TODO - OpenGrok cross reference for /linux-6.12.1/drivers/gpio/TODO

1 This is a place for planning the ongoing long-term work in the GPIO
5 GPIO descriptors
7 Starting with commit 79a9becda894 the GPIO subsystem embarked on a journey
8 to move away from the global GPIO numberspace and toward a descriptor-based
9 approach. This means that GPIO consumers, drivers and machine descriptions
10 ideally have no use or idea of the global GPIO numberspace that has/was
11 used in the inception of the GPIO subsystem.
16 The underlying motivation for this is that the GPIO numberspace has become
18 establish the numberspace at compile-time, making it hard to add any numbers
19 in the middle (such as if you missed a pin on a chip) without the numberspace
22 Machine descriptions such as device tree or ACPI does not have a concept of the
23 Linux GPIO number as those descriptions are external to the Linux kernel
24 and treat GPIO lines as abstract entities.
26 The runtime-assigned GPIO numberspace (what you get if you assign the GPIO
27 base as -1 in struct gpio_chip) has also became unpredictable due to factors
28 such as probe ordering and the introduction of -EPROBE_DEFER making probe
29 ordering of independent GPIO chips essentially unpredictable, as their base
30 number will be assigned on a first come first serve basis.
32 The best way to get out of the problem is to make the global GPIO numbers
33 unimportant by simply not using them. GPIO descriptors deal with this.
37 - Convert all GPIO device drivers to only #include <linux/gpio/driver.h>
39 - Convert all consumer drivers to only #include <linux/gpio/consumer.h>
41 - Convert all machine descriptors in "boardfiles" to only
42   #include <linux/gpio/machine.h>, the other option being to convert it
43   to a machine description such as device tree, ACPI or fwnode that
44   implicitly does not use global GPIO numbers.
46 - When this work is complete (will require some of the items in the
48   numberspace accessors from <linux/gpio.h> and eventually delete
49   <linux/gpio.h> altogether.
55 driver infrastructure for doing simpler MMIO GPIO devices and there was
58 the device tree back-end. It is legacy and should not be used in new code.
62 - Change all consumer drivers that #include <linux/of_gpio.h> to
63   #include <linux/gpio/consumer.h> and stop doing custom parsing of the
64   GPIO lines from the device tree. This can be tricky and often ivolves
67 - Pull semantics for legacy device tree (OF) GPIO lookups into
68   gpiolib-of.c: in some cases subsystems are doing custom flags and
75 - Delete <linux/of_gpio.h> when all the above is complete and everything
76   uses <linux/gpio/consumer.h> or <linux/gpio/driver.h> instead.
79 Get rid of <linux/gpio/legacy-of-mm-gpiochip.h>
83 - Get rid of struct of_mm_gpio_chip altogether: use the generic  MMIO
84   GPIO for all current users (see below). Delete struct of_mm_gpio_chip,
89 Get rid of <linux/gpio.h>
91 This legacy header is a one stop shop for anything GPIO is closely tied
92 to the global GPIO numberspace. The endgame of the above refactorings will
93 be the removal of <linux/gpio.h> and from that point only the specialized
94 headers under <linux/gpio/*.h> will be used. This requires all the above to
95 be completed and is expected to take a long time.
100 Collect GPIO drivers from arch/* and other places that should be placed
101 in drivers/gpio/gpio-*. Augment platforms to create platform devices or
102 similar and probe a proper driver in the gpiolib subsystem.
104 In some cases it makes sense to create a GPIO chip from the local driver
105 for a few GPIOs. Those should stay where they are.
108 new coming drivers. For example, gpio-ml-ioh should be incorporated into
109 gpio-pch.
112 Generic MMIO GPIO
114 The GPIO drivers can utilize the generic MMIO helper library in many
116 drivers. (drivers/gpio/gpio-mmio.c)
120 - Look over and identify any remaining easily converted drivers and
121   dry-code conversions to MMIO GPIO for maintainers to test
123 - Expand the MMIO GPIO or write a new library for regmap-based I/O
124   helpers for GPIO drivers on regmap that simply use offsets
125   0..n in some register to drive GPIO lines
127 - Expand the MMIO GPIO or write a new library for port-mapped I/O
128   helpers (x86 inb()/outb()) and convert port-mapped I/O drivers to use
129   this with dry-coding and sending to maintainers to test
132 Generic regmap GPIO
134 In the very similar way to Generic MMIO GPIO convert the users which can
136 MMIO case the regmap MMIO with gpio-regmap.c is preferable over gpio-mmio.c.
141 The GPIOLIB irqchip is a helper irqchip for "simple cases" that should
142 try to cover any generic kind of irqchip cascaded from a GPIO.
144 - Look over and identify any remaining easily converted drivers and
145   dry-code conversions to gpiolib irqchip for maintainers to test
150 There are already ways to use pin control as back-end for GPIO and
153 use of the global GPIO numbers. Once the above is complete, it may
155 multiplexing, pin configuration, GPIO, etc selectable options in one
156 and the same pin control and GPIO subsystem.
164 root filesystems where you only have a minimal set of tools such
168 as it relies on the global GPIO numberspace that assume a strict
169 order of global GPIO numbers that do not change between boots
173 and development, implement a debugfs interface to manipulate
174 GPIO lines that can do everything that sysfs can do today: one
189 discussed but the idea is to provide a low-level access point
197 Most of the gpio chips implementing interrupt support rely on gpiolib
199 from being made read-only and forcing duplication of structures that
204 core code know about that by exposing a flag (IRQCHIP_IMMUTABLE) in
208 A small number of drivers have been converted (pl061, tegra186, msm,