190 lines
7.4 KiB
Plaintext
190 lines
7.4 KiB
Plaintext
This is a place for planning the ongoing long-term work in the GPIO
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subsystem.
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GPIO descriptors
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Starting with commit 79a9becda894 the GPIO subsystem embarked on a journey
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to move away from the global GPIO numberspace and toward a decriptor-based
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approach. This means that GPIO consumers, drivers and machine descriptions
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ideally have no use or idea of the global GPIO numberspace that has/was
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used in the inception of the GPIO subsystem.
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The numberspace issue is the same as to why irq is moving away from irq
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numbers to IRQ descriptors.
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The underlying motivation for this is that the GPIO numberspace has become
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unmanageable: machine board files tend to become full of macros trying to
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establish the numberspace at compile-time, making it hard to add any numbers
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in the middle (such as if you missed a pin on a chip) without the numberspace
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breaking.
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Machine descriptions such as device tree or ACPI does not have a concept of the
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Linux GPIO number as those descriptions are external to the Linux kernel
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and treat GPIO lines as abstract entities.
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The runtime-assigned GPIO numberspace (what you get if you assign the GPIO
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base as -1 in struct gpio_chip) has also became unpredictable due to factors
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such as probe ordering and the introduction of -EPROBE_DEFER making probe
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ordering of independent GPIO chips essentially unpredictable, as their base
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number will be assigned on a first come first serve basis.
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The best way to get out of the problem is to make the global GPIO numbers
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unimportant by simply not using them. GPIO descriptors deal with this.
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Work items:
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- Convert all GPIO device drivers to only #include <linux/gpio/driver.h>
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- Convert all consumer drivers to only #include <linux/gpio/consumer.h>
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- Convert all machine descriptors in "boardfiles" to only
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#include <linux/gpio/machine.h>, the other option being to convert it
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to a machine description such as device tree, ACPI or fwnode that
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implicitly does not use global GPIO numbers.
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- When this work is complete (will require some of the items in the
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following ongoing work as well) we can delete the old global
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numberspace accessors from <linux/gpio.h> and eventually delete
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<linux/gpio.h> altogether.
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Get rid of <linux/of_gpio.h>
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This header and helpers appeared at one point when there was no proper
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driver infrastructure for doing simpler MMIO GPIO devices and there was
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no core support for parsing device tree GPIOs from the core library with
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the [devm_]gpiod_get() calls we have today that will implicitly go into
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the device tree back-end. It is legacy and should not be used in new code.
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Work items:
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- Get rid of struct of_mm_gpio_chip altogether: use the generic MMIO
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GPIO for all current users (see below). Delete struct of_mm_gpio_chip,
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to_of_mm_gpio_chip(), of_mm_gpiochip_add_data(), of_mm_gpiochip_add()
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of_mm_gpiochip_remove() from the kernel.
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- Change all consumer drivers that #include <linux/of_gpio.h> to
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#include <linux/gpio/consumer.h> and stop doing custom parsing of the
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GPIO lines from the device tree. This can be tricky and often ivolves
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changing boardfiles, etc.
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- Pull semantics for legacy device tree (OF) GPIO lookups into
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gpiolib-of.c: in some cases subsystems are doing custom flags and
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lookups for polarity inversion, open drain and what not. As we now
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handle this with generic OF bindings, pull all legacy handling into
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gpiolib so the library API becomes narrow and deep and handle all
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legacy bindings internally. (See e.g. commits 6953c57ab172,
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6a537d48461d etc)
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- Delete <linux/of_gpio.h> when all the above is complete and everything
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uses <linux/gpio/consumer.h> or <linux/gpio/driver.h> instead.
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Get rid of <linux/gpio.h>
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This legacy header is a one stop shop for anything GPIO is closely tied
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to the global GPIO numberspace. The endgame of the above refactorings will
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be the removal of <linux/gpio.h> and from that point only the specialized
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headers under <linux/gpio/*.h> will be used. This requires all the above to
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be completed and is expected to take a long time.
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Collect drivers
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Collect GPIO drivers from arch/* and other places that should be placed
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in drivers/gpio/gpio-*. Augment platforms to create platform devices or
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similar and probe a proper driver in the gpiolib subsystem.
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In some cases it makes sense to create a GPIO chip from the local driver
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for a few GPIOs. Those should stay where they are.
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Generic MMIO GPIO
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The GPIO drivers can utilize the generic MMIO helper library in many
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cases, and the helper library should be as helpful as possible for MMIO
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drivers. (drivers/gpio/gpio-mmio.c)
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Work items:
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- Look over and identify any remaining easily converted drivers and
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dry-code conversions to MMIO GPIO for maintainers to test
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- Expand the MMIO GPIO or write a new library for regmap-based I/O
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helpers for GPIO drivers on regmap that simply use offsets
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0..n in some register to drive GPIO lines
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- Expand the MMIO GPIO or write a new library for port-mapped I/O
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helpers (x86 inb()/outb()) and convert port-mapped I/O drivers to use
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this with dry-coding and sending to maintainers to test
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GPIOLIB irqchip
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The GPIOLIB irqchip is a helper irqchip for "simple cases" that should
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try to cover any generic kind of irqchip cascaded from a GPIO.
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- Convert all the GPIOLIB_IRQCHIP users to pass an irqchip template,
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parent and flags before calling [devm_]gpiochip_add[_data]().
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Currently we set up the irqchip after setting up the gpiochip
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using gpiochip_irqchip_add() and gpiochip_set_[chained|nested]_irqchip().
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This is too complex, so convert all users over to just set up
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the irqchip before registering the gpio_chip, typical example:
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/* Typical state container with dynamic irqchip */
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struct my_gpio {
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struct gpio_chip gc;
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struct irq_chip irq;
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};
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int irq; /* from platform etc */
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struct my_gpio *g;
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struct gpio_irq_chip *girq;
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/* Set up the irqchip dynamically */
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g->irq.name = "my_gpio_irq";
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g->irq.irq_ack = my_gpio_ack_irq;
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g->irq.irq_mask = my_gpio_mask_irq;
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g->irq.irq_unmask = my_gpio_unmask_irq;
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g->irq.irq_set_type = my_gpio_set_irq_type;
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/* Get a pointer to the gpio_irq_chip */
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girq = &g->gc.irq;
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girq->chip = &g->irq;
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girq->parent_handler = ftgpio_gpio_irq_handler;
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girq->num_parents = 1;
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girq->parents = devm_kcalloc(dev, 1, sizeof(*girq->parents),
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GFP_KERNEL);
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if (!girq->parents)
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return -ENOMEM;
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girq->default_type = IRQ_TYPE_NONE;
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girq->handler = handle_bad_irq;
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girq->parents[0] = irq;
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When this is done, we will delete the old APIs for instatiating
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GPIOLIB_IRQCHIP and simplify the code.
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- Look over and identify any remaining easily converted drivers and
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dry-code conversions to gpiolib irqchip for maintainers to test
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- Drop gpiochip_set_chained_irqchip() when all the chained irqchips
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have been converted to the above infrastructure.
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- Add more infrastructure to make it possible to also pass a threaded
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irqchip in struct gpio_irq_chip.
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- Drop gpiochip_irqchip_add_nested() when all the chained irqchips
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have been converted to the above infrastructure.
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Increase integration with pin control
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There are already ways to use pin control as back-end for GPIO and
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it may make sense to bring these subsystems closer. One reason for
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creating pin control as its own subsystem was that we could avoid any
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use of the global GPIO numbers. Once the above is complete, it may
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make sense to simply join the subsystems into one and make pin
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multiplexing, pin configuration, GPIO, etc selectable options in one
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and the same pin control and GPIO subsystem.
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