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- 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
65  changing boardfiles, etc.
66
67- Pull semantics for legacy device tree (OF) GPIO lookups into
68  gpiolib-of.c: in some cases subsystems are doing custom flags and
69  lookups for polarity inversion, open drain and what not. As we now
70  handle this with generic OF bindings, pull all legacy handling into
71  gpiolib so the library API becomes narrow and deep and handle all
72  legacy bindings internally. (See e.g. commits 6953c57ab172,
73  6a537d48461d etc)
74
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.
77
78
79Get rid of <linux/gpio/legacy-of-mm-gpiochip.h>
80
81Work items:
82
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,
85  to_of_mm_gpio_chip(), of_mm_gpiochip_add_data(), of_mm_gpiochip_remove(),
86  CONFIG_OF_GPIO_MM_GPIOCHIP from the kernel.
87
88
89Get rid of <linux/gpio.h>
90
91This legacy header is a one stop shop for anything GPIO is closely tied
92to the global GPIO numberspace. The endgame of the above refactorings will
93be the removal of <linux/gpio.h> and from that point only the specialized
94headers under <linux/gpio/*.h> will be used. This requires all the above to
95be completed and is expected to take a long time.
96
97
98Collect drivers
99
100Collect GPIO drivers from arch/* and other places that should be placed
101in drivers/gpio/gpio-*. Augment platforms to create platform devices or
102similar and probe a proper driver in the gpiolib subsystem.
103
104In some cases it makes sense to create a GPIO chip from the local driver
105for a few GPIOs. Those should stay where they are.
106
107At the same time it makes sense to get rid of code duplication in existing or
108new coming drivers. For example, gpio-ml-ioh should be incorporated into
109gpio-pch.
110
111
112Generic MMIO GPIO
113
114The GPIO drivers can utilize the generic MMIO helper library in many
115cases, and the helper library should be as helpful as possible for MMIO
116drivers. (drivers/gpio/gpio-mmio.c)
117
118Work items:
119
120- Look over and identify any remaining easily converted drivers and
121  dry-code conversions to MMIO GPIO for maintainers to test
122
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
126
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
130
131
132Generic regmap GPIO
133
134In the very similar way to Generic MMIO GPIO convert the users which can
135take advantage of using regmap over direct IO accessors. Note, even in
136MMIO case the regmap MMIO with gpio-regmap.c is preferable over gpio-mmio.c.
137
138
139GPIOLIB irqchip
140
141The GPIOLIB irqchip is a helper irqchip for "simple cases" that should
142try to cover any generic kind of irqchip cascaded from a GPIO.
143
144- Look over and identify any remaining easily converted drivers and
145  dry-code conversions to gpiolib irqchip for maintainers to test
146
147
148Increase integration with pin control
149
150There are already ways to use pin control as back-end for GPIO and
151it may make sense to bring these subsystems closer. One reason for
152creating pin control as its own subsystem was that we could avoid any
153use of the global GPIO numbers. Once the above is complete, it may
154make sense to simply join the subsystems into one and make pin
155multiplexing, pin configuration, GPIO, etc selectable options in one
156and the same pin control and GPIO subsystem.
157
158
159Debugfs in place of sysfs
160
161The old sysfs code that enables simple uses of GPIOs from the
162command line is still popular despite the existance of the proper
163character device. The reason is that it is simple to use on
164root filesystems where you only have a minimal set of tools such
165as "cat", "echo" etc.
166
167The old sysfs still need to be strongly deprecated and removed
168as it relies on the global GPIO numberspace that assume a strict
169order of global GPIO numbers that do not change between boots
170and is independent of probe order.
171
172To solve this and provide an ABI that people can use for hacks
173and development, implement a debugfs interface to manipulate
174GPIO lines that can do everything that sysfs can do today: one
175directory per gpiochip and one file entry per line:
176
177/sys/kernel/debug/gpiochip/gpiochip0
178/sys/kernel/debug/gpiochip/gpiochip0/gpio0
179/sys/kernel/debug/gpiochip/gpiochip0/gpio1
180/sys/kernel/debug/gpiochip/gpiochip0/gpio2
181/sys/kernel/debug/gpiochip/gpiochip0/gpio3
182...
183/sys/kernel/debug/gpiochip/gpiochip1
184/sys/kernel/debug/gpiochip/gpiochip1/gpio0
185/sys/kernel/debug/gpiochip/gpiochip1/gpio1
186...
187
188The exact files and design of the debugfs interface can be
189discussed but the idea is to provide a low-level access point
190for debugging and hacking and to expose all lines without the
191need of any exporting. Also provide ample ammunition to shoot
192oneself in the foot, because this is debugfs after all.
193
194
195Moving over to immutable irq_chip structures
196
197Most of the gpio chips implementing interrupt support rely on gpiolib
198intercepting some of the irq_chip callbacks, preventing the structures
199from being made read-only and forcing duplication of structures that
200should otherwise be unique.
201
202The solution is to call into the gpiolib code when needed (resource
203management, enable/disable or unmask/mask callbacks), and to let the
204core code know about that by exposing a flag (IRQCHIP_IMMUTABLE) in
205the irq_chip structure. The irq_chip structure can then be made unique
206and const.
207
208A small number of drivers have been converted (pl061, tegra186, msm,
209amd, apple), and can be used as examples of how to proceed with this
210conversion. Note that drivers using the generic irqchip framework
211cannot be converted yet, but watch this space!
212