205 lines
8.1 KiB
Plaintext
205 lines
8.1 KiB
Plaintext
This is a place for planning the ongoing long-term work in the GPIO
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subsystem.
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GPIO descriptors
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Starting with commit 79a9becda894 the GPIO subsystem embarked on a journey
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to move away from the global GPIO numberspace and toward a descriptor-based
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approach. This means that GPIO consumers, drivers and machine descriptions
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ideally have no use or idea of the global GPIO numberspace that has/was
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used in the inception of the GPIO subsystem.
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The numberspace issue is the same as to why irq is moving away from irq
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numbers to IRQ descriptors.
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The underlying motivation for this is that the GPIO numberspace has become
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unmanageable: machine board files tend to become full of macros trying to
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establish the numberspace at compile-time, making it hard to add any numbers
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in the middle (such as if you missed a pin on a chip) without the numberspace
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breaking.
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Machine descriptions such as device tree or ACPI does not have a concept of the
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Linux GPIO number as those descriptions are external to the Linux kernel
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and treat GPIO lines as abstract entities.
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The runtime-assigned GPIO numberspace (what you get if you assign the GPIO
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base as -1 in struct gpio_chip) has also became unpredictable due to factors
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such as probe ordering and the introduction of -EPROBE_DEFER making probe
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ordering of independent GPIO chips essentially unpredictable, as their base
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number will be assigned on a first come first serve basis.
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The best way to get out of the problem is to make the global GPIO numbers
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unimportant by simply not using them. GPIO descriptors deal with this.
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Work items:
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- Convert all GPIO device drivers to only #include <linux/gpio/driver.h>
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- Convert all consumer drivers to only #include <linux/gpio/consumer.h>
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- Convert all machine descriptors in "boardfiles" to only
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#include <linux/gpio/machine.h>, the other option being to convert it
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to a machine description such as device tree, ACPI or fwnode that
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implicitly does not use global GPIO numbers.
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- When this work is complete (will require some of the items in the
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following ongoing work as well) we can delete the old global
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numberspace accessors from <linux/gpio.h> and eventually delete
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<linux/gpio.h> altogether.
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Get rid of <linux/of_gpio.h>
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This header and helpers appeared at one point when there was no proper
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driver infrastructure for doing simpler MMIO GPIO devices and there was
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no core support for parsing device tree GPIOs from the core library with
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the [devm_]gpiod_get() calls we have today that will implicitly go into
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the device tree back-end. It is legacy and should not be used in new code.
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Work items:
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- Change all consumer drivers that #include <linux/of_gpio.h> to
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#include <linux/gpio/consumer.h> and stop doing custom parsing of the
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GPIO lines from the device tree. This can be tricky and often ivolves
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changing boardfiles, etc.
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- Pull semantics for legacy device tree (OF) GPIO lookups into
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gpiolib-of.c: in some cases subsystems are doing custom flags and
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lookups for polarity inversion, open drain and what not. As we now
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handle this with generic OF bindings, pull all legacy handling into
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gpiolib so the library API becomes narrow and deep and handle all
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legacy bindings internally. (See e.g. commits 6953c57ab172,
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6a537d48461d etc)
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- Delete <linux/of_gpio.h> when all the above is complete and everything
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uses <linux/gpio/consumer.h> or <linux/gpio/driver.h> instead.
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Get rid of <linux/gpio/legacy-of-mm-gpiochip.h>
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Work items:
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- Get rid of struct of_mm_gpio_chip altogether: use the generic MMIO
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GPIO for all current users (see below). Delete struct of_mm_gpio_chip,
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to_of_mm_gpio_chip(), of_mm_gpiochip_add_data(), of_mm_gpiochip_remove(),
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CONFIG_OF_GPIO_MM_GPIOCHIP from the kernel.
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Get rid of <linux/gpio.h>
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This legacy header is a one stop shop for anything GPIO is closely tied
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to the global GPIO numberspace. The endgame of the above refactorings will
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be the removal of <linux/gpio.h> and from that point only the specialized
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headers under <linux/gpio/*.h> will be used. This requires all the above to
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be completed and is expected to take a long time.
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Collect drivers
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Collect GPIO drivers from arch/* and other places that should be placed
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in drivers/gpio/gpio-*. Augment platforms to create platform devices or
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similar and probe a proper driver in the gpiolib subsystem.
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In some cases it makes sense to create a GPIO chip from the local driver
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for a few GPIOs. Those should stay where they are.
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At the same time it makes sense to get rid of code duplication in existing or
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new coming drivers. For example, gpio-ml-ioh should be incorporated into
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gpio-pch.
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Generic MMIO GPIO
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The GPIO drivers can utilize the generic MMIO helper library in many
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cases, and the helper library should be as helpful as possible for MMIO
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drivers. (drivers/gpio/gpio-mmio.c)
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Work items:
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- Look over and identify any remaining easily converted drivers and
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dry-code conversions to MMIO GPIO for maintainers to test
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- Expand the MMIO GPIO or write a new library for regmap-based I/O
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helpers for GPIO drivers on regmap that simply use offsets
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0..n in some register to drive GPIO lines
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- Expand the MMIO GPIO or write a new library for port-mapped I/O
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helpers (x86 inb()/outb()) and convert port-mapped I/O drivers to use
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this with dry-coding and sending to maintainers to test
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GPIOLIB irqchip
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The GPIOLIB irqchip is a helper irqchip for "simple cases" that should
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try to cover any generic kind of irqchip cascaded from a GPIO.
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- Look over and identify any remaining easily converted drivers and
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dry-code conversions to gpiolib irqchip for maintainers to test
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Increase integration with pin control
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There are already ways to use pin control as back-end for GPIO and
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it may make sense to bring these subsystems closer. One reason for
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creating pin control as its own subsystem was that we could avoid any
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use of the global GPIO numbers. Once the above is complete, it may
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make sense to simply join the subsystems into one and make pin
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multiplexing, pin configuration, GPIO, etc selectable options in one
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and the same pin control and GPIO subsystem.
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Debugfs in place of sysfs
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The old sysfs code that enables simple uses of GPIOs from the
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command line is still popular despite the existance of the proper
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character device. The reason is that it is simple to use on
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root filesystems where you only have a minimal set of tools such
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as "cat", "echo" etc.
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The old sysfs still need to be strongly deprecated and removed
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as it relies on the global GPIO numberspace that assume a strict
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order of global GPIO numbers that do not change between boots
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and is independent of probe order.
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To solve this and provide an ABI that people can use for hacks
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and development, implement a debugfs interface to manipulate
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GPIO lines that can do everything that sysfs can do today: one
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directory per gpiochip and one file entry per line:
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/sys/kernel/debug/gpiochip/gpiochip0
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/sys/kernel/debug/gpiochip/gpiochip0/gpio0
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/sys/kernel/debug/gpiochip/gpiochip0/gpio1
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/sys/kernel/debug/gpiochip/gpiochip0/gpio2
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/sys/kernel/debug/gpiochip/gpiochip0/gpio3
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...
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/sys/kernel/debug/gpiochip/gpiochip1
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/sys/kernel/debug/gpiochip/gpiochip1/gpio0
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/sys/kernel/debug/gpiochip/gpiochip1/gpio1
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...
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The exact files and design of the debugfs interface can be
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discussed but the idea is to provide a low-level access point
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for debugging and hacking and to expose all lines without the
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need of any exporting. Also provide ample ammunition to shoot
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oneself in the foot, because this is debugfs after all.
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Moving over to immutable irq_chip structures
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Most of the gpio chips implementing interrupt support rely on gpiolib
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intercepting some of the irq_chip callbacks, preventing the structures
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from being made read-only and forcing duplication of structures that
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should otherwise be unique.
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The solution is to call into the gpiolib code when needed (resource
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management, enable/disable or unmask/mask callbacks), and to let the
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core code know about that by exposing a flag (IRQCHIP_IMMUTABLE) in
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the irq_chip structure. The irq_chip structure can then be made unique
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and const.
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A small number of drivers have been converted (pl061, tegra186, msm,
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amd, apple), and can be used as examples of how to proceed with this
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conversion. Note that drivers using the generic irqchip framework
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cannot be converted yet, but watch this space!
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