3 This provides an overview of GPIO access conventions on Linux.
8 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
9 digital signal. They are provided from many kinds of chip, and are familiar
10 to Linux developers working with embedded and custom hardware. Each GPIO
11 represents a bit connected to a particular pin, or "ball" on Ball Grid Array
12 (BGA) packages. Board schematics show which external hardware connects to
13 which GPIOs. Drivers can be written generically, so that board setup code
14 passes such pin configuration data to drivers.
16 System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
17 non-dedicated pin can be configured as a GPIO; and most chips have at least
18 several dozen of them. Programmable logic devices (like FPGAs) can easily
19 provide GPIOs; multifunction chips like power managers, and audio codecs
20 often have a few such pins to help with pin scarcity on SOCs; and there are
21 also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
22 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
23 firmware knowing how they're used).
25 The exact capabilities of GPIOs vary between systems. Common options:
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
30 for the other value (notably, "open drain" signaling).
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.
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.
41 - Usually a GPIO will be configurable as either input or output, as needed
42 by different product boards; single direction ones exist too.
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.
47 On a given board each GPIO is used for one specific purpose like monitoring
48 MMC/SD card insertion/removal, detecting card writeprotect status, driving
49 a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
50 watchdog, sensing a switch, and so on.
55 Note that this is called a "convention" because you don't need to do it this
56 way, and it's no crime if you don't. There **are** cases where portability
57 is not the main issue; GPIOs are often used for the kind of board-specific
58 glue logic that may even change between board revisions, and can't ever be
59 used on a board that's wired differently. Only least-common-denominator
60 functionality can be very portable. Other features are platform-specific,
61 and that can be critical for glue logic.
63 Plus, this doesn't define an implementation framework, just an interface.
64 One platform might implement it as simple inline functions accessing chip
65 registers; another might implement it by delegating through abstractions
66 used for several very different kinds of GPIO controller.
68 That said, if the convention is supported on their platform, drivers should
69 use it when possible. Platforms should declare GENERIC_GPIO support in
70 Kconfig (boolean true), which multi-platform drivers can depend on when
71 using the include file:
75 If you stick to this convention then it'll be easier for other developers to
76 see what your code is doing, and help maintain it.
81 GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
82 reserves "negative" numbers for other purposes like marking signals as
83 "not available on this board", or indicating faults. Code that doesn't
84 touch the underlying hardware treats these integers as opaque cookies.
86 Platforms define how they use those integers, and usually #define symbols
87 for the GPIO lines so that board-specific setup code directly corresponds
88 to the relevant schematics. In contrast, drivers should only use GPIO
89 numbers passed to them from that setup code, using platform_data to hold
90 board-specific pin configuration data (along with other board specific
91 data they need). That avoids portability problems.
93 So for example one platform uses numbers 32-159 for GPIOs; while another
94 uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
95 type of GPIO controller, and on one particular board 80-95 with an FPGA.
96 The numbers need not be contiguous; either of those platforms could also
97 use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
99 Whether a platform supports multiple GPIO controllers is currently a
100 platform-specific implementation issue.
105 One of the first things to do with a GPIO, often in board setup code when
106 setting up a platform_device using the GPIO, is mark its direction:
108 /* set as input or output, returning 0 or negative errno */
109 int gpio_direction_input(unsigned gpio);
110 int gpio_direction_output(unsigned gpio, int value);
112 The return value is zero for success, else a negative errno. It should
113 be checked, since the get/set calls don't have error returns and since
114 misconfiguration is possible. You should normally issue these calls from
115 a task context. However, for spinlock-safe GPIOs it's OK to use them
116 before tasking is enabled, as part of early board setup.
118 For output GPIOs, the value provided becomes the initial output value.
119 This helps avoid signal glitching during system startup.
121 Setting the direction can fail if the GPIO number is invalid, or when
122 that particular GPIO can't be used in that mode. It's generally a bad
123 idea to rely on boot firmware to have set the direction correctly, since
124 it probably wasn't validated to do more than boot Linux. (Similarly,
125 that board setup code probably needs to multiplex that pin as a GPIO,
126 and configure pullups/pulldowns appropriately.)
129 Spinlock-Safe GPIO access
130 -------------------------
131 Most GPIO controllers can be accessed with memory read/write instructions.
132 That doesn't need to sleep, and can safely be done from inside IRQ handlers.
134 Use these calls to access such GPIOs:
136 /* GPIO INPUT: return zero or nonzero */
137 int gpio_get_value(unsigned gpio);
140 void gpio_set_value(unsigned gpio, int value);
142 The values are boolean, zero for low, nonzero for high. When reading the
143 value of an output pin, the value returned should be what's seen on the
144 pin ... that won't always match the specified output value, because of
145 issues including wire-OR and output latencies.
147 The get/set calls have no error returns because "invalid GPIO" should have
148 been reported earlier in gpio_set_direction(). However, note that not all
149 platforms can read the value of output pins; those that can't should always
150 return zero. Also, using these calls for GPIOs that can't safely be accessed
151 without sleeping (see below) is an error.
153 Platform-specific implementations are encouraged to optimize the two
154 calls to access the GPIO value in cases where the GPIO number (and for
155 output, value) are constant. It's normal for them to need only a couple
156 of instructions in such cases (reading or writing a hardware register),
157 and not to need spinlocks. Such optimized calls can make bitbanging
158 applications a lot more efficient (in both space and time) than spending
159 dozens of instructions on subroutine calls.
162 GPIO access that may sleep
163 --------------------------
164 Some GPIO controllers must be accessed using message based busses like I2C
165 or SPI. Commands to read or write those GPIO values require waiting to
166 get to the head of a queue to transmit a command and get its response.
167 This requires sleeping, which can't be done from inside IRQ handlers.
169 Platforms that support this type of GPIO distinguish them from other GPIOs
170 by returning nonzero from this call:
172 int gpio_cansleep(unsigned gpio);
174 To access such GPIOs, a different set of accessors is defined:
176 /* GPIO INPUT: return zero or nonzero, might sleep */
177 int gpio_get_value_cansleep(unsigned gpio);
179 /* GPIO OUTPUT, might sleep */
180 void gpio_set_value_cansleep(unsigned gpio, int value);
182 Other than the fact that these calls might sleep, and will not be ignored
183 for GPIOs that can't be accessed from IRQ handlers, these calls act the
184 same as the spinlock-safe calls.
187 Claiming and Releasing GPIOs (OPTIONAL)
188 ---------------------------------------
189 To help catch system configuration errors, two calls are defined.
190 However, many platforms don't currently support this mechanism.
192 /* request GPIO, returning 0 or negative errno.
193 * non-null labels may be useful for diagnostics.
195 int gpio_request(unsigned gpio, const char *label);
197 /* release previously-claimed GPIO */
198 void gpio_free(unsigned gpio);
200 Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
201 GPIOs that have already been claimed with that call. The return value of
202 gpio_request() must be checked. You should normally issue these calls from
203 a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
204 before tasking is enabled, as part of early board setup.
206 These calls serve two basic purposes. One is marking the signals which
207 are actually in use as GPIOs, for better diagnostics; systems may have
208 several hundred potential GPIOs, but often only a dozen are used on any
209 given board. Another is to catch conflicts between drivers, reporting
210 errors when drivers wrongly think they have exclusive use of that signal.
212 These two calls are optional because not not all current Linux platforms
213 offer such functionality in their GPIO support; a valid implementation
214 could return success for all gpio_request() calls. Unlike the other calls,
215 the state they represent doesn't normally match anything from a hardware
216 register; it's just a software bitmap which clearly is not necessary for
217 correct operation of hardware or (bug free) drivers.
219 Note that requesting a GPIO does NOT cause it to be configured in any
220 way; it just marks that GPIO as in use. Separate code must handle any
221 pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
226 GPIO numbers are unsigned integers; so are IRQ numbers. These make up
227 two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
228 map between them using calls like:
230 /* map GPIO numbers to IRQ numbers */
231 int gpio_to_irq(unsigned gpio);
233 /* map IRQ numbers to GPIO numbers */
234 int irq_to_gpio(unsigned irq);
236 Those return either the corresponding number in the other namespace, or
237 else a negative errno code if the mapping can't be done. (For example,
238 some GPIOs can't used as IRQs.) It is an unchecked error to use a GPIO
239 number that hasn't been marked as an input using gpio_set_direction(), or
240 to use an IRQ number that didn't originally come from gpio_to_irq().
242 These two mapping calls are expected to cost on the order of a single
243 addition or subtraction. They're not allowed to sleep.
245 Non-error values returned from gpio_to_irq() can be passed to request_irq()
246 or free_irq(). They will often be stored into IRQ resources for platform
247 devices, by the board-specific initialization code. Note that IRQ trigger
248 options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
249 system wakeup capabilities.
251 Non-error values returned from irq_to_gpio() would most commonly be used
252 with gpio_get_value(), for example to initialize or update driver state
253 when the IRQ is edge-triggered.
256 Emulating Open Drain Signals
257 ----------------------------
258 Sometimes shared signals need to use "open drain" signaling, where only the
259 low signal level is actually driven. (That term applies to CMOS transistors;
260 "open collector" is used for TTL.) A pullup resistor causes the high signal
261 level. This is sometimes called a "wire-AND"; or more practically, from the
262 negative logic (low=true) perspective this is a "wire-OR".
264 One common example of an open drain signal is a shared active-low IRQ line.
265 Also, bidirectional data bus signals sometimes use open drain signals.
267 Some GPIO controllers directly support open drain outputs; many don't. When
268 you need open drain signaling but your hardware doesn't directly support it,
269 there's a common idiom you can use to emulate it with any GPIO pin that can
270 be used as either an input or an output:
272 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
273 and overrides the pullup.
275 HIGH: gpio_direction_input(gpio) ... this turns off the output,
276 so the pullup (or some other device) controls the signal.
278 If you are "driving" the signal high but gpio_get_value(gpio) reports a low
279 value (after the appropriate rise time passes), you know some other component
280 is driving the shared signal low. That's not necessarily an error. As one
281 common example, that's how I2C clocks are stretched: a slave that needs a
282 slower clock delays the rising edge of SCK, and the I2C master adjusts its
283 signaling rate accordingly.
286 What do these conventions omit?
287 ===============================
288 One of the biggest things these conventions omit is pin multiplexing, since
289 this is highly chip-specific and nonportable. One platform might not need
290 explicit multiplexing; another might have just two options for use of any
291 given pin; another might have eight options per pin; another might be able
292 to route a given GPIO to any one of several pins. (Yes, those examples all
293 come from systems that run Linux today.)
295 Related to multiplexing is configuration and enabling of the pullups or
296 pulldowns integrated on some platforms. Not all platforms support them,
297 or support them in the same way; and any given board might use external
298 pullups (or pulldowns) so that the on-chip ones should not be used.
300 There are other system-specific mechanisms that are not specified here,
301 like the aforementioned options for input de-glitching and wire-OR output.
302 Hardware may support reading or writing GPIOs in gangs, but that's usually
303 configuration dependent: for GPIOs sharing the same bank. (GPIOs are
304 commonly grouped in banks of 16 or 32, with a given SOC having several such
305 banks.) Some systems can trigger IRQs from output GPIOs. Code relying on
306 such mechanisms will necessarily be nonportable.
308 Dynamic definition of GPIOs is not currently supported; for example, as
309 a side effect of configuring an add-on board with some GPIO expanders.
311 These calls are purely for kernel space, but a userspace API could be built
projects / linux-2.6 / blob