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Diffstat (limited to 'drivers/gpio/TODO')
| -rw-r--r-- | drivers/gpio/TODO | 211 |
1 files changed, 211 insertions, 0 deletions
diff --git a/drivers/gpio/TODO b/drivers/gpio/TODO new file mode 100644 index 0000000000..189c3abe7e --- /dev/null +++ b/drivers/gpio/TODO @@ -0,0 +1,211 @@ +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: + +- 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/legacy-of-mm-gpiochip.h> + +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_remove(), + CONFIG_OF_GPIO_MM_GPIOCHIP from the kernel. + + +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. + + +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 + + +Generic regmap GPIO + +In the very similar way to Generic MMIO GPIO convert the users which can +take advantage of using regmap over direct IO accessors. Note, even in +MMIO case the regmap MMIO with gpio-regmap.c is preferable over gpio-mmio.c. + + +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. + +- Look over and identify any remaining easily converted drivers and + dry-code conversions to gpiolib irqchip for maintainers to test + + +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. + + +Debugfs in place of sysfs + +The old sysfs code that enables simple uses of GPIOs from the +command line is still popular despite the existance of the proper +character device. The reason is that it is simple to use on +root filesystems where you only have a minimal set of tools such +as "cat", "echo" etc. + +The old sysfs still need to be strongly deprecated and removed +as it relies on the global GPIO numberspace that assume a strict +order of global GPIO numbers that do not change between boots +and is independent of probe order. + +To solve this and provide an ABI that people can use for hacks +and development, implement a debugfs interface to manipulate +GPIO lines that can do everything that sysfs can do today: one +directory per gpiochip and one file entry per line: + +/sys/kernel/debug/gpiochip/gpiochip0 +/sys/kernel/debug/gpiochip/gpiochip0/gpio0 +/sys/kernel/debug/gpiochip/gpiochip0/gpio1 +/sys/kernel/debug/gpiochip/gpiochip0/gpio2 +/sys/kernel/debug/gpiochip/gpiochip0/gpio3 +... +/sys/kernel/debug/gpiochip/gpiochip1 +/sys/kernel/debug/gpiochip/gpiochip1/gpio0 +/sys/kernel/debug/gpiochip/gpiochip1/gpio1 +... + +The exact files and design of the debugfs interface can be +discussed but the idea is to provide a low-level access point +for debugging and hacking and to expose all lines without the +need of any exporting. Also provide ample ammunition to shoot +oneself in the foot, because this is debugfs after all. + + +Moving over to immutable irq_chip structures + +Most of the gpio chips implementing interrupt support rely on gpiolib +intercepting some of the irq_chip callbacks, preventing the structures +from being made read-only and forcing duplication of structures that +should otherwise be unique. + +The solution is to call into the gpiolib code when needed (resource +management, enable/disable or unmask/mask callbacks), and to let the +core code know about that by exposing a flag (IRQCHIP_IMMUTABLE) in +the irq_chip structure. The irq_chip structure can then be made unique +and const. + +A small number of drivers have been converted (pl061, tegra186, msm, +amd, apple), and can be used as examples of how to proceed with this +conversion. Note that drivers using the generic irqchip framework +cannot be converted yet, but watch this space! |