Re: [PATCH v2 2/2] counter: add GPIO based pulse counters

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Oleksij,

On 22.01.21 10:33, Oleksij Rempel wrote:
> Add simple GPIO base pulse counter. This device is used to measure
> rotation speed of some agricultural devices, so no high frequency on the
> counter pin is expected.
> 
> The maximal measurement frequency depends on the CPU and system load. On
> the idle iMX6S I was able to measure up to 20kHz without count drops.
> 
> Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> ---
>  drivers/counter/Kconfig          |   9 ++
>  drivers/counter/Makefile         |   1 +
>  drivers/counter/gpio-pulse-cnt.c | 244 +++++++++++++++++++++++++++++++
>  3 files changed, 254 insertions(+)
>  create mode 100644 drivers/counter/gpio-pulse-cnt.c
> 
> diff --git a/drivers/counter/Kconfig b/drivers/counter/Kconfig
> index 2de53ab0dd25..9ad1d9d49dd1 100644
> --- a/drivers/counter/Kconfig
> +++ b/drivers/counter/Kconfig
> @@ -29,6 +29,15 @@ config 104_QUAD_8
>  	  The base port addresses for the devices may be configured via the base
>  	  array module parameter.
>  
> +config GPIO_PULSE_CNT
> +	tristate "GPIO pulse counter driver"
> +	depends on GPIOLIB
> +	help
> +	  Select this option to enable GPIO pulse counter driver.
> +
> +	  To compile this driver as a module, choose M here: the
> +	  module will be called gpio-pulse-cnt.
> +
>  config STM32_TIMER_CNT
>  	tristate "STM32 Timer encoder counter driver"
>  	depends on MFD_STM32_TIMERS || COMPILE_TEST
> diff --git a/drivers/counter/Makefile b/drivers/counter/Makefile
> index 0a393f71e481..6a5c3fc6f2a0 100644
> --- a/drivers/counter/Makefile
> +++ b/drivers/counter/Makefile
> @@ -6,6 +6,7 @@
>  obj-$(CONFIG_COUNTER) += counter.o
>  
>  obj-$(CONFIG_104_QUAD_8)	+= 104-quad-8.o
> +obj-$(CONFIG_GPIO_PULSE_CNT)	+= gpio-pulse-cnt.o
>  obj-$(CONFIG_STM32_TIMER_CNT)	+= stm32-timer-cnt.o
>  obj-$(CONFIG_STM32_LPTIMER_CNT)	+= stm32-lptimer-cnt.o
>  obj-$(CONFIG_TI_EQEP)		+= ti-eqep.o
> diff --git a/drivers/counter/gpio-pulse-cnt.c b/drivers/counter/gpio-pulse-cnt.c
> new file mode 100644
> index 000000000000..cb42656a55b5
> --- /dev/null
> +++ b/drivers/counter/gpio-pulse-cnt.c
> @@ -0,0 +1,244 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2021 Pengutronix, Oleksij Rempel <kernel@pengutronix.de>
> + */
> +
> +#include <linux/counter.h>
> +#include <linux/gpio/consumer.h>
> +#include <linux/interrupt.h>
> +#include <linux/module.h>
> +#include <linux/of_device.h>
> +#include <linux/platform_device.h>
> +
> +#define GPIO_PULSE_NAME		"gpio-pulse-counter"
> +
> +struct gpio_pulse_priv {
> +	struct counter_device counter;
> +	struct gpio_desc *gpio;
> +	int irq;
> +	bool enabled;
> +	unsigned long count;
> +};
> +
> +static irqreturn_t gpio_pulse_irq_isr(int irq, void *dev_id)
> +{
> +	struct gpio_pulse_priv *priv = dev_id;
> +
> +	if (!priv->enabled)
> +		return IRQ_NONE;
> +
> +	priv->count++;

Memory locations that are concurrently accessed needs to be
marked as such, otherwise the compiler is allowed to funky stuff:
https://lore.kernel.org/lkml/CAGzjT4ez+gWr3BFQsEr-wma+vs6UZNJ+mRARx_BWoAKEJSsN=w@mail.gmail.com/

This should be at least WRITE_ONCE(priv->count, READ_ONCE(priv->count) + 1);
(assuming you disable IRQs during concurrent write, see below)

> +
> +	return IRQ_HANDLED;
> +}
> +
> +static ssize_t gpio_pulse_count_enable_read(struct counter_device *counter,
> +					    struct counter_count *count,
> +					    void *private, char *buf)
> +{
> +	struct gpio_pulse_priv *priv = counter->priv;
> +
> +	return sysfs_emit(buf, "%d\n", priv->enabled);
> +}
> +
> +static ssize_t gpio_pulse_count_enable_write(struct counter_device *counter,
> +					     struct counter_count *count,
> +					     void *private,
> +					     const char *buf, size_t len)
> +{
> +	struct gpio_pulse_priv *priv = counter->priv;
> +	bool enable;
> +	ssize_t ret;
> +
> +	ret = kstrtobool(buf, &enable);
> +	if (ret)
> +		return ret;
> +
> +	if (priv->enabled == enable)
> +		return len;
> +
> +	priv->enabled = enable;
> +
> +	if (enable)
> +		enable_irq(priv->irq);
> +	else
> +		disable_irq(priv->irq);
> +
> +	return len;
> +}
> +
> +static const struct counter_count_ext gpio_pulse_count_ext[] = {
> +	{
> +		.name = "enable",
> +		.read = gpio_pulse_count_enable_read,
> +		.write = gpio_pulse_count_enable_write,
> +	},
> +};
> +
> +static enum counter_synapse_action gpio_pulse_synapse_actions[] = {
> +	COUNTER_SYNAPSE_ACTION_RISING_EDGE,
> +};
> +
> +static int gpio_pulse_action_get(struct counter_device *counter,
> +			    struct counter_count *count,
> +			    struct counter_synapse *synapse,
> +			    size_t *action)
> +{
> +	*action = COUNTER_SYNAPSE_ACTION_RISING_EDGE;
> +
> +	return 0;
> +}
> +
> +static int gpio_pulse_count_read(struct counter_device *counter,
> +				 struct counter_count *count,
> +				 unsigned long *val)
> +{
> +	struct gpio_pulse_priv *priv = counter->priv;
> +
> +	*val = priv->count;

READ_ONCE(priv->count)

> +
> +	return 0;
> +}
> +
> +static int gpio_pulse_count_write(struct counter_device *counter,
> +				  struct counter_count *count,
> +				  const unsigned long val)
> +{
> +	struct gpio_pulse_priv *priv = counter->priv;
> +
> +	priv->count = val;
Users would expect after the write is done that the count is >= val.
But this races with the irq handler and you could find the value
here overwritten.

Either disable the irq while writing priv->count (and use WRITE_ONCE)
or switch to atomic_inc in the irq handler.

> +
> +	return 0;
> +}
> +
> +static int gpio_pulse_count_function_get(struct counter_device *counter,
> +					 struct counter_count *count,
> +					 size_t *function)
> +{
> +	*function = COUNTER_COUNT_FUNCTION_INCREASE;
> +
> +	return 0;
> +}
> +
> +static int gpio_pulse_count_signal_read(struct counter_device *counter,
> +					struct counter_signal *signal,
> +					enum counter_signal_value *val)
> +{
> +	struct gpio_pulse_priv *priv = counter->priv;
> +	int ret;
> +
> +	ret = gpiod_get_value(priv->gpio);
> +	if (ret < 0)
> +		return ret;
> +
> +	*val = ret ? COUNTER_SIGNAL_HIGH : COUNTER_SIGNAL_LOW;
> +
> +	return 0;
> +}
> +
> +static const struct counter_ops gpio_pulse_cnt_ops = {
> +	.action_get = gpio_pulse_action_get,
> +	.count_read = gpio_pulse_count_read,
> +	.count_write = gpio_pulse_count_write,
> +	.function_get = gpio_pulse_count_function_get,
> +	.signal_read  = gpio_pulse_count_signal_read,
> +};
> +
> +static struct counter_signal gpio_pulse_signals[] = {
> +	{
> +		.id = 0,
> +		.name = "Channel 0 signal",
> +	},
> +};
> +
> +static struct counter_synapse gpio_pulse_count_synapses[] = {
> +	{
> +		.actions_list = gpio_pulse_synapse_actions,
> +		.num_actions = ARRAY_SIZE(gpio_pulse_synapse_actions),
> +		.signal = &gpio_pulse_signals[0]
> +	},
> +};
> +
> +static enum counter_count_function gpio_pulse_count_functions[] = {
> +	COUNTER_COUNT_FUNCTION_INCREASE,
> +};
> +
> +static struct counter_count gpio_pulse_counts[] = {
> +	{
> +		.id = 0,
> +		.name = "Channel 1 Count",
> +		.functions_list = gpio_pulse_count_functions,
> +		.num_functions = ARRAY_SIZE(gpio_pulse_count_functions),
> +		.synapses = gpio_pulse_count_synapses,
> +		.num_synapses = ARRAY_SIZE(gpio_pulse_count_synapses),
> +		.ext = gpio_pulse_count_ext,
> +		.num_ext = ARRAY_SIZE(gpio_pulse_count_ext),
> +	},
> +};
> +
> +static int gpio_pulse_cnt_probe(struct platform_device *pdev)
> +{
> +	struct device *dev = &pdev->dev;
> +	struct gpio_pulse_priv *priv;
> +	int ret;
> +
> +	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +
> +	if (gpiod_count(dev, NULL) != 1) {
> +		dev_err(dev, "Error, need exactly 1 gpio for device\n");
> +		return -EINVAL;
> +	}
> +
> +	priv->gpio = devm_gpiod_get(dev, NULL, GPIOD_IN);
> +	if (IS_ERR(priv->gpio))
> +		return dev_err_probe(dev, PTR_ERR(priv->gpio), "failed to get gpio\n");
> +
> +	priv->irq = gpiod_to_irq(priv->gpio);
> +	if (priv->irq < 0) {
> +		dev_err(dev, "failed to map GPIO to IRQ: %d\n", priv->irq);
> +		return priv->irq;
> +	}
> +
> +	priv->counter.name = dev_name(dev);
> +	priv->counter.parent = dev;
> +	priv->counter.ops = &gpio_pulse_cnt_ops;
> +	priv->counter.counts = gpio_pulse_counts;
> +	priv->counter.num_counts = ARRAY_SIZE(gpio_pulse_counts);
> +	priv->counter.signals = gpio_pulse_signals;
> +	priv->counter.num_signals = ARRAY_SIZE(gpio_pulse_signals);
> +	priv->counter.priv = priv;
> +
> +	ret = devm_request_irq(dev, priv->irq, gpio_pulse_irq_isr,
> +			       IRQF_TRIGGER_RISING | IRQF_NO_THREAD,
> +			       GPIO_PULSE_NAME, priv);
> +	if (ret)
> +		return ret;
> +
> +	disable_irq(priv->irq);
> +
> +	platform_set_drvdata(pdev, priv);
> +
> +	return devm_counter_register(dev, &priv->counter);
> +}
> +
> +static const struct of_device_id gpio_pulse_cnt_of_match[] = {
> +	{ .compatible = "virtual,gpio-pulse-counter", },
> +	{}
> +};
> +MODULE_DEVICE_TABLE(of, gpio_pulse_cnt_of_match);
> +
> +static struct platform_driver gpio_pulse_cnt_driver = {
> +	.probe = gpio_pulse_cnt_probe,
> +	.driver = {
> +		.name = GPIO_PULSE_NAME,
> +		.of_match_table = gpio_pulse_cnt_of_match,
> +	},
> +};
> +module_platform_driver(gpio_pulse_cnt_driver);
> +
> +MODULE_ALIAS("platform:gpio-pulse-counter");
> +MODULE_AUTHOR("Oleksij Rempel <o.rempel@pengutronix.de>");
> +MODULE_DESCRIPTION("GPIO pulse counter driver");
> +MODULE_LICENSE("GPL v2");
> 

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