default to OFF */
365			{ ... },
366		};
367	
368		err = gpio_request_one(31, GPIOF_IN, "Reset Button");
369		if (err)
370			...
371	
372		err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
373		if (err)
374			...
375	
376		gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
377	
378	
379	GPIOs mapped to IRQs
380	--------------------
381	GPIO numbers are unsigned integers; so are IRQ numbers.  These make up
382	two logically distinct namespaces (GPIO 0 need not use IRQ 0).  You can
383	map between them using calls like:
384	
385		/* map GPIO numbers to IRQ numbers */
386		int gpio_to_irq(unsigned gpio);
387	
388		/* map IRQ numbers to GPIO numbers (avoid using this) */
389		int irq_to_gpio(unsigned irq);
390	
391	Those return either the corresponding number in the other namespace, or
392	else a negative errno code if the mapping can't be done.  (For example,
393	some GPIOs can't be used as IRQs.)  It is an unchecked error to use a GPIO
394	number that wasn't set up as an input using gpio_direction_input(), or
395	to use an IRQ number that didn't originally come from gpio_to_irq().
396	
397	These two mapping calls are expected to cost on the order of a single
398	addition or subtraction.  They're not allowed to sleep.
399	
400	Non-error values returned from gpio_to_irq() can be passed to request_irq()
401	or free_irq().  They will often be stored into IRQ resources for platform
402	devices, by the board-specific initialization code.  Note that IRQ trigger
403	options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
404	system wakeup capabilities.
405	
406	Non-error values returned from irq_to_gpio() would most commonly be used
407	with gpio_get_value(), for example to initialize or update driver state
408	when the IRQ is edge-triggered.  Note that some platforms don't support
409	this reverse mapping, so you should avoid using it.
410	
411	
412	Emulating Open Drain Signals
413	----------------------------
414	Sometimes shared signals need to use "open drain" signaling, where only the
415	low signal level is actually driven.  (That term applies to CMOS transistors;
416	"open collector" is used for TTL.)  A pullup resistor causes the high signal
417	level.  This is sometimes called a "wire-AND"; or more practically, from the
418	negative logic (low=true) perspective this is a "wire-OR".
419	
420	One common example of an open drain signal is a shared active-low IRQ line.
421	Also, bidirectional data bus signals sometimes use open drain signals.
422	
423	Some GPIO controllers directly support open drain outputs; many don't.  When
424	you need open drain signaling but your hardware doesn't directly support it,
425	there's a common idiom you can use to emulate it with any GPIO pin that can
426	be used as either an input or an output:
427	
428	 LOW:	gpio_direction_output(gpio, 0) ... this drives the signal
429		and overrides the pullup.
430	
431	 HIGH:	gpio_direction_input(gpio) ... this turns off the output,
432		so the pullup (or some other device) controls the signal.
433	
434	If you are "driving" the signal high but gpio_get_value(gpio) reports a low
435	value (after the appropriate rise time passes), you know some other component
436	is driving the shared signal low.  That's not necessarily an error.  As one
437	common example, that's how I2C clocks are stretched:  a slave that needs a
438	slower clock delays the rising edge of SCK, and the I2C master adjusts its
439	signaling rate accordingly.
440	
441	
442	GPIO controllers and the pinctrl subsystem
443	------------------------------------------
444	
445	A GPIO controller on a SOC might be tightly coupled with the pinctrl
446	subsystem, in the sense that the pins can be used by other functions
447	together with an optional gpio feature. We have already covered the
448	case where e.g. a GPIO controller need to reserve a pin or set the
449	direction of a pin by calling any of:
450	
451	pinctrl_request_gpio()
452	pinctrl_free_gpio()
453	pinctrl_gpio_direction_input()
454	pinctrl_gpio_direction_output()
455	
456	But how does the pin control subsystem cross-correlate the GPIO
457	numbers (which are a global business) to a certain pin on a certain
458	pin controller?
459	
460	This is done by registering "ranges" of pins, which are essentially
461	cross-reference tables. These are described in
462	Documentation/pinctrl.txt
463	
464	While the pin allocation is totally managed by the pinctrl subsystem,
465	gpio (under gpiolib) is still maintained by gpio drivers. It may happen
466	that different pin ranges in a SoC is managed by different gpio drivers.
467	
468	This makes it logical to let gpio drivers announce their pin ranges to
469	the pin ctrl subsystem before it will call 'pinctrl_request_gpio' in order
470	to request the corresponding pin to be prepared by the pinctrl subsystem
471	before any gpio usage.
472	
473	For this, the gpio controller can register its pin range with pinctrl
474	subsystem. There are two ways of doing it currently: with or without DT.
475	
476	For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt.
477	
478	For non-DT support, user can call gpiochip_add_pin_range() with appropriate
479	parameters to register a range of gpio pins with a pinctrl driver. For this
480	exact name string of pinctrl device has to be passed as one of the
481	argument to this routine.
482	
483	
484	What do these conventions omit?
485	===============================
486	One of the biggest things these conventions omit is pin multiplexing, since
487	this is highly chip-specific and nonportable.  One platform might not need
488	explicit multiplexing; another might have just two options for use of any
489	given pin; another might have eight options per pin; another might be able
490	to route a given GPIO to any one of several pins.  (Yes, those examples all
491	come from systems that run Linux today.)
492	
493	Related to multiplexing is configuration and enabling of the pullups or
494	pulldowns integrated on some platforms.  Not all platforms support them,
495	or support them in the same way; and any given board might use external
496	pullups (or pulldowns) so that the on-chip ones should not be used.
497	(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
498	Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
499	platform-specific issue, as are models like (not) having a one-to-one
500	correspondence between configurable pins and GPIOs.
501	
502	There are other system-specific mechanisms that are not specified here,
503	like the aforementioned options for input de-glitching and wire-OR output.
504	Hardware may support reading or writing GPIOs in gangs, but that's usually
505	configuration dependent:  for GPIOs sharing the same bank.  (GPIOs are
506	commonly grouped in banks of 16 or 32, with a given SOC having several such
507	banks.)  Some systems can trigger IRQs from output GPIOs, or read values
508	from pins not managed as GPIOs.  Code relying on such mechanisms will
509	necessarily be nonportable.
510	
511	Dynamic definition of GPIOs is not currently standard; for example, as
512	a side effect of configuring an add-on board with some GPIO expanders.
513	
514	
515	GPIO implementor's framework (OPTIONAL)
516	=======================================
517	As noted earlier, there is an optional implementation framework making it
518	easier for platforms to support different kinds of GPIO controller using
519	the same programming interface.  This framework is called "gpiolib".
520	
521	As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
522	will be found there.  That will list all the controllers registered through
523	this framework, and the state of the GPIOs currently in use.
524	
525	
526	Controller Drivers: gpio_chip
527	-----------------------------
528	In this framework each GPIO controller is packaged as a "struct gpio_chip"
529	with information common to each controller of that type:
530	
531	 - methods to establish GPIO direction
532	 - methods used to access GPIO values
533	 - flag saying whether calls to its methods may sleep
534	 - optional debugfs dump method (showing extra state like pullup config)
535	 - label for diagnostics
536	
537	There is also per-instance data, which may come from device.platform_data:
538	the number of its first GPIO, and how many GPIOs it exposes.
539	
540	The code implementing a gpio_chip should support multiple instances of the
541	controller, possibly using the driver model.  That code will configure each
542	gpio_chip and issue gpiochip_add().  Removing a GPIO controller should be
543	rare; use gpiochip_remove() when it is unavoidable.
544	
545	Most often a gpio_chip is part of an instance-specific structure with state
546	not exposed by the GPIO interfaces, such as addressing, power management,
547	and more.  Chips such as codecs will have complex non-GPIO state.
548	
549	Any debugfs dump method should normally ignore signals which haven't been
550	requested as GPIOs.  They can use gpiochip_is_requested(), which returns
551	either NULL or the label associated with that GPIO when it was requested.
552	
553	
554	Platform Support
555	----------------
556	To support this framework, a platform's Kconfig will "select" either
557	ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
558	and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
559	three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
560	
561	It may also provide a custom value for ARCH_NR_GPIOS, so that it better
562	reflects the number of GPIOs in actual use on that platform, without
563	wasting static table space.  (It should count both built-in/SoC GPIOs and
564	also ones on GPIO expanders.
565	
566	ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
567	into the kernel on that architecture.
568	
569	ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user
570	can enable it and build it into the kernel optionally.
571	
572	If neither of these options are selected, the platform does not support
573	GPIOs through GPIO-lib and the code cannot be enabled by the user.
574	
575	Trivial implementations of those functions can directly use framework
576	code, which always dispatches through the gpio_chip:
577	
578	  #define gpio_get_value	__gpio_get_value
579	  #define gpio_set_value	__gpio_set_value
580	  #define gpio_cansleep		__gpio_cansleep
581	
582	Fancier implementations could instead define those as inline functions with
583	logic optimizing access to specific SOC-based GPIOs.  For example, if the
584	referenced GPIO is the constant "12", getting or setting its value could
585	cost as little as two or three instructions, never sleeping.  When such an
586	optimization is not possible those calls must delegate to the framework
587	code, costing at least a few dozen instructions.  For bitbanged I/O, such
588	instruction savings can be significant.
589	
590	For SOCs, platform-specific code defines and registers gpio_chip instances
591	for each bank of on-chip GPIOs.  Those GPIOs should be numbered/labeled to
592	match chip vendor documentation, and directly match board schematics.  They
593	may well start at zero and go up to a platform-specific limit.  Such GPIOs
594	are normally integrated into platform initialization to make them always be
595	available, from arch_initcall() or earlier; they can often serve as IRQs.
596	
597	
598	Board Support
599	-------------
600	For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
601	function devices, FPGAs or CPLDs -- most often board-specific code handles
602	registering controller devices and ensures that their drivers know what GPIO
603	numbers to use with gpiochip_add().  Their numbers often start right after
604	platform-specific GPIOs.
605	
606	For example, board setup code could create structures identifying the range
607	of GPIOs that chip will expose, and passes them to each GPIO expander chip
608	using platform_data.  Then the chip driver's probe() routine could pass that
609	data to gpiochip_add().
610	
611	Initialization order can be important.  For example, when a device relies on
612	an I2C-based GPIO, its probe() routine should only be called after that GPIO
613	becomes available.  That may mean the device should not be registered until
614	calls for that GPIO can work.  One way to address such dependencies is for
615	such gpio_chip controllers to provide setup() and teardown() callbacks to
616	board specific code; those board specific callbacks would register devices
617	once all the necessary resources are available, and remove them later when
618	the GPIO controller device becomes unavailable.
619	
620	
621	Sysfs Interface for Userspace (OPTIONAL)
622	========================================
623	Platforms which use the "gpiolib" implementors framework may choose to
624	configure a sysfs user interface to GPIOs.  This is different from the
625	debugfs interface, since it provides control over GPIO direction and
626	value instead of just showing a gpio state summary.  Plus, it could be
627	present on production systems without debugging support.
628	
629	Given appropriate hardware documentation for the system, userspace could
630	know for example that GPIO #23 controls the write protect line used to
631	protect boot loader segments in flash memory.  System upgrade procedures
632	may need to temporarily remove that protection, first importing a GPIO,
633	then changing its output state, then updating the code before re-enabling
634	the write protection.  In normal use, GPIO #23 would never be touched,
635	and the kernel would have no need to know about it.
636	
637	Again depending on appropriate hardware documentation, on some systems
638	userspace GPIO can be used to determine system configuration data that
639	standard kernels won't know about.  And for some tasks, simple userspace
640	GPIO drivers could be all that the system really needs.
641	
642	Note that standard kernel drivers exist for common "LEDs and Buttons"
643	GPIO tasks:  "leds-gpio" and "gpio_keys", respectively.  Use those
644	instead of talking directly to the GPIOs; they integrate with kernel
645	frameworks better than your userspace code could.
646	
647	
648	Paths in Sysfs
649	--------------
650	There are three kinds of entry in /sys/class/gpio:
651	
652	   -	Control interfaces used to get userspace control over GPIOs;
653	
654	   -	GPIOs themselves; and
655	
656	   -	GPIO controllers ("gpio_chip" instances).
657	
658	That's in addition to standard files including the "device" symlink.
659	
660	The control interfaces are write-only:
661	
662	    /sys/class/gpio/
663	
664	    	"export" ... Userspace may ask the kernel to export control of
665			a GPIO to userspace by writing its number to this file.
666	
667			Example:  "echo 19 > export" will create a "gpio19" node
668			for GPIO #19, if that's not requested by kernel code.
669	
670	    	"unexport" ... Reverses the effect of exporting to userspace.
671	
672			Example:  "echo 19 > unexport" will remove a "gpio19"
673			node exported using the "export" file.
674	
675	GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
676	and have the following read/write attributes:
677	
678	    /sys/class/gpio/gpioN/
679	
680		"direction" ... reads as either "in" or "out".  This value may
681			normally be written.  Writing as "out" defaults to
682			initializing the value as low.  To ensure glitch free
683			operation, values "low" and "high" may be written to
684			configure the GPIO as an output with that initial value.
685	
686			Note that this attribute *will not exist* if the kernel
687			doesn't support changing the direction of a GPIO, or
688			it was exported by kernel code that didn't explicitly
689			allow userspace to reconfigure this GPIO's direction.
690	
691		"value" ... reads as either 0 (low) or 1 (high).  If the GPIO
692			is configured as an output, this value may be written;
693			any nonzero value is treated as high.
694	
695			If the pin can be configured as interrupt-generating interrupt
696			and if it has been configured to generate interrupts (see the
697			description of "edge"), you can poll(2) on that file and
698			poll(2) will return whenever the interrupt was triggered. If
699			you use poll(2), set the events POLLPRI and POLLERR. If you
700			use select(2), set the file descriptor in exceptfds. After
701			poll(2) returns, either lseek(2) to the beginning of the sysfs
702			file and read the new value or close the file and re-open it
703			to read the value.
704	
705		"edge" ... reads as either "none", "rising", "falling", or
706			"both". Write these strings to select the signal edge(s)
707			that will make poll(2) on the "value" file return.
708	
709			This file exists only if the pin can be configured as an
710			interrupt generating input pin.
711	
712		"active_low" ... reads as either 0 (false) or 1 (true).  Write
713			any nonzero value to invert the value attribute both
714			for reading and writing.  Existing and subsequent
715			poll(2) support configuration via the edge attribute
716			for "rising" and "falling" edges will follow this
717			setting.
718	
719	GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
720	controller implementing GPIOs starting at #42) and have the following
721	read-only attributes:
722	
723	    /sys/class/gpio/gpiochipN/
724	
725	    	"base" ... same as N, the first GPIO managed by this chip
726	
727	    	"label" ... provided for diagnostics (not always unique)
728	
729	    	"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
730	
731	Board documentation should in most cases cover what GPIOs are used for
732	what purposes.  However, those numbers are not always stable; GPIOs on
733	a daughtercard might be different depending on the base board being used,
734	or other cards in the stack.  In such cases, you may need to use the
735	gpiochip nodes (possibly in conjunction with schematics) to determine
736	the correct GPIO number to use for a given signal.
737	
738	
739	Exporting from Kernel code
740	--------------------------
741	Kernel code can explicitly manage exports of GPIOs which have already been
742	requested using gpio_request():
743	
744		/* export the GPIO to userspace */
745		int gpio_export(unsigned gpio, bool direction_may_change);
746	
747		/* reverse gpio_export() */
748		void gpio_unexport();
749	
750		/* create a sysfs link to an exported GPIO node */
751		int gpio_export_link(struct device *dev, const char *name,
752			unsigned gpio)
753	
754		/* change the polarity of a GPIO node in sysfs */
755		int gpio_sysfs_set_active_low(unsigned gpio, int value);
756	
757	After a kernel driver requests a GPIO, it may only be made available in
758	the sysfs interface by gpio_export().  The driver can control whether the
759	signal direction may change.  This helps drivers prevent userspace code
760	from accidentally clobbering important system state.
761	
762	This explicit exporting can help with debugging (by making some kinds
763	of experiments easier), or can provide an always-there interface that's
764	suitable for documenting as part of a board support package.
765	
766	After the GPIO has been exported, gpio_export_link() allows creating
767	symlinks from elsewhere in sysfs to the GPIO sysfs node.  Drivers can
768	use this to provide the interface under their own device in sysfs with
769	a descriptive name.
770	
771	Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
772	differences between boards from user space.  This only affects the
773	sysfs interface.  Polarity change can be done both before and after
774	gpio_export(), and previously enabled poll(2) support for either
775	rising or falling edge will be reconfigured to follow this setting.
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