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1============================
2Subsystem drivers using GPIO
3============================
4
5Note that standard kernel drivers exist for common GPIO tasks and will provide
6the right in-kernel and userspace APIs/ABIs for the job, and that these
7drivers can quite easily interconnect with other kernel subsystems using
8hardware descriptions such as device tree or ACPI:
9
10- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO
11 lines, giving you the LED sysfs interface
12
13- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
14 i.e. a LED will turn on/off in response to a GPIO line going high or low
15 (and that LED may in turn use the leds-gpio as per above).
16
17- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
18 can generate interrupts in response to a key press. Also supports debounce.
19
20- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
21 GPIO line cannot generate interrupts, so it needs to be periodically polled
22 by a timer.
23
24- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
25 up to three buttons by simply using GPIOs and no mouse port. You can cut the
26 mouse cable and connect the wires to GPIO lines or solder a mouse connector
27 to the lines for a more permanent solution of this type.
28
29- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
30 an external speaker connected to a GPIO line.
31
32- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
33 external connector status, such as a headset line for an audio driver or an
34 HDMI connector. It will provide a better userspace sysfs interface than GPIO.
35
36- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
37 the system by pulling a GPIO line and will register a restart handler so
38 userspace can issue the right system call to restart the system.
39
40- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
41 system down by pulling a GPIO line and will register a pm_power_off()
42 callback so that userspace can issue the right system call to power down the
43 system.
44
45- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
46 (off/on) that uses a GPIO, and integrated with the clock subsystem.
47
48- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
49 (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
50 appear as any other I2C bus to the system and makes it possible to connect
51 drivers for the I2C devices on the bus like any other I2C bus driver.
52
53- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
54 of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
55 GPIO hammering (bitbang). It will appear as any other SPI bus on the system
56 and makes it possible to connect drivers for SPI devices on the bus like
57 any other SPI bus driver. For example any MMC/SD card can then be connected
58 to this SPI by using the mmc_spi host from the MMC/SD card subsystem.
59
60- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
61 a GPIO line, integrating with the W1 subsystem and handling devices on
62 the bus like any other W1 device.
63
64- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
65 system, connected to a GPIO line (and optionally a GPIO alarm line),
66 presenting all the right in-kernel and sysfs interfaces to make your system
67 not overheat.
68
69- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
70 regulator providing a certain voltage by pulling a GPIO line, integrating
71 with the regulator subsystem and giving you all the right interfaces.
72
73- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
74 that will periodically "ping" a hardware connected to a GPIO line by toggling
75 it from 1-to-0-to-1. If that hardware does not receive its "ping"
76 periodically, it will reset the system.
77
78- gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip
79 to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
80 NAND flash MTD subsystem and provides chip access and partition parsing like
81 any other NAND driving hardware.
82
83- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
84 bus, data and clock line, by bit banging two GPIO lines. It will appear as
85 any other serio bus to the system and makes it possible to connect drivers
86 for e.g. keyboards and other PS/2 protocol based devices.
87
88- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
89 Consumer Electronics Control bus using only GPIO. It is used to communicate
90 with devices on the HDMI bus.
91
92- gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do
93 battery charging and all you have to go by to check the presence of the
94 AC charger or more complex tasks such as indicating charging status using
95 nothing but GPIO lines, this driver provides that and also a clearly defined
96 way to pass the charging parameters from hardware descriptions such as the
97 device tree.
98
99- gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using
100 n GPIO lines such that you can mux in 2^n different devices by activating
101 different GPIO lines. Often the GPIOs are on a SoC and the devices are
102 some SoC-external entities, such as different components on a PCB that
103 can be selectively enabled.
104
105Apart from this there are special GPIO drivers in subsystems like MMC/SD to
106read card detect and write protect GPIO lines, and in the TTY serial subsystem
107to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
108MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
109usually connected directly to the flash.
110
111Use those instead of talking directly to the GPIOs from userspace; they
112integrate with kernel frameworks better than your userspace code could.
113Needless to say, just using the appropriate kernel drivers will simplify and
114speed up your embedded hacking in particular by providing ready-made components.
1============================
2Subsystem drivers using GPIO
3============================
4
5Note that standard kernel drivers exist for common GPIO tasks and will provide
6the right in-kernel and userspace APIs/ABIs for the job, and that these
7drivers can quite easily interconnect with other kernel subsystems using
8hardware descriptions such as device tree or ACPI:
9
10- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO
11 lines, giving you the LED sysfs interface
12
13- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
14 i.e. a LED will turn on/off in response to a GPIO line going high or low
15 (and that LED may in turn use the leds-gpio as per above).
16
17- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
18 can generate interrupts in response to a key press. Also supports debounce.
19
20- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
21 GPIO line cannot generate interrupts, so it needs to be periodically polled
22 by a timer.
23
24- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
25 up to three buttons by simply using GPIOs and no mouse port. You can cut the
26 mouse cable and connect the wires to GPIO lines or solder a mouse connector
27 to the lines for a more permanent solution of this type.
28
29- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
30 an external speaker connected to a GPIO line.
31
32- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
33 external connector status, such as a headset line for an audio driver or an
34 HDMI connector. It will provide a better userspace sysfs interface than GPIO.
35
36- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
37 the system by pulling a GPIO line and will register a restart handler so
38 userspace can issue the right system call to restart the system.
39
40- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
41 system down by pulling a GPIO line and will register a pm_power_off()
42 callback so that userspace can issue the right system call to power down the
43 system.
44
45- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
46 (off/on) that uses a GPIO, and integrated with the clock subsystem.
47
48- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
49 (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
50 appear as any other I2C bus to the system and makes it possible to connect
51 drivers for the I2C devices on the bus like any other I2C bus driver.
52
53- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
54 of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
55 GPIO hammering (bitbang). It will appear as any other SPI bus on the system
56 and makes it possible to connect drivers for SPI devices on the bus like
57 any other SPI bus driver. For example any MMC/SD card can then be connected
58 to this SPI by using the mmc_spi host from the MMC/SD card subsystem.
59
60- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
61 a GPIO line, integrating with the W1 subsystem and handling devices on
62 the bus like any other W1 device.
63
64- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
65 system, connected to a GPIO line (and optionally a GPIO alarm line),
66 presenting all the right in-kernel and sysfs interfaces to make your system
67 not overheat.
68
69- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
70 regulator providing a certain voltage by pulling a GPIO line, integrating
71 with the regulator subsystem and giving you all the right interfaces.
72
73- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
74 that will periodically "ping" a hardware connected to a GPIO line by toggling
75 it from 1-to-0-to-1. If that hardware does not receive its "ping"
76 periodically, it will reset the system.
77
78- gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip
79 to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
80 NAND flash MTD subsystem and provides chip access and partition parsing like
81 any other NAND driving hardware.
82
83- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
84 bus, data and clock line, by bit banging two GPIO lines. It will appear as
85 any other serio bus to the system and makes it possible to connect drivers
86 for e.g. keyboards and other PS/2 protocol based devices.
87
88- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
89 Consumer Electronics Control bus using only GPIO. It is used to communicate
90 with devices on the HDMI bus.
91
92- gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do
93 battery charging and all you have to go by to check the presence of the
94 AC charger or more complex tasks such as indicating charging status using
95 nothing but GPIO lines, this driver provides that and also a clearly defined
96 way to pass the charging parameters from hardware descriptions such as the
97 device tree.
98
99- gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using
100 n GPIO lines such that you can mux in 2^n different devices by activating
101 different GPIO lines. Often the GPIOs are on a SoC and the devices are
102 some SoC-external entities, such as different components on a PCB that
103 can be selectively enabled.
104
105Apart from this there are special GPIO drivers in subsystems like MMC/SD to
106read card detect and write protect GPIO lines, and in the TTY serial subsystem
107to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
108MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
109usually connected directly to the flash.
110
111Use those instead of talking directly to the GPIOs from userspace; they
112integrate with kernel frameworks better than your userspace code could.
113Needless to say, just using the appropriate kernel drivers will simplify and
114speed up your embedded hacking in particular by providing ready-made components.