Re: [PATCH RFC 3/3] hwmon: pwm-fan: Add RPM support via external interrupt

[Guenter Roeck <linux@roeck-us.net>]

On Wed, Jan 30, 2019 at 04:07:07PM +0100, Stefan Wahren wrote:
> This adds RPM support to the pwm-fan driver in order to use with
> fancontrol/pwmconfig. This feature is intended for 2 pulse-per-revolution
> fans which provides a tachometer output signal.
> 
> Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
> ---
>  drivers/hwmon/pwm-fan.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 55 insertions(+)
> 
> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> index 9e0591e..731fdc6 100644
> --- a/drivers/hwmon/pwm-fan.c
> +++ b/drivers/hwmon/pwm-fan.c
> @@ -18,6 +18,7 @@
>  
>  #include <linux/hwmon.h>
>  #include <linux/hwmon-sysfs.h>
> +#include <linux/interrupt.h>
>  #include <linux/module.h>
>  #include <linux/mutex.h>
>  #include <linux/of.h>
> @@ -26,13 +27,22 @@
>  #include <linux/regulator/consumer.h>
>  #include <linux/sysfs.h>
>  #include <linux/thermal.h>
> +#include <linux/timer.h>
>  
>  #define MAX_PWM 255
> +#define SAMPLE_TIME		1 /* seconds */
> +#define PULSE_PER_REVOLUTION	2
>  
>  struct pwm_fan_ctx {
>  	struct mutex lock;
>  	struct pwm_device *pwm;
>  	struct regulator *reg_en;
> +
> +	int irq;
> +	unsigned int pulses;
> +	unsigned int rpm;
> +	struct timer_list sample_timer;
> +
>  	unsigned int pwm_value;
>  	unsigned int pwm_fan_state;
>  	unsigned int pwm_fan_max_state;
> @@ -40,6 +50,27 @@ struct pwm_fan_ctx {
>  	struct thermal_cooling_device *cdev;
>  };
>  
> +static irqreturn_t pulse_handler(int irq, void *dev_id)
> +{
> +	struct pwm_fan_ctx *ctx = dev_id;
> +
> +	if (ctx->pulses < INT_MAX / 2)
> +		ctx->pulses++;

Please explain the limit. With this code, the maximum number
for pulses is INT_MAX / 2. The calculation below multiplies
that with 60, which is well above UINT_MAX. Either use a 
realistic limit or drop the if statement (it is kind of
unlikely to get that many interrupts per second).

> +
> +	return IRQ_HANDLED;

This assumes a self resetting edge triggered interrupt ?
You might want to mention that somewhere.

> +}
> +
> +static void sample_timer(struct timer_list *t)
> +{
> +	struct pwm_fan_ctx *ctx = from_timer(ctx, t, sample_timer);
> +	unsigned int pulses;
> +
> +	pulses = ctx->pulses;
> +	ctx->pulses -= pulses;

This is racy; the substract operation is not atomic, and neither
is the addition in the interrupt handler.

> +	ctx->rpm = pulses * 60 / SAMPLE_TIME / PULSE_PER_REVOLUTION;

The calculation assumes that sample_timer is called exactly after one
second. Have to tested how much variation this causes on a loaded system ?

> +	mod_timer(&ctx->sample_timer, jiffies + (HZ * SAMPLE_TIME));
> +}
> +
>  static int  __set_pwm(struct pwm_fan_ctx *ctx, unsigned long pwm)
>  {
>  	unsigned long period;
> @@ -100,11 +131,20 @@ static ssize_t pwm_show(struct device *dev, struct device_attribute *attr,
>  	return sprintf(buf, "%u\n", ctx->pwm_value);
>  }
>  
> +static ssize_t rpm_show(struct device *dev,
> +			struct device_attribute *attr, char *buf)
> +{
> +	struct pwm_fan_ctx *ctx = dev_get_drvdata(dev);
> +
> +	return sprintf(buf, "%u\n", ctx->rpm);
> +}
>  
>  static SENSOR_DEVICE_ATTR_RW(pwm1, pwm, 0);
> +static SENSOR_DEVICE_ATTR_RO(fan1_input, rpm, 0);
>  
>  static struct attribute *pwm_fan_attrs[] = {
>  	&sensor_dev_attr_pwm1.dev_attr.attr,
> +	&sensor_dev_attr_fan1_input.dev_attr.attr,

This attribute is visible even if not supported, meaning it will
report a fan speed of 0 rpm in that case. Please make the attribute
conditional.

>  	NULL,
>  };
>  
> @@ -263,6 +303,19 @@ static int pwm_fan_probe(struct platform_device *pdev)
>  		}
>  	}
>  
> +	timer_setup(&ctx->sample_timer, sample_timer, 0);
> +
> +	ctx->irq = platform_get_irq(pdev, 0);
> +	if (ctx->irq >= 0) {
> +		ret = devm_request_irq(&pdev->dev, ctx->irq, pulse_handler, 0,
> +				       pdev->name, ctx);
> +		if (ret) {
> +			dev_err(&pdev->dev, "Can't get interrupt working.\n");
> +			return ret;
> +		}
> +		mod_timer(&ctx->sample_timer, jiffies + (HZ * SAMPLE_TIME));
> +	}
> +
>  	hwmon = devm_hwmon_device_register_with_groups(&pdev->dev, "pwmfan",
>  						       ctx, pwm_fan_groups);
>  	if (IS_ERR(hwmon)) {
> @@ -304,6 +357,8 @@ static int pwm_fan_remove(struct platform_device *pdev)
>  	struct pwm_fan_ctx *ctx = platform_get_drvdata(pdev);
>  
>  	thermal_cooling_device_unregister(ctx->cdev);
> +	del_timer_sync(&ctx->sample_timer);
> +
>  	if (ctx->pwm_value)
>  		pwm_disable(ctx->pwm);
>