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-rw-r--r--Documentation/gpio.txt271
-rw-r--r--include/asm-arm/gpio.h7
-rw-r--r--include/asm-generic/gpio.h25
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diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
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1GPIO 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
30 for the other value.
31
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
69use it when possible:
70
71 #include <asm/gpio.h>
72
73If you stick to this convention then it'll be easier for other developers to
74see what your code is doing, and help maintain it.
75
76
77Identifying GPIOs
78-----------------
79GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
80reserves "negative" numbers for other purposes like marking signals as
81"not available on this board", or indicating faults.
82
83Platforms define how they use those integers, and usually #define symbols
84for the GPIO lines so that board-specific setup code directly corresponds
85to the relevant schematics. In contrast, drivers should only use GPIO
86numbers passed to them from that setup code, using platform_data to hold
87board-specific pin configuration data (along with other board specific
88data they need). That avoids portability problems.
89
90So for example one platform uses numbers 32-159 for GPIOs; while another
91uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
92type of GPIO controller, and on one particular board 80-95 with an FPGA.
93The numbers need not be contiguous; either of those platforms could also
94use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
95
96Whether a platform supports multiple GPIO controllers is currently a
97platform-specific implementation issue.
98
99
100Using GPIOs
101-----------
102One of the first things to do with a GPIO, often in board setup code when
103setting up a platform_device using the GPIO, is mark its direction:
104
105 /* set as input or output, returning 0 or negative errno */
106 int gpio_direction_input(unsigned gpio);
107 int gpio_direction_output(unsigned gpio);
108
109The return value is zero for success, else a negative errno. It should
110be checked, since the get/set calls don't have error returns and since
111misconfiguration is possible. (These calls could sleep.)
112
113Setting the direction can fail if the GPIO number is invalid, or when
114that particular GPIO can't be used in that mode. It's generally a bad
115idea to rely on boot firmware to have set the direction correctly, since
116it probably wasn't validated to do more than boot Linux. (Similarly,
117that board setup code probably needs to multiplex that pin as a GPIO,
118and configure pullups/pulldowns appropriately.)
119
120
121Spinlock-Safe GPIO access
122-------------------------
123Most GPIO controllers can be accessed with memory read/write instructions.
124That doesn't need to sleep, and can safely be done from inside IRQ handlers.
125
126Use these calls to access such GPIOs:
127
128 /* GPIO INPUT: return zero or nonzero */
129 int gpio_get_value(unsigned gpio);
130
131 /* GPIO OUTPUT */
132 void gpio_set_value(unsigned gpio, int value);
133
134The values are boolean, zero for low, nonzero for high. When reading the
135value of an output pin, the value returned should be what's seen on the
136pin ... that won't always match the specified output value, because of
137issues including wire-OR and output latencies.
138
139The get/set calls have no error returns because "invalid GPIO" should have
140been reported earlier in gpio_set_direction(). However, note that not all
141platforms can read the value of output pins; those that can't should always
142return zero. Also, these calls will be ignored for GPIOs that can't safely
143be accessed wihtout sleeping (see below).
144
145Platform-specific implementations are encouraged to optimise the two
146calls to access the GPIO value in cases where the GPIO number (and for
147output, value) are constant. It's normal for them to need only a couple
148of instructions in such cases (reading or writing a hardware register),
149and not to need spinlocks. Such optimized calls can make bitbanging
150applications a lot more efficient (in both space and time) than spending
151dozens of instructions on subroutine calls.
152
153
154GPIO access that may sleep
155--------------------------
156Some GPIO controllers must be accessed using message based busses like I2C
157or SPI. Commands to read or write those GPIO values require waiting to
158get to the head of a queue to transmit a command and get its response.
159This requires sleeping, which can't be done from inside IRQ handlers.
160
161Platforms that support this type of GPIO distinguish them from other GPIOs
162by returning nonzero from this call:
163
164 int gpio_cansleep(unsigned gpio);
165
166To access such GPIOs, a different set of accessors is defined:
167
168 /* GPIO INPUT: return zero or nonzero, might sleep */
169 int gpio_get_value_cansleep(unsigned gpio);
170
171 /* GPIO OUTPUT, might sleep */
172 void gpio_set_value_cansleep(unsigned gpio, int value);
173
174Other than the fact that these calls might sleep, and will not be ignored
175for GPIOs that can't be accessed from IRQ handlers, these calls act the
176same as the spinlock-safe calls.
177
178
179Claiming and Releasing GPIOs (OPTIONAL)
180---------------------------------------
181To help catch system configuration errors, two calls are defined.
182However, many platforms don't currently support this mechanism.
183
184 /* request GPIO, returning 0 or negative errno.
185 * non-null labels may be useful for diagnostics.
186 */
187 int gpio_request(unsigned gpio, const char *label);
188
189 /* release previously-claimed GPIO */
190 void gpio_free(unsigned gpio);
191
192Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
193GPIOs that have already been claimed with that call. The return value of
194gpio_request() must be checked. (These calls could sleep.)
195
196These calls serve two basic purposes. One is marking the signals which
197are actually in use as GPIOs, for better diagnostics; systems may have
198several hundred potential GPIOs, but often only a dozen are used on any
199given board. Another is to catch conflicts between drivers, reporting
200errors when drivers wrongly think they have exclusive use of that signal.
201
202These two calls are optional because not not all current Linux platforms
203offer such functionality in their GPIO support; a valid implementation
204could return success for all gpio_request() calls. Unlike the other calls,
205the state they represent doesn't normally match anything from a hardware
206register; it's just a software bitmap which clearly is not necessary for
207correct operation of hardware or (bug free) drivers.
208
209Note that requesting a GPIO does NOT cause it to be configured in any
210way; it just marks that GPIO as in use. Separate code must handle any
211pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
212
213
214GPIOs mapped to IRQs
215--------------------
216GPIO numbers are unsigned integers; so are IRQ numbers. These make up
217two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
218map between them using calls like:
219
220 /* map GPIO numbers to IRQ numbers */
221 int gpio_to_irq(unsigned gpio);
222
223 /* map IRQ numbers to GPIO numbers */
224 int irq_to_gpio(unsigned irq);
225
226Those return either the corresponding number in the other namespace, or
227else a negative errno code if the mapping can't be done. (For example,
228some GPIOs can't used as IRQs.) It is an unchecked error to use a GPIO
229number that hasn't been marked as an input using gpio_set_direction(), or
230to use an IRQ number that didn't originally come from gpio_to_irq().
231
232These two mapping calls are expected to cost on the order of a single
233addition or subtraction. They're not allowed to sleep.
234
235Non-error values returned from gpio_to_irq() can be passed to request_irq()
236or free_irq(). They will often be stored into IRQ resources for platform
237devices, by the board-specific initialization code. Note that IRQ trigger
238options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
239system wakeup capabilities.
240
241Non-error values returned from irq_to_gpio() would most commonly be used
242with gpio_get_value().
243
244
245
246What do these conventions omit?
247===============================
248One of the biggest things these conventions omit is pin multiplexing, since
249this is highly chip-specific and nonportable. One platform might not need
250explicit multiplexing; another might have just two options for use of any
251given pin; another might have eight options per pin; another might be able
252to route a given GPIO to any one of several pins. (Yes, those examples all
253come from systems that run Linux today.)
254
255Related to multiplexing is configuration and enabling of the pullups or
256pulldowns integrated on some platforms. Not all platforms support them,
257or support them in the same way; and any given board might use external
258pullups (or pulldowns) so that the on-chip ones should not be used.
259
260There are other system-specific mechanisms that are not specified here,
261like the aforementioned options for input de-glitching and wire-OR output.
262Hardware may support reading or writing GPIOs in gangs, but that's usually
263configuration dependednt: for GPIOs sharing the same bank. (GPIOs are
264commonly grouped in banks of 16 or 32, with a given SOC having several such
265banks.) Code relying on such mechanisms will necessarily be nonportable.
266
267Dynamic definition of GPIOs is not currently supported; for example, as
268a side effect of configuring an add-on board with some GPIO expanders.
269
270These calls are purely for kernel space, but a userspace API could be built
271on top of it.
diff --git a/include/asm-arm/gpio.h b/include/asm-arm/gpio.h
new file mode 100644
index 000000000000..fff4f800ee42
--- /dev/null
+++ b/include/asm-arm/gpio.h
@@ -0,0 +1,7 @@
1#ifndef _ARCH_ARM_GPIO_H
2#define _ARCH_ARM_GPIO_H
3
4/* not all ARM platforms necessarily support this API ... */
5#include <asm/arch/gpio.h>
6
7#endif /* _ARCH_ARM_GPIO_H */
diff --git a/include/asm-generic/gpio.h b/include/asm-generic/gpio.h
new file mode 100644
index 000000000000..2d0aab1d8611
--- /dev/null
+++ b/include/asm-generic/gpio.h
@@ -0,0 +1,25 @@
1#ifndef _ASM_GENERIC_GPIO_H
2#define _ASM_GENERIC_GPIO_H
3
4/* platforms that don't directly support access to GPIOs through I2C, SPI,
5 * or other blocking infrastructure can use these wrappers.
6 */
7
8static inline int gpio_cansleep(unsigned gpio)
9{
10 return 0;
11}
12
13static inline int gpio_get_value_cansleep(unsigned gpio)
14{
15 might_sleep();
16 return gpio_get_value(gpio);
17}
18
19static inline void gpio_set_value_cansleep(unsigned gpio, int value)
20{
21 might_sleep();
22 gpio_set_value(gpio, value);
23}
24
25#endif /* _ASM_GENERIC_GPIO_H */