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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
5
6What is a GPIO?
7===============
8A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
9digital signal. They are provided from many kinds of chip, and are familiar
10to Linux developers working with embedded and custom hardware. Each GPIO
11represents a bit connected to a particular pin, or "ball" on Ball Grid Array
12(BGA) packages. Board schematics show which external hardware connects to
13which GPIOs. Drivers can be written generically, so that board setup code
14passes such pin configuration data to drivers.
15
16System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
17non-dedicated pin can be configured as a GPIO; and most chips have at least
18several dozen of them. Programmable logic devices (like FPGAs) can easily
19provide GPIOs; multifunction chips like power managers, and audio codecs
20often have a few such pins to help with pin scarcity on SOCs; and there are
21also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
22Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
23firmware knowing how they're used).
24
25The exact capabilities of GPIOs vary between systems. Common options:
26
27 - Output values are writable (high=1, low=0). Some chips also have
28 options about how that value is driven, so that for example only one
29 value might be driven ... supporting "wire-OR" and similar schemes
David Brownell1668be72007-04-11 23:28:42 -070030 for the other value (notably, "open drain" signaling).
David Brownell4c203862007-02-12 00:53:11 -080031
32 - Input values are likewise readable (1, 0). Some chips support readback
33 of pins configured as "output", which is very useful in such "wire-OR"
34 cases (to support bidirectional signaling). GPIO controllers may have
35 input de-glitch logic, sometimes with software controls.
36
37 - Inputs can often be used as IRQ signals, often edge triggered but
38 sometimes level triggered. Such IRQs may be configurable as system
39 wakeup events, to wake the system from a low power state.
40
41 - Usually a GPIO will be configurable as either input or output, as needed
42 by different product boards; single direction ones exist too.
43
44 - Most GPIOs can be accessed while holding spinlocks, but those accessed
45 through a serial bus normally can't. Some systems support both types.
46
47On a given board each GPIO is used for one specific purpose like monitoring
48MMC/SD card insertion/removal, detecting card writeprotect status, driving
49a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
50watchdog, sensing a switch, and so on.
51
52
53GPIO conventions
54================
55Note that this is called a "convention" because you don't need to do it this
56way, and it's no crime if you don't. There **are** cases where portability
57is not the main issue; GPIOs are often used for the kind of board-specific
58glue logic that may even change between board revisions, and can't ever be
59used on a board that's wired differently. Only least-common-denominator
60functionality can be very portable. Other features are platform-specific,
61and that can be critical for glue logic.
62
63Plus, this doesn't define an implementation framework, just an interface.
64One platform might implement it as simple inline functions accessing chip
65registers; another might implement it by delegating through abstractions
66used for several very different kinds of GPIO controller.
67
68That said, if the convention is supported on their platform, drivers should
David Brownell32993b72007-05-10 22:22:17 -070069use it when possible. Platforms should declare GENERIC_GPIO support in
70Kconfig (boolean true), which multi-platform drivers can depend on when
71using the include file:
David Brownell4c203862007-02-12 00:53:11 -080072
73 #include <asm/gpio.h>
74
75If you stick to this convention then it'll be easier for other developers to
76see what your code is doing, and help maintain it.
77
David Brownella0a99832007-07-19 01:47:52 -070078Note that these operations include I/O barriers on platforms which need to
79use them; drivers don't need to add them explicitly.
80
David Brownell4c203862007-02-12 00:53:11 -080081
82Identifying GPIOs
83-----------------
84GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
85reserves "negative" numbers for other purposes like marking signals as
David Brownellf5de6112007-02-16 01:27:14 -080086"not available on this board", or indicating faults. Code that doesn't
87touch the underlying hardware treats these integers as opaque cookies.
David Brownell4c203862007-02-12 00:53:11 -080088
89Platforms define how they use those integers, and usually #define symbols
90for the GPIO lines so that board-specific setup code directly corresponds
91to the relevant schematics. In contrast, drivers should only use GPIO
92numbers passed to them from that setup code, using platform_data to hold
93board-specific pin configuration data (along with other board specific
94data they need). That avoids portability problems.
95
96So for example one platform uses numbers 32-159 for GPIOs; while another
97uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
98type of GPIO controller, and on one particular board 80-95 with an FPGA.
99The numbers need not be contiguous; either of those platforms could also
100use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
101
102Whether a platform supports multiple GPIO controllers is currently a
103platform-specific implementation issue.
104
105
106Using GPIOs
107-----------
108One of the first things to do with a GPIO, often in board setup code when
109setting up a platform_device using the GPIO, is mark its direction:
110
111 /* set as input or output, returning 0 or negative errno */
112 int gpio_direction_input(unsigned gpio);
David Brownell28735a72007-03-16 13:38:14 -0800113 int gpio_direction_output(unsigned gpio, int value);
David Brownell4c203862007-02-12 00:53:11 -0800114
115The return value is zero for success, else a negative errno. It should
116be checked, since the get/set calls don't have error returns and since
David Brownell83c65902007-05-16 22:11:13 -0700117misconfiguration is possible. You should normally issue these calls from
118a task context. However, for spinlock-safe GPIOs it's OK to use them
119before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800120
David Brownell28735a72007-03-16 13:38:14 -0800121For output GPIOs, the value provided becomes the initial output value.
122This helps avoid signal glitching during system startup.
123
David Brownell4c203862007-02-12 00:53:11 -0800124Setting the direction can fail if the GPIO number is invalid, or when
125that particular GPIO can't be used in that mode. It's generally a bad
126idea to rely on boot firmware to have set the direction correctly, since
127it probably wasn't validated to do more than boot Linux. (Similarly,
128that board setup code probably needs to multiplex that pin as a GPIO,
129and configure pullups/pulldowns appropriately.)
130
131
132Spinlock-Safe GPIO access
133-------------------------
134Most GPIO controllers can be accessed with memory read/write instructions.
135That doesn't need to sleep, and can safely be done from inside IRQ handlers.
136
137Use these calls to access such GPIOs:
138
139 /* GPIO INPUT: return zero or nonzero */
140 int gpio_get_value(unsigned gpio);
141
142 /* GPIO OUTPUT */
143 void gpio_set_value(unsigned gpio, int value);
144
145The values are boolean, zero for low, nonzero for high. When reading the
146value of an output pin, the value returned should be what's seen on the
147pin ... that won't always match the specified output value, because of
148issues including wire-OR and output latencies.
149
150The get/set calls have no error returns because "invalid GPIO" should have
151been reported earlier in gpio_set_direction(). However, note that not all
152platforms can read the value of output pins; those that can't should always
David Brownellf5de6112007-02-16 01:27:14 -0800153return zero. Also, using these calls for GPIOs that can't safely be accessed
154without sleeping (see below) is an error.
David Brownell4c203862007-02-12 00:53:11 -0800155
David Brownellf5de6112007-02-16 01:27:14 -0800156Platform-specific implementations are encouraged to optimize the two
David Brownell4c203862007-02-12 00:53:11 -0800157calls to access the GPIO value in cases where the GPIO number (and for
158output, value) are constant. It's normal for them to need only a couple
159of instructions in such cases (reading or writing a hardware register),
160and not to need spinlocks. Such optimized calls can make bitbanging
161applications a lot more efficient (in both space and time) than spending
162dozens of instructions on subroutine calls.
163
164
165GPIO access that may sleep
166--------------------------
167Some GPIO controllers must be accessed using message based busses like I2C
168or SPI. Commands to read or write those GPIO values require waiting to
169get to the head of a queue to transmit a command and get its response.
170This requires sleeping, which can't be done from inside IRQ handlers.
171
172Platforms that support this type of GPIO distinguish them from other GPIOs
173by returning nonzero from this call:
174
175 int gpio_cansleep(unsigned gpio);
176
177To access such GPIOs, a different set of accessors is defined:
178
179 /* GPIO INPUT: return zero or nonzero, might sleep */
180 int gpio_get_value_cansleep(unsigned gpio);
181
182 /* GPIO OUTPUT, might sleep */
183 void gpio_set_value_cansleep(unsigned gpio, int value);
184
185Other than the fact that these calls might sleep, and will not be ignored
186for GPIOs that can't be accessed from IRQ handlers, these calls act the
187same as the spinlock-safe calls.
188
189
190Claiming and Releasing GPIOs (OPTIONAL)
191---------------------------------------
192To help catch system configuration errors, two calls are defined.
193However, many platforms don't currently support this mechanism.
194
195 /* request GPIO, returning 0 or negative errno.
196 * non-null labels may be useful for diagnostics.
197 */
198 int gpio_request(unsigned gpio, const char *label);
199
200 /* release previously-claimed GPIO */
201 void gpio_free(unsigned gpio);
202
203Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
204GPIOs that have already been claimed with that call. The return value of
David Brownell83c65902007-05-16 22:11:13 -0700205gpio_request() must be checked. You should normally issue these calls from
206a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
207before tasking is enabled, as part of early board setup.
David Brownell4c203862007-02-12 00:53:11 -0800208
209These calls serve two basic purposes. One is marking the signals which
210are actually in use as GPIOs, for better diagnostics; systems may have
211several hundred potential GPIOs, but often only a dozen are used on any
212given board. Another is to catch conflicts between drivers, reporting
213errors when drivers wrongly think they have exclusive use of that signal.
214
215These two calls are optional because not not all current Linux platforms
216offer such functionality in their GPIO support; a valid implementation
217could return success for all gpio_request() calls. Unlike the other calls,
218the state they represent doesn't normally match anything from a hardware
219register; it's just a software bitmap which clearly is not necessary for
220correct operation of hardware or (bug free) drivers.
221
222Note that requesting a GPIO does NOT cause it to be configured in any
223way; it just marks that GPIO as in use. Separate code must handle any
224pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
225
226
227GPIOs mapped to IRQs
228--------------------
229GPIO numbers are unsigned integers; so are IRQ numbers. These make up
230two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
231map between them using calls like:
232
233 /* map GPIO numbers to IRQ numbers */
234 int gpio_to_irq(unsigned gpio);
235
236 /* map IRQ numbers to GPIO numbers */
237 int irq_to_gpio(unsigned irq);
238
239Those return either the corresponding number in the other namespace, or
240else a negative errno code if the mapping can't be done. (For example,
241some GPIOs can't used as IRQs.) It is an unchecked error to use a GPIO
242number that hasn't been marked as an input using gpio_set_direction(), or
243to use an IRQ number that didn't originally come from gpio_to_irq().
244
245These two mapping calls are expected to cost on the order of a single
246addition or subtraction. They're not allowed to sleep.
247
248Non-error values returned from gpio_to_irq() can be passed to request_irq()
249or free_irq(). They will often be stored into IRQ resources for platform
250devices, by the board-specific initialization code. Note that IRQ trigger
251options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
252system wakeup capabilities.
253
254Non-error values returned from irq_to_gpio() would most commonly be used
David Brownellf5de6112007-02-16 01:27:14 -0800255with gpio_get_value(), for example to initialize or update driver state
256when the IRQ is edge-triggered.
David Brownell4c203862007-02-12 00:53:11 -0800257
258
David Brownell1668be72007-04-11 23:28:42 -0700259Emulating Open Drain Signals
260----------------------------
261Sometimes shared signals need to use "open drain" signaling, where only the
262low signal level is actually driven. (That term applies to CMOS transistors;
263"open collector" is used for TTL.) A pullup resistor causes the high signal
264level. This is sometimes called a "wire-AND"; or more practically, from the
265negative logic (low=true) perspective this is a "wire-OR".
266
267One common example of an open drain signal is a shared active-low IRQ line.
268Also, bidirectional data bus signals sometimes use open drain signals.
269
270Some GPIO controllers directly support open drain outputs; many don't. When
271you need open drain signaling but your hardware doesn't directly support it,
272there's a common idiom you can use to emulate it with any GPIO pin that can
273be used as either an input or an output:
274
275 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
276 and overrides the pullup.
277
278 HIGH: gpio_direction_input(gpio) ... this turns off the output,
279 so the pullup (or some other device) controls the signal.
280
281If you are "driving" the signal high but gpio_get_value(gpio) reports a low
282value (after the appropriate rise time passes), you know some other component
283is driving the shared signal low. That's not necessarily an error. As one
284common example, that's how I2C clocks are stretched: a slave that needs a
285slower clock delays the rising edge of SCK, and the I2C master adjusts its
286signaling rate accordingly.
287
David Brownell4c203862007-02-12 00:53:11 -0800288
289What do these conventions omit?
290===============================
291One of the biggest things these conventions omit is pin multiplexing, since
292this is highly chip-specific and nonportable. One platform might not need
293explicit multiplexing; another might have just two options for use of any
294given pin; another might have eight options per pin; another might be able
295to route a given GPIO to any one of several pins. (Yes, those examples all
296come from systems that run Linux today.)
297
298Related to multiplexing is configuration and enabling of the pullups or
299pulldowns integrated on some platforms. Not all platforms support them,
300or support them in the same way; and any given board might use external
301pullups (or pulldowns) so that the on-chip ones should not be used.
302
303There are other system-specific mechanisms that are not specified here,
304like the aforementioned options for input de-glitching and wire-OR output.
305Hardware may support reading or writing GPIOs in gangs, but that's usually
David Brownellf5de6112007-02-16 01:27:14 -0800306configuration dependent: for GPIOs sharing the same bank. (GPIOs are
David Brownell4c203862007-02-12 00:53:11 -0800307commonly grouped in banks of 16 or 32, with a given SOC having several such
David Brownellf5de6112007-02-16 01:27:14 -0800308banks.) Some systems can trigger IRQs from output GPIOs. Code relying on
309such mechanisms will necessarily be nonportable.
David Brownell4c203862007-02-12 00:53:11 -0800310
311Dynamic definition of GPIOs is not currently supported; for example, as
312a side effect of configuring an add-on board with some GPIO expanders.
313
314These calls are purely for kernel space, but a userspace API could be built
315on top of it.