Re: [RFC gpio/for-next 2/2] gpio: gpio-mux-input: add generic gpio input multiplexer

[Drew Fustini <drew@beagleboard.org>]

On Thu, Mar 25, 2021 at 02:28:32PM +0200, Mauri Sandberg wrote:
> Suppport for a general GPIO multiplexer. To drive the multiplexer a
> mux-controller is needed. The output pin of the multiplexer is a GPIO
> pin
> 
> Signed-off-by: Mauri Sandberg <sandberg@mailfence.com>

Thanks for posting the RFC so we can take a look at the code and discuss
how it works.

> ---
>  drivers/gpio/Kconfig          |  11 +++
>  drivers/gpio/Makefile         |   1 +
>  drivers/gpio/gpio-mux-input.c | 143 ++++++++++++++++++++++++++++++++++
>  3 files changed, 155 insertions(+)
>  create mode 100644 drivers/gpio/gpio-mux-input.c
> 
> diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
> index c70f46e80a3b..41062d8f7d93 100644
> --- a/drivers/gpio/Kconfig
> +++ b/drivers/gpio/Kconfig
> @@ -1641,4 +1641,15 @@ config GPIO_MOCKUP
>  
>  endmenu
>  
> +comment "Other GPIO expanders"
> +
> +config GPIO_MUX_INPUT
> +	tristate "General GPIO input multiplexer"
> +	select MULTIPLEXER
> +	select MUX_GPIO
> +	depends on OF_GPIO
> +	help
> +	  Say yes here to enable support for generic GPIO input multiplexer. This
> +	  needs a multiplexer controller to drive the select pins.
> +
>  endif
> diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
> index 35e3b6026665..00f7576ce23f 100644
> --- a/drivers/gpio/Makefile
> +++ b/drivers/gpio/Makefile
> @@ -105,6 +105,7 @@ obj-$(CONFIG_GPIO_MPC8XXX)		+= gpio-mpc8xxx.o
>  obj-$(CONFIG_GPIO_MSC313)		+= gpio-msc313.o
>  obj-$(CONFIG_GPIO_MSIC)			+= gpio-msic.o
>  obj-$(CONFIG_GPIO_MT7621)		+= gpio-mt7621.o
> +obj-$(CONFIG_GPIO_MUX_INPUT)		+= gpio-mux-input.o
>  obj-$(CONFIG_GPIO_MVEBU)		+= gpio-mvebu.o
>  obj-$(CONFIG_GPIO_MXC)			+= gpio-mxc.o
>  obj-$(CONFIG_GPIO_MXS)			+= gpio-mxs.o

This does not apply to mainline. I've added it manually to my
drivers/gpio/Makefile but something to fix in v2.

> diff --git a/drivers/gpio/gpio-mux-input.c b/drivers/gpio/gpio-mux-input.c
> new file mode 100644
> index 000000000000..ec0c7acbab2f
> --- /dev/null
> +++ b/drivers/gpio/gpio-mux-input.c
> @@ -0,0 +1,143 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + *  A generic GPIO input multiplexer driver
> + *
> + *  Copyright (C) 2021 Mauri Sandberg <sandberg@mailfence.com>
> + *
> + */
> +
> +#include <linux/module.h>
> +#include <linux/gpio/consumer.h>
> +#include <linux/gpio/driver.h>
> +#include <linux/slab.h>
> +#include <linux/platform_device.h>
> +#include <linux/mux/consumer.h>
> +
> +struct gpio_mux_input {
> +	struct device		*parent;
> +	struct gpio_chip	gpio_chip;
> +	struct mux_control	*mux_control;
> +	struct gpio_desc	*mux_pin;
> +};
> +
> +static struct gpio_mux_input *gpio_to_mux(struct gpio_chip *gc)
> +{
> +	return container_of(gc, struct gpio_mux_input, gpio_chip);
> +}
> +
> +static int gpio_mux_input_direction_input(struct gpio_chip *gc,
> +				       unsigned int offset)
> +{
> +	return 0;
> +}
> +
> +static int gpio_mux_input_direction_output(struct gpio_chip *gc,
> +					unsigned int offset, int val)
> +{
> +	return -EINVAL;
> +}
> +
> +static int gpio_mux_input_get_value(struct gpio_chip *gc, unsigned int offset)
> +{
> +	struct gpio_mux_input *mux;
> +	int ret;
> +
> +	mux = gpio_to_mux(gc);
> +	ret = mux_control_select(mux->mux_control, offset);
> +	if (ret)
> +		return ret;
> +
> +	ret = gpiod_get_value(mux->mux_pin);

I'm not too familiar with how mux_control works but does there need to
be locking here?

Or is not possible for mux_pin to change to another offset before if
gpiod_get_value() if gpio_mux_input_get_value() runs concurrently?

> +	mux_control_deselect(mux->mux_control);
> +	return ret;
> +}
> +
> +static void gpio_mux_input_set_value(struct gpio_chip *gc,
> +				  unsigned int offset, int val)
> +{
> +	/* not supported */

I'm not sure but maybe it is better not to define gc->set in the probe?

> +}
> +
> +static int gpio_mux_input_probe(struct platform_device *pdev)
> +{
> +	struct device_node *np = pdev->dev.of_node;
> +	struct gpio_mux_input *mux;
> +	struct gpio_chip *gc;
> +	struct mux_control *mc;
> +	struct gpio_desc *pin;
> +	int err;
> +
> +	mux = kzalloc(sizeof(struct gpio_mux_input), GFP_KERNEL);
> +	if (mux == NULL)
> +		return -ENOMEM;
> +
> +	mc = mux_control_get(&pdev->dev, NULL);
> +	if (IS_ERR(mc)) {
> +		err = (int) PTR_ERR(mc);
> +		if (err != -EPROBE_DEFER)
> +			dev_err(&pdev->dev, "unable to get mux-control: %d\n",
> +				err);
> +		goto err_free_mux;
> +	}
> +
> +	mux->mux_control = mc;
> +	pin = gpiod_get(&pdev->dev, "pin",  GPIOD_IN);
> +	if (IS_ERR(pin)) {
> +		err = (int) PTR_ERR(pin);
> +		dev_err(&pdev->dev, "unable to claim pin GPIOs: %d\n", err);
> +		goto err_free_mc;
> +	}
> +
> +	mux->mux_pin = pin;
> +	mux->parent = &pdev->dev;
> +
> +	gc = &mux->gpio_chip;
> +	gc->direction_input  = gpio_mux_input_direction_input;
> +	gc->direction_output = gpio_mux_input_direction_output;
> +	gc->get = gpio_mux_input_get_value;
> +	gc->set = gpio_mux_input_set_value;
> +	gc->can_sleep = 1;
> +
> +	gc->base = -1;
> +	gc->ngpio = mux_control_states(mc);
> +	gc->label = dev_name(mux->parent);
> +	gc->parent = mux->parent;
> +	gc->owner = THIS_MODULE;
> +	gc->of_node = np;
> +
> +	err = gpiochip_add(&mux->gpio_chip);
> +	if (err) {
> +		dev_err(&pdev->dev, "unable to add gpio chip, err=%d\n", err);
> +		goto err_free_pin;
> +	}
> +
> +	platform_set_drvdata(pdev, mux);
> +	return 0;
> +
> +err_free_pin:
> +	gpiod_put(pin);
> +err_free_mc:
> +	mux_control_put(mc);
> +err_free_mux:
> +	kfree(mux);
> +	return err;
> +}
> +
> +static const struct of_device_id gpio_mux_input_id[] = {
> +	{
> +		.compatible = "gpio-mux-input",
> +		.data = NULL,
> +	},
> +	{ /* sentinel */ }
> +};
> +MODULE_DEVICE_TABLE(of, gpio_mux_input_id);
> +
> +static struct platform_driver gpio_mux_input_driver = {
> +	.driver	= {
> +		.name		= "gpio-mux-input",
> +		.owner		= THIS_MODULE,
> +		.of_match_table = gpio_mux_input_id,
> +	},
> +	.probe	= gpio_mux_input_probe,
> +};
> +module_platform_driver(gpio_mux_input_driver);

I believe you need to add:

  MODULE_AUTHOR("...");
  MODULE_DESCRIPTION("...");
  MODULE_LICENSE("GPL");

My build failed with:

  ERROR: modpost: missing MODULE_LICENSE() in drivers/gpio/gpio-mux-input.o
  LZMA    arch/arm/boot/compressed/piggy_data
  make[1]: *** [scripts/Makefile.modpost:132: Module.symvers] Error 1
  make[1]: *** Deleting file 'Module.symvers'
  make: *** [Makefile:1442: modules] Error 2
  make: *** Waiting for unfinished jobs....
  AS      arch/arm/boot/compressed/piggy.o
  LD      arch/arm/boot/compressed/vmlinux
  OBJCOPY arch/arm/boot/zImage
  Kernel: arch/arm/boot/zImage is ready

I added those lines and it compiled successfully.


-Drew