xref: /openbmc/linux/drivers/gpio/TODO (revision 842ed298)
1This is a place for planning the ongoing long-term work in the GPIO
2subsystem.
3
4
5GPIO descriptors
6
7Starting with commit 79a9becda894 the GPIO subsystem embarked on a journey
8to move away from the global GPIO numberspace and toward a descriptor-based
9approach. This means that GPIO consumers, drivers and machine descriptions
10ideally have no use or idea of the global GPIO numberspace that has/was
11used in the inception of the GPIO subsystem.
12
13The numberspace issue is the same as to why irq is moving away from irq
14numbers to IRQ descriptors.
15
16The underlying motivation for this is that the GPIO numberspace has become
17unmanageable: machine board files tend to become full of macros trying to
18establish the numberspace at compile-time, making it hard to add any numbers
19in the middle (such as if you missed a pin on a chip) without the numberspace
20breaking.
21
22Machine descriptions such as device tree or ACPI does not have a concept of the
23Linux GPIO number as those descriptions are external to the Linux kernel
24and treat GPIO lines as abstract entities.
25
26The runtime-assigned GPIO numberspace (what you get if you assign the GPIO
27base as -1 in struct gpio_chip) has also became unpredictable due to factors
28such as probe ordering and the introduction of -EPROBE_DEFER making probe
29ordering of independent GPIO chips essentially unpredictable, as their base
30number will be assigned on a first come first serve basis.
31
32The best way to get out of the problem is to make the global GPIO numbers
33unimportant by simply not using them. GPIO descriptors deal with this.
34
35Work items:
36
37- Convert all GPIO device drivers to only #include <linux/gpio/driver.h>
38
39- Convert all consumer drivers to only #include <linux/gpio/consumer.h>
40
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.
45
46- When this work is complete (will require some of the items in the
47  following ongoing work as well) we can delete the old global
48  numberspace accessors from <linux/gpio.h> and eventually delete
49  <linux/gpio.h> altogether.
50
51
52Get rid of <linux/of_gpio.h>
53
54This header and helpers appeared at one point when there was no proper
55driver infrastructure for doing simpler MMIO GPIO devices and there was
56no core support for parsing device tree GPIOs from the core library with
57the [devm_]gpiod_get() calls we have today that will implicitly go into
58the device tree back-end. It is legacy and should not be used in new code.
59
60Work items:
61
62- Get rid of struct of_mm_gpio_chip altogether: use the generic  MMIO
63  GPIO for all current users (see below). Delete struct of_mm_gpio_chip,
64  to_of_mm_gpio_chip(), of_mm_gpiochip_add_data(), of_mm_gpiochip_add()
65  of_mm_gpiochip_remove() from the kernel.
66
67- Change all consumer drivers that #include <linux/of_gpio.h> to
68  #include <linux/gpio/consumer.h> and stop doing custom parsing of the
69  GPIO lines from the device tree. This can be tricky and often ivolves
70  changing boardfiles, etc.
71
72- Pull semantics for legacy device tree (OF) GPIO lookups into
73  gpiolib-of.c: in some cases subsystems are doing custom flags and
74  lookups for polarity inversion, open drain and what not. As we now
75  handle this with generic OF bindings, pull all legacy handling into
76  gpiolib so the library API becomes narrow and deep and handle all
77  legacy bindings internally. (See e.g. commits 6953c57ab172,
78  6a537d48461d etc)
79
80- Delete <linux/of_gpio.h> when all the above is complete and everything
81  uses <linux/gpio/consumer.h> or <linux/gpio/driver.h> instead.
82
83
84Get rid of <linux/gpio.h>
85
86This legacy header is a one stop shop for anything GPIO is closely tied
87to the global GPIO numberspace. The endgame of the above refactorings will
88be the removal of <linux/gpio.h> and from that point only the specialized
89headers under <linux/gpio/*.h> will be used. This requires all the above to
90be completed and is expected to take a long time.
91
92
93Collect drivers
94
95Collect GPIO drivers from arch/* and other places that should be placed
96in drivers/gpio/gpio-*. Augment platforms to create platform devices or
97similar and probe a proper driver in the gpiolib subsystem.
98
99In some cases it makes sense to create a GPIO chip from the local driver
100for a few GPIOs. Those should stay where they are.
101
102At the same time it makes sense to get rid of code duplication in existing or
103new coming drivers. For example, gpio-ml-ioh should be incorporated into
104gpio-pch. In similar way gpio-intel-mid into gpio-pxa.
105
106
107Generic MMIO GPIO
108
109The GPIO drivers can utilize the generic MMIO helper library in many
110cases, and the helper library should be as helpful as possible for MMIO
111drivers. (drivers/gpio/gpio-mmio.c)
112
113Work items:
114
115- Look over and identify any remaining easily converted drivers and
116  dry-code conversions to MMIO GPIO for maintainers to test
117
118- Expand the MMIO GPIO or write a new library for regmap-based I/O
119  helpers for GPIO drivers on regmap that simply use offsets
120  0..n in some register to drive GPIO lines
121
122- Expand the MMIO GPIO or write a new library for port-mapped I/O
123  helpers (x86 inb()/outb()) and convert port-mapped I/O drivers to use
124  this with dry-coding and sending to maintainers to test
125
126
127GPIOLIB irqchip
128
129The GPIOLIB irqchip is a helper irqchip for "simple cases" that should
130try to cover any generic kind of irqchip cascaded from a GPIO.
131
132- Look over and identify any remaining easily converted drivers and
133  dry-code conversions to gpiolib irqchip for maintainers to test
134
135
136Increase integration with pin control
137
138There are already ways to use pin control as back-end for GPIO and
139it may make sense to bring these subsystems closer. One reason for
140creating pin control as its own subsystem was that we could avoid any
141use of the global GPIO numbers. Once the above is complete, it may
142make sense to simply join the subsystems into one and make pin
143multiplexing, pin configuration, GPIO, etc selectable options in one
144and the same pin control and GPIO subsystem.
145
146
147Debugfs in place of sysfs
148
149The old sysfs code that enables simple uses of GPIOs from the
150command line is still popular despite the existance of the proper
151character device. The reason is that it is simple to use on
152root filesystems where you only have a minimal set of tools such
153as "cat", "echo" etc.
154
155The old sysfs still need to be strongly deprecated and removed
156as it relies on the global GPIO numberspace that assume a strict
157order of global GPIO numbers that do not change between boots
158and is independent of probe order.
159
160To solve this and provide an ABI that people can use for hacks
161and development, implement a debugfs interface to manipulate
162GPIO lines that can do everything that sysfs can do today: one
163directory per gpiochip and one file entry per line:
164
165/sys/kernel/debug/gpiochip/gpiochip0
166/sys/kernel/debug/gpiochip/gpiochip0/gpio0
167/sys/kernel/debug/gpiochip/gpiochip0/gpio1
168/sys/kernel/debug/gpiochip/gpiochip0/gpio2
169/sys/kernel/debug/gpiochip/gpiochip0/gpio3
170...
171/sys/kernel/debug/gpiochip/gpiochip1
172/sys/kernel/debug/gpiochip/gpiochip1/gpio0
173/sys/kernel/debug/gpiochip/gpiochip1/gpio1
174...
175
176The exact files and design of the debugfs interface can be
177discussed but the idea is to provide a low-level access point
178for debugging and hacking and to expose all lines without the
179need of any exporting. Also provide ample ammunition to shoot
180oneself in the foot, because this is debugfs after all.
181