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