125 lines
5.8 KiB
ReStructuredText
125 lines
5.8 KiB
ReStructuredText
============
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Introduction
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============
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GPIO Interfaces
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===============
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The documents in this directory give detailed instructions on how to access
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GPIOs in drivers, and how to write a driver for a device that provides GPIOs
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itself.
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Due to the history of GPIO interfaces in the kernel, there are two different
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ways to obtain and use GPIOs:
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- The descriptor-based interface is the preferred way to manipulate GPIOs,
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and is described by all the files in this directory excepted gpio-legacy.txt.
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- The legacy integer-based interface which is considered deprecated (but still
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usable for compatibility reasons) is documented in gpio-legacy.txt.
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The remainder of this document applies to the new descriptor-based interface.
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gpio-legacy.txt contains the same information applied to the legacy
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integer-based interface.
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What is a GPIO?
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===============
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A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
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digital signal. They are provided from many kinds of chip, and are familiar
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to Linux developers working with embedded and custom hardware. Each GPIO
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represents a bit connected to a particular pin, or "ball" on Ball Grid Array
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(BGA) packages. Board schematics show which external hardware connects to
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which GPIOs. Drivers can be written generically, so that board setup code
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passes such pin configuration data to drivers.
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System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
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non-dedicated pin can be configured as a GPIO; and most chips have at least
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several dozen of them. Programmable logic devices (like FPGAs) can easily
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provide GPIOs; multifunction chips like power managers, and audio codecs
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often have a few such pins to help with pin scarcity on SOCs; and there are
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also "GPIO Expander" chips that connect using the I2C or SPI serial buses.
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Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
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firmware knowing how they're used).
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The exact capabilities of GPIOs vary between systems. Common options:
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- Output values are writable (high=1, low=0). Some chips also have
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options about how that value is driven, so that for example only one
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value might be driven, supporting "wire-OR" and similar schemes for the
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other value (notably, "open drain" signaling).
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- Input values are likewise readable (1, 0). Some chips support readback
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of pins configured as "output", which is very useful in such "wire-OR"
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cases (to support bidirectional signaling). GPIO controllers may have
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input de-glitch/debounce logic, sometimes with software controls.
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- Inputs can often be used as IRQ signals, often edge triggered but
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sometimes level triggered. Such IRQs may be configurable as system
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wakeup events, to wake the system from a low power state.
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- Usually a GPIO will be configurable as either input or output, as needed
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by different product boards; single direction ones exist too.
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- Most GPIOs can be accessed while holding spinlocks, but those accessed
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through a serial bus normally can't. Some systems support both types.
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On a given board each GPIO is used for one specific purpose like monitoring
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MMC/SD card insertion/removal, detecting card write-protect status, driving
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a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware
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watchdog, sensing a switch, and so on.
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Common GPIO Properties
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======================
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These properties are met through all the other documents of the GPIO interface
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and it is useful to understand them, especially if you need to define GPIO
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mappings.
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Active-High and Active-Low
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--------------------------
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It is natural to assume that a GPIO is "active" when its output signal is 1
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("high"), and inactive when it is 0 ("low"). However in practice the signal of a
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GPIO may be inverted before is reaches its destination, or a device could decide
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to have different conventions about what "active" means. Such decisions should
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be transparent to device drivers, therefore it is possible to define a GPIO as
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being either active-high ("1" means "active", the default) or active-low ("0"
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means "active") so that drivers only need to worry about the logical signal and
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not about what happens at the line level.
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Open Drain and Open Source
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--------------------------
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Sometimes shared signals need to use "open drain" (where only the low signal
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level is actually driven), or "open source" (where only the high signal level is
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driven) signaling. That term applies to CMOS transistors; "open collector" is
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used for TTL. A pullup or pulldown resistor causes the high or low signal level.
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This is sometimes called a "wire-AND"; or more practically, from the negative
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logic (low=true) perspective this is a "wire-OR".
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One common example of an open drain signal is a shared active-low IRQ line.
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Also, bidirectional data bus signals sometimes use open drain signals.
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Some GPIO controllers directly support open drain and open source outputs; many
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don't. When you need open drain signaling but your hardware doesn't directly
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support it, there's a common idiom you can use to emulate it with any GPIO pin
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that can be used as either an input or an output:
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LOW: gpiod_direction_output(gpio, 0) ... this drives the signal and overrides
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the pullup.
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HIGH: gpiod_direction_input(gpio) ... this turns off the output, so the pullup
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(or some other device) controls the signal.
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The same logic can be applied to emulate open source signaling, by driving the
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high signal and configuring the GPIO as input for low. This open drain/open
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source emulation can be handled transparently by the GPIO framework.
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If you are "driving" the signal high but gpiod_get_value(gpio) reports a low
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value (after the appropriate rise time passes), you know some other component is
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driving the shared signal low. That's not necessarily an error. As one common
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example, that's how I2C clocks are stretched: a slave that needs a slower clock
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delays the rising edge of SCK, and the I2C master adjusts its signaling rate
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accordingly.
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