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Commit | Line | Data |
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2646b90d LW |
1 | This is a place for planning the ongoing long-term work in the GPIO |
2 | subsystem. | |
3 | ||
4 | ||
5 | GPIO descriptors | |
6 | ||
7 | Starting with commit 79a9becda894 the GPIO subsystem embarked on a journey | |
3abda79a | 8 | to move away from the global GPIO numberspace and toward a descriptor-based |
2646b90d LW |
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. | |
12 | ||
97082890 LW |
13 | The numberspace issue is the same as to why irq is moving away from irq |
14 | numbers to IRQ descriptors. | |
15 | ||
16 | The underlying motivation for this is that the GPIO numberspace has become | |
17 | unmanageable: machine board files tend to become full of macros trying to | |
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 | |
20 | breaking. | |
21 | ||
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. | |
25 | ||
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. | |
31 | ||
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. | |
34 | ||
2646b90d LW |
35 | Work 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 | ||
52 | Get rid of <linux/of_gpio.h> | |
53 | ||
54 | This header and helpers appeared at one point when there was no proper | |
55 | driver infrastructure for doing simpler MMIO GPIO devices and there was | |
56 | no core support for parsing device tree GPIOs from the core library with | |
57 | the [devm_]gpiod_get() calls we have today that will implicitly go into | |
97082890 | 58 | the device tree back-end. It is legacy and should not be used in new code. |
2646b90d LW |
59 | |
60 | Work 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 | ||
97082890 LW |
84 | Get rid of <linux/gpio.h> |
85 | ||
86 | This legacy header is a one stop shop for anything GPIO is closely tied | |
87 | to the global GPIO numberspace. The endgame of the above refactorings will | |
88 | be the removal of <linux/gpio.h> and from that point only the specialized | |
89 | headers under <linux/gpio/*.h> will be used. This requires all the above to | |
90 | be completed and is expected to take a long time. | |
91 | ||
92 | ||
2646b90d LW |
93 | Collect drivers |
94 | ||
95 | Collect GPIO drivers from arch/* and other places that should be placed | |
96 | in drivers/gpio/gpio-*. Augment platforms to create platform devices or | |
97 | similar and probe a proper driver in the gpiolib subsystem. | |
98 | ||
99 | In some cases it makes sense to create a GPIO chip from the local driver | |
100 | for a few GPIOs. Those should stay where they are. | |
101 | ||
85a94ff8 AS |
102 | At the same time it makes sense to get rid of code duplication in existing or |
103 | new coming drivers. For example, gpio-ml-ioh should be incorporated into | |
5f7582aa | 104 | gpio-pch. |
85a94ff8 | 105 | |
2646b90d LW |
106 | |
107 | Generic MMIO GPIO | |
108 | ||
109 | The GPIO drivers can utilize the generic MMIO helper library in many | |
110 | cases, and the helper library should be as helpful as possible for MMIO | |
111 | drivers. (drivers/gpio/gpio-mmio.c) | |
112 | ||
113 | Work 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 | ||
41c4616b LW |
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 | ||
2646b90d LW |
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 | ||
127 | GPIOLIB irqchip | |
128 | ||
129 | The GPIOLIB irqchip is a helper irqchip for "simple cases" that should | |
130 | try 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 | ||
2646b90d LW |
135 | |
136 | Increase integration with pin control | |
137 | ||
138 | There are already ways to use pin control as back-end for GPIO and | |
139 | it may make sense to bring these subsystems closer. One reason for | |
140 | creating pin control as its own subsystem was that we could avoid any | |
141 | use of the global GPIO numbers. Once the above is complete, it may | |
142 | make sense to simply join the subsystems into one and make pin | |
143 | multiplexing, pin configuration, GPIO, etc selectable options in one | |
144 | and the same pin control and GPIO subsystem. | |
dd0fa811 LW |
145 | |
146 | ||
147 | Debugfs in place of sysfs | |
148 | ||
149 | The old sysfs code that enables simple uses of GPIOs from the | |
150 | command line is still popular despite the existance of the proper | |
151 | character device. The reason is that it is simple to use on | |
152 | root filesystems where you only have a minimal set of tools such | |
153 | as "cat", "echo" etc. | |
154 | ||
155 | The old sysfs still need to be strongly deprecated and removed | |
156 | as it relies on the global GPIO numberspace that assume a strict | |
157 | order of global GPIO numbers that do not change between boots | |
158 | and is independent of probe order. | |
159 | ||
160 | To solve this and provide an ABI that people can use for hacks | |
161 | and development, implement a debugfs interface to manipulate | |
162 | GPIO lines that can do everything that sysfs can do today: one | |
163 | directory 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 | ||
176 | The exact files and design of the debugfs interface can be | |
177 | discussed but the idea is to provide a low-level access point | |
178 | for debugging and hacking and to expose all lines without the | |
179 | need of any exporting. Also provide ample ammunition to shoot | |
180 | oneself in the foot, because this is debugfs after all. |