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Date:   Wed, 25 Nov 2020 18:05:36 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Dongjin Kim <tobetter@...il.com>
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: (pwm-fan) add fan pwm1_enable attribute

On Thu, Nov 26, 2020 at 01:32:42AM +0900, Dongjin Kim wrote:
> This patch adds to new attribute 'pwm1_enable' to change the fan speed
> control method as documented in 'Documentation/hwmon/sysfs-interface'.
> 
> Signed-off-by: Dongjin Kim <tobetter@...il.com>

The new attribute needs to be documented in
Documentation/hwmon/pwm-fan.rst, with supported values.

> ---
>  drivers/hwmon/pwm-fan.c | 52 ++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> index 1f63807c0399..834275309421 100644
> --- a/drivers/hwmon/pwm-fan.c
> +++ b/drivers/hwmon/pwm-fan.c
> @@ -39,6 +39,7 @@ struct pwm_fan_ctx {
>  	unsigned int pwm_fan_max_state;
>  	unsigned int *pwm_fan_cooling_levels;
>  	struct thermal_cooling_device *cdev;
> +	int enable;
>  };
>  
>  /* This handler assumes self resetting edge triggered interrupt. */
> @@ -76,6 +77,10 @@ static int  __set_pwm(struct pwm_fan_ctx *ctx, unsigned long pwm)
>  	struct pwm_state state = { };
>  
>  	mutex_lock(&ctx->lock);
> +
> +	if (ctx->enable == 0)
> +		pwm = MAX_PWM;
> +
>  	if (ctx->pwm_value == pwm)
>  		goto exit_set_pwm_err;
>  
> @@ -137,11 +142,42 @@ static ssize_t rpm_show(struct device *dev,
>  	return sprintf(buf, "%u\n", ctx->rpm);
>  }
>  
> +static ssize_t enable_store(struct device *dev,
> +		struct device_attribute *attr,
> +		const char *buf, size_t count)
> +{
> +	struct pwm_fan_ctx *ctx = dev_get_drvdata(dev);
> +	int err;
> +	unsigned long val;
> +
> +	err = kstrtoul(buf, 10, &val);
> +	if (err)
> +		return err;
> +

'val' must be validated and only accept permitted values.

> +	mutex_lock(&ctx->lock);
> +	ctx->enable = val;
> +	mutex_unlock(&ctx->lock);
> +
> +	err = __set_pwm(ctx, ctx->pwm_fan_cooling_levels[ctx->pwm_fan_state]);
> +
> +	return err ? err : count;
> +}
> +
> +static ssize_t enable_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->enable);
> +}
> +
>  static SENSOR_DEVICE_ATTR_RW(pwm1, pwm, 0);
> +static SENSOR_DEVICE_ATTR_RW(pwm1_enable, enable, 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_pwm1_enable.dev_attr.attr,
>  	&sensor_dev_attr_fan1_input.dev_attr.attr,
>  	NULL,
>  };
> @@ -153,7 +189,7 @@ static umode_t pwm_fan_attrs_visible(struct kobject *kobj, struct attribute *a,
>  	struct pwm_fan_ctx *ctx = dev_get_drvdata(dev);
>  
>  	/* Hide fan_input in case no interrupt is available  */
> -	if (n == 1 && ctx->irq <= 0)
> +	if (n == 2 && ctx->irq <= 0)
>  		return 0;
>  
>  	return a->mode;
> @@ -200,7 +236,7 @@ static int
>  pwm_fan_set_cur_state(struct thermal_cooling_device *cdev, unsigned long state)
>  {
>  	struct pwm_fan_ctx *ctx = cdev->devdata;
> -	int ret;
> +	int ret = 0;
>  
>  	if (!ctx || (state > ctx->pwm_fan_max_state))
>  		return -EINVAL;
> @@ -208,10 +244,12 @@ pwm_fan_set_cur_state(struct thermal_cooling_device *cdev, unsigned long state)
>  	if (state == ctx->pwm_fan_state)
>  		return 0;
>  
> -	ret = __set_pwm(ctx, ctx->pwm_fan_cooling_levels[state]);
> -	if (ret) {
> -		dev_err(&cdev->device, "Cannot set pwm!\n");
> -		return ret;
> +	if (ctx->enable >= 2) {

What is "automatic" here ? I don't see how this driver or the underlying
pwm controller/chip would support automatic fan speed control. This is
completely independent of thermal control: thermal device support does _not_
imply or suggest "automatic" fan speed control from a pwm chip perspective.

This makes me also very concerned about this attribute in the first please:
All it does, in its current implementation, is to disable thermal device 
control. That is not the idea here, and it doesn't make any sense to me.
On the contrary, all I can find is that it is dangerous.

Please describe in detail what you think this attribute is supposed to
accomplish, and why you think it is needed or even makes sense. Overriding
thermal control doesn't make sense to me: If you want that, just disable
thermal control, or don't register this device as cooling device in the
first place.  Other than that, the whole driver implies manual pwm control.
That means pwm can already be "disabled" by writing a 0 pwm value. An
additional attributes doesn't add value and only makes the driver ABI
unnecessarily complex. So please prepare a detailed rationale to convince
me otherwise.

We can have a separate discussion if disabling a pwm controller using the
hwmon ABI should disable that pwm controller from a thermal (cooling) device
perspective. If so, we'll need to think about the implications. We can not
just disable thermal cooling device support without telling the thermal
subsystem that this means of temperature control doesn't work anymore.
That should, however, be a completely separate discussion, independent
of this driver, and it should include thermal subsystem maintainers
and the thermal subsystem mailing list.

Thanks,
Guenter

> +		ret = __set_pwm(ctx, ctx->pwm_fan_cooling_levels[state]);
> +		if (ret) {
> +			dev_err(&cdev->device, "Cannot set pwm!\n");
> +			return ret;
> +		}
>  	}
>  
>  	ctx->pwm_fan_state = state;
> @@ -298,6 +336,8 @@ static int pwm_fan_probe(struct platform_device *pdev)
>  	if (IS_ERR(ctx->pwm))
>  		return dev_err_probe(dev, PTR_ERR(ctx->pwm), "Could not get PWM\n");
>  
> +	ctx->enable = 2;
> +
>  	platform_set_drvdata(pdev, ctx);
>  
>  	ctx->irq = platform_get_irq_optional(pdev, 0);
> -- 
> 2.25.1
> 

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