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. In similar way gpio-intel-mid into gpio-pxa.
|
|
|
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
|
|
|
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.
|
|
- Convert all the GPIOLIB_IRQCHIP users to pass an irqchip template,
|
parent and flags before calling [devm_]gpiochip_add[_data]().
|
Currently we set up the irqchip after setting up the gpiochip
|
using gpiochip_irqchip_add() and gpiochip_set_[chained|nested]_irqchip().
|
This is too complex, so convert all users over to just set up
|
the irqchip before registering the gpio_chip, typical example:
|
|
/* Typical state container with dynamic irqchip */
|
struct my_gpio {
|
struct gpio_chip gc;
|
struct irq_chip irq;
|
};
|
|
int irq; /* from platform etc */
|
struct my_gpio *g;
|
struct gpio_irq_chip *girq;
|
|
/* Set up the irqchip dynamically */
|
g->irq.name = "my_gpio_irq";
|
g->irq.irq_ack = my_gpio_ack_irq;
|
g->irq.irq_mask = my_gpio_mask_irq;
|
g->irq.irq_unmask = my_gpio_unmask_irq;
|
g->irq.irq_set_type = my_gpio_set_irq_type;
|
|
/* Get a pointer to the gpio_irq_chip */
|
girq = &g->gc.irq;
|
girq->chip = &g->irq;
|
girq->parent_handler = ftgpio_gpio_irq_handler;
|
girq->num_parents = 1;
|
girq->parents = devm_kcalloc(dev, 1, sizeof(*girq->parents),
|
GFP_KERNEL);
|
if (!girq->parents)
|
return -ENOMEM;
|
girq->default_type = IRQ_TYPE_NONE;
|
girq->handler = handle_bad_irq;
|
girq->parents[0] = irq;
|
|
When this is done, we will delete the old APIs for instatiating
|
GPIOLIB_IRQCHIP and simplify the code.
|
|
- Look over and identify any remaining easily converted drivers and
|
dry-code conversions to gpiolib irqchip for maintainers to test
|
|
- Drop gpiochip_set_chained_irqchip() when all the chained irqchips
|
have been converted to the above infrastructure.
|
|
- Add more infrastructure to make it possible to also pass a threaded
|
irqchip in struct gpio_irq_chip.
|
|
- Drop gpiochip_irqchip_add_nested() when all the chained irqchips
|
have been converted to the above infrastructure.
|
|
|
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.
|
