Re: [PATCH v4 1/2] gpio: Add gpio latch driver

[Serge Semin]

On Thu, Oct 06, 2022 at 10:30:30AM +0200, Sascha Hauer wrote:
> This driver implements a GPIO multiplexer based on latches connected to
> other GPIOs. A set of data GPIOs is connected to the data input of
> multiple latches. The clock input of each latch is driven by another
> set of GPIOs. With two 8-bit latches 10 GPIOs can be multiplexed into
> 16 GPIOs. GPOs might be a better term as in fact the multiplexed pins
> are output only.
> 
> Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
> ---
> 
> Notes:
>     Changes since v3:
>     - Introduce delays between GPIO toggles as suggested by Serge Semin
>     
>     Changes since v2:
>     - Fix inconsistent licensing
>     - document locks
>     - use regular bit operations
>     - include linux/mod_devicetable.h rather than linux/of_device.h
>     - Put spinlock and mutex into a union to make clear that only one of them is used
>     - rename __gpio_latch_set to gpio_latch_set_unlocked
>     
>     Changes since v1:
>     - Use gpiod_set_value_cansleep when the underlying GPIOs might sleep
>     - Move MODULE_DEVICE_TABLE near to the end
> 
>  drivers/gpio/Kconfig      |   6 ++
>  drivers/gpio/Makefile     |   1 +
>  drivers/gpio/gpio-latch.c | 220 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 227 insertions(+)
>  create mode 100644 drivers/gpio/gpio-latch.c
> 
> diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
> index 0642f579196f2..e4603810ec910 100644
> --- a/drivers/gpio/Kconfig
> +++ b/drivers/gpio/Kconfig
> @@ -1690,6 +1690,12 @@ config GPIO_AGGREGATOR
>  	      industrial control context, to be operated from userspace using
>  	      the GPIO chardev interface.
>  
> +config GPIO_LATCH
> +	tristate "GPIO latch driver"
> +	help
> +	  Say yes here to enable a driver for GPIO multiplexers based on latches
> +	  connected to other GPIOs.
> +
>  config GPIO_MOCKUP
>  	tristate "GPIO Testing Driver"
>  	select IRQ_SIM
> diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
> index a0985d30f51bb..310fa08decc69 100644
> --- a/drivers/gpio/Makefile
> +++ b/drivers/gpio/Makefile
> @@ -75,6 +75,7 @@ obj-$(CONFIG_GPIO_IT87)			+= gpio-it87.o
>  obj-$(CONFIG_GPIO_IXP4XX)		+= gpio-ixp4xx.o
>  obj-$(CONFIG_GPIO_JANZ_TTL)		+= gpio-janz-ttl.o
>  obj-$(CONFIG_GPIO_KEMPLD)		+= gpio-kempld.o
> +obj-$(CONFIG_GPIO_LATCH)		+= gpio-latch.o
>  obj-$(CONFIG_GPIO_LOGICVC)		+= gpio-logicvc.o
>  obj-$(CONFIG_GPIO_LOONGSON1)		+= gpio-loongson1.o
>  obj-$(CONFIG_GPIO_LOONGSON)		+= gpio-loongson.o
> diff --git a/drivers/gpio/gpio-latch.c b/drivers/gpio/gpio-latch.c
> new file mode 100644
> index 0000000000000..a4ab8f1240450
> --- /dev/null
> +++ b/drivers/gpio/gpio-latch.c
> @@ -0,0 +1,220 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * GPIO latch driver
> + *
> + *  Copyright (C) 2022 Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
> + *
> + * This driver implements a GPIO (or better GPO as there is no input)
> + * multiplexer based on latches like this:
> + *
> + * CLK0 ----------------------.        ,--------.
> + * CLK1 -------------------.  `--------|>    #0 |
> + *                         |           |        |
> + * OUT0 ----------------+--|-----------|D0    Q0|-----|<
> + * OUT1 --------------+-|--|-----------|D1    Q1|-----|<
> + * OUT2 ------------+-|-|--|-----------|D2    Q2|-----|<
> + * OUT3 ----------+-|-|-|--|-----------|D3    Q3|-----|<
> + * OUT4 --------+-|-|-|-|--|-----------|D4    Q4|-----|<
> + * OUT5 ------+-|-|-|-|-|--|-----------|D5    Q5|-----|<
> + * OUT6 ----+-|-|-|-|-|-|--|-----------|D6    Q6|-----|<
> + * OUT7 --+-|-|-|-|-|-|-|--|-----------|D7    Q7|-----|<
> + *        | | | | | | | |  |           `--------'
> + *        | | | | | | | |  |
> + *        | | | | | | | |  |           ,--------.
> + *        | | | | | | | |  `-----------|>    #1 |
> + *        | | | | | | | |              |        |
> + *        | | | | | | | `--------------|D0    Q0|-----|<
> + *        | | | | | | `----------------|D1    Q1|-----|<
> + *        | | | | | `------------------|D2    Q2|-----|<
> + *        | | | | `--------------------|D3    Q3|-----|<
> + *        | | | `----------------------|D4    Q4|-----|<
> + *        | | `------------------------|D5    Q5|-----|<
> + *        | `--------------------------|D6    Q6|-----|<
> + *        `----------------------------|D7    Q7|-----|<
> + *                                     `--------'
> + *
> + * The above is just an example. The actual number of number of latches and
> + * the number of inputs per latch is derived from the number of GPIOs given
> + * in the corresponding device tree properties.
> + */
> +
> +#include <linux/err.h>
> +#include <linux/gpio/consumer.h>
> +#include <linux/gpio/driver.h>
> +#include <linux/module.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/platform_device.h>
> +#include <linux/delay.h>
> +
> +#include "gpiolib.h"
> +
> +struct gpio_latch_priv {
> +	struct gpio_chip gc;
> +	struct gpio_descs *clk_gpios;
> +	struct gpio_descs *latched_gpios;
> +	int n_latched_gpios;
> +	unsigned int setup_duration_ns;
> +	unsigned int clock_duration_ns;
> +	unsigned long *shadow;
> +	/*
> +	 * Depending on whether any of the underlying GPIOs may sleep we either
> +	 * use a mutex or a spinlock to protect our shadow map.
> +	 */
> +	union {
> +		struct mutex mutex; /* protects @shadow */
> +		spinlock_t spinlock; /* protects @shadow */
> +	};
> +};
> +
> +static int gpio_latch_get_direction(struct gpio_chip *gc, unsigned int offset)
> +{
> +	return GPIO_LINE_DIRECTION_OUT;
> +}
> +
> +static void gpio_latch_set_unlocked(struct gpio_latch_priv *priv,
> +				    void (*set)(struct gpio_desc *desc, int value),
> +				    unsigned int offset, bool val)
> +{
> +	int latch = offset / priv->n_latched_gpios;
> +	int i;
> +
> +	assign_bit(offset, priv->shadow, val);
> +
> +	for (i = 0; i < priv->n_latched_gpios; i++)
> +		set(priv->latched_gpios->desc[i],
> +		    test_bit(latch * priv->n_latched_gpios + i, priv->shadow));
> +
> +	ndelay(priv->setup_duration_ns);
> +	set(priv->clk_gpios->desc[latch], 1);

> +	udelay(priv->clock_duration_ns);

These are supposed to be !n!delay()'s. Aren't they?

> +	set(priv->clk_gpios->desc[latch], 0);

> +	udelay(priv->clock_duration_ns);

Why do you need the second clock-duration? AFAICS at least from the
TI SNx4LV74A specification the outputs get updated on the positive
edge of the pulse. So the clock-duration value shall contain both
the "CLK pulse duration" and "CLK hold time", which should be enough
for the latches states being perceived by the device.

> +}
> +
> +static void gpio_latch_set(struct gpio_chip *gc, unsigned int offset, int val)
> +{
> +	struct gpio_latch_priv *priv = gpiochip_get_data(gc);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&priv->spinlock, flags);
> +
> +	gpio_latch_set_unlocked(priv, gpiod_set_value, offset, val);
> +
> +	spin_unlock_irqrestore(&priv->spinlock, flags);
> +}
> +
> +static void gpio_latch_set_can_sleep(struct gpio_chip *gc, unsigned int offset, int val)
> +{
> +	struct gpio_latch_priv *priv = gpiochip_get_data(gc);
> +
> +	mutex_lock(&priv->mutex);
> +
> +	gpio_latch_set_unlocked(priv, gpiod_set_value_cansleep, offset, val);
> +
> +	mutex_unlock(&priv->mutex);
> +}
> +
> +static bool gpio_latch_can_sleep(struct gpio_latch_priv *priv, unsigned int n_latches)
> +{
> +	int i;
> +
> +	for (i = 0; i < n_latches; i++)
> +		if (gpiod_cansleep(priv->clk_gpios->desc[i]))
> +			return true;
> +
> +	for (i = 0; i < priv->n_latched_gpios; i++)
> +		if (gpiod_cansleep(priv->latched_gpios->desc[i]))
> +			return true;
> +
> +	return false;
> +}
> +
> +static int gpio_latch_probe(struct platform_device *pdev)
> +{
> +	struct gpio_latch_priv *priv;
> +	unsigned int n_latches;
> +	struct device_node *np = pdev->dev.of_node;

> +	/*
> +	 * Some value which is still acceptable to delay in atomic context.
> +	 * If we need to go higher we might have to switch to usleep_range(),
> +	 * but that cannot ne used in atomic context and the driver would have
> +	 * to be adjusted to support that.
> +	 */
> +	unsigned int duration_ns_max = 5000;

I don't see you changing this parameter. So it's better to be a macro.

-Sergey

> +
> +	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +
> +	priv->clk_gpios = devm_gpiod_get_array(&pdev->dev, "clk", GPIOD_OUT_LOW);
> +	if (IS_ERR(priv->clk_gpios))
> +		return PTR_ERR(priv->clk_gpios);
> +
> +	priv->latched_gpios = devm_gpiod_get_array(&pdev->dev, "latched", GPIOD_OUT_LOW);
> +	if (IS_ERR(priv->latched_gpios))
> +		return PTR_ERR(priv->latched_gpios);
> +
> +	n_latches = priv->clk_gpios->ndescs;
> +	priv->n_latched_gpios = priv->latched_gpios->ndescs;
> +
> +	priv->shadow = devm_bitmap_zalloc(&pdev->dev, n_latches * priv->n_latched_gpios,
> +					  GFP_KERNEL);
> +	if (!priv->shadow)
> +		return -ENOMEM;
> +
> +	if (gpio_latch_can_sleep(priv, n_latches)) {
> +		priv->gc.can_sleep = true;
> +		priv->gc.set = gpio_latch_set_can_sleep;
> +		mutex_init(&priv->mutex);
> +	} else {
> +		priv->gc.can_sleep = false;
> +		priv->gc.set = gpio_latch_set;
> +		spin_lock_init(&priv->spinlock);
> +	}
> +
> +	of_property_read_u32(np, "setup-duration-ns", &priv->setup_duration_ns);
> +	if (priv->setup_duration_ns > duration_ns_max) {
> +		dev_warn(&pdev->dev, "setup-duration-ns too high, limit to %d\n",
> +			 duration_ns_max);
> +		priv->setup_duration_ns = duration_ns_max;
> +	}
> +
> +	of_property_read_u32(np, "clock-duration-ns", &priv->clock_duration_ns);
> +	if (priv->clock_duration_ns > duration_ns_max) {
> +		dev_warn(&pdev->dev, "clock-duration-ns too high, limit to %d\n",
> +			 duration_ns_max);
> +		priv->clock_duration_ns = duration_ns_max;
> +	}
> +
> +	priv->gc.get_direction = gpio_latch_get_direction;
> +	priv->gc.ngpio = n_latches * priv->n_latched_gpios;
> +	priv->gc.owner = THIS_MODULE;
> +	priv->gc.base = -1;
> +	priv->gc.parent = &pdev->dev;
> +
> +	platform_set_drvdata(pdev, priv);
> +
> +	return devm_gpiochip_add_data(&pdev->dev, &priv->gc, priv);
> +}
> +
> +static const struct of_device_id gpio_latch_ids[] = {
> +	{
> +		.compatible	= "gpio-latch",
> +	}, {
> +		/* sentinel */
> +	}
> +};
> +MODULE_DEVICE_TABLE(of, gpio_latch_ids);
> +
> +static struct platform_driver gpio_latch_driver = {
> +	.driver	= {
> +		.name		= "gpio-latch",
> +		.of_match_table	= gpio_latch_ids,
> +	},
> +	.probe	= gpio_latch_probe,
> +};
> +module_platform_driver(gpio_latch_driver);
> +
> +MODULE_LICENSE("GPL v2");
> +MODULE_AUTHOR("Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>");
> +MODULE_DESCRIPTION("GPIO latch driver");
> -- 
> 2.30.2
>