Re: [PATCH v4 3/4] devfreq: Factor out devfreq_set_governor()

[Chanwoo Choi <cwchoi00@gmail.com>]

On 7/29/2026 7:23 PM, Jie Zhan wrote:
> governor_store() and devfreq_add_device() contain similar logic for setting
> a governor when devfreq->governor is NULL.  Merge this into a common
> function, devfreq_set_governor(), to reduce code duplication and unify the
> entry of setting governors.
> 
> This also prepares for further changes that get / put a module refcount of
> the active governor and prevent the governor module from being unloaded
> while it's in use.
> 
> Signed-off-by: Jie Zhan <zhanjie9@hisilicon.com>
> ---
>  drivers/devfreq/devfreq.c | 128 ++++++++++++++++++++------------------
>  1 file changed, 67 insertions(+), 61 deletions(-)
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 53c40d795a13..959f0b08ed36 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -318,6 +318,70 @@ static struct devfreq_governor *try_then_request_governor(const char *name)
>  	return governor;
>  }
>  
> +static int devfreq_set_governor(struct devfreq *df,
> +				const struct devfreq_governor *new_gov)
> +{
> +	const struct devfreq_governor *old_gov;
> +	struct device *dev;
> +	int ret;
> +
> +	lockdep_assert_held(&devfreq_list_lock);
> +
> +	old_gov = df->governor;
> +	dev = &df->dev;
> +
> +	if (old_gov) {
> +		if (old_gov == new_gov)
> +			return 0;
> +
> +		if (IS_SUPPORTED_FLAG(old_gov->flags, IMMUTABLE) ||
> +		    IS_SUPPORTED_FLAG(new_gov->flags, IMMUTABLE))
> +			return -EINVAL;
> +
> +		/* Stop the current governor */
> +		ret = df->governor->event_handler(df, DEVFREQ_GOV_STOP, NULL);
> +		if (ret) {
> +			dev_warn(dev, "%s: Governor %s not stopped(%d)\n",
> +				 __func__, df->governor->name, ret);
> +			return ret;
> +		}
> +	}
> +
> +	/* Start the new governor */
> +	df->governor = new_gov;
> +	ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
> +	if (ret) {
> +		dev_warn(dev, "%s: Governor %s not started(%d)\n",
> +			 __func__, df->governor->name, ret);
> +
> +		/* Restore previous governor */
> +		df->governor = old_gov;
> +		if (!df->governor)
> +			return ret;
> +
> +		ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
> +		if (ret) {
> +			dev_err(dev, "%s: restore Governor %s failed (%d)\n",
> +				__func__, df->governor->name, ret);
> +			df->governor = NULL;
> +			return ret;
> +		}
> +
> +		ret = sysfs_update_group(&df->dev.kobj, &gov_attr_group);
> +		if (ret)
> +			dev_warn(dev, "sysfs update failed (%d)\n", ret);
> +
> +		/*
> +		 * Old governor restored successfully, but the new governor
> +		 * failed to start.  Return a distinct error so the caller is
> +		 * not misled into thinking the switch succeeded.
> +		 */
> +		return -EAGAIN;
> +	}
> +
> +	return sysfs_update_group(&df->dev.kobj, &gov_attr_group);
> +}
> +
>  static int devfreq_notify_transition(struct devfreq *devfreq,
>  		struct devfreq_freqs *freqs, unsigned int state)
>  {
> @@ -942,9 +1006,7 @@ struct devfreq *devfreq_add_device(struct device *dev,
>  		goto err_init;
>  	}
>  
> -	devfreq->governor = governor;
> -	err = devfreq->governor->event_handler(devfreq, DEVFREQ_GOV_START,
> -						NULL);
> +	err = devfreq_set_governor(devfreq, governor);
>  	if (err) {
>  		dev_err_probe(dev, err,
>  			"%s: Unable to start governor for the device\n",
> @@ -952,10 +1014,6 @@ struct devfreq *devfreq_add_device(struct device *dev,
>  		goto err_init;
>  	}
>  
> -	err = sysfs_update_group(&devfreq->dev.kobj, &gov_attr_group);
> -	if (err)
> -		goto err_init;
> -
>  	list_add(&devfreq->node, &devfreq_list);
>  
>  	mutex_unlock(&devfreq_list_lock);
> @@ -1380,7 +1438,7 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
>  	struct devfreq *df = to_devfreq(dev);
>  	int ret;
>  	char str_governor[DEVFREQ_NAME_LEN + 1];
> -	const struct devfreq_governor *governor, *prev_governor;
> +	const struct devfreq_governor *governor;
>  
>  	ret = sscanf(buf, "%" __stringify(DEVFREQ_NAME_LEN) "s", str_governor);
>  	if (ret != 1)
> @@ -1391,59 +1449,7 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
>  	if (IS_ERR(governor))
>  		return PTR_ERR(governor);
>  
> -	if (!df->governor)
> -		goto start_new_governor;
> -
> -	if (df->governor == governor)
> -		return count;
> -
> -	if (IS_SUPPORTED_FLAG(df->governor->flags, IMMUTABLE) ||
> -	    IS_SUPPORTED_FLAG(governor->flags, IMMUTABLE))
> -		return -EINVAL;
> -
> -	/*
> -	 * Stop the current governor and remove the specific sysfs files
> -	 * which depend on current governor.
> -	 */
> -	ret = df->governor->event_handler(df, DEVFREQ_GOV_STOP, NULL);
> -	if (ret) {
> -		dev_warn(dev, "%s: Governor %s not stopped(%d)\n",
> -			 __func__, df->governor->name, ret);
> -		return ret;
> -	}
> -
> -start_new_governor:
> -	/*
> -	 * Start the new governor and create the specific sysfs files
> -	 * which depend on the new governor.
> -	 */
> -	prev_governor = df->governor;
> -	df->governor = governor;
> -	ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
> -	if (ret) {
> -		dev_warn(dev, "%s: Governor %s not started(%d)\n",
> -			 __func__, df->governor->name, ret);
> -
> -		/* Restore previous governor */
> -		df->governor = prev_governor;
> -		if (!df->governor)
> -			return ret;
> -
> -		ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
> -		if (ret) {
> -			dev_err(dev,
> -				"%s: reverting to Governor %s failed (%d)\n",
> -				__func__, prev_governor->name, ret);
> -			df->governor = NULL;
> -			return ret;
> -		}
> -	}
> -
> -	/*
> -	 * Create the sysfs files for the new governor. But if failed to start
> -	 * the new governor, restore the sysfs files of previous governor.
> -	 */
> -	ret = sysfs_update_group(&df->dev.kobj, &gov_attr_group);
> +	ret = devfreq_set_governor(df, governor);
>  	if (ret)
>  		return ret;
>  

Actually, I didn't want to merge it becaus there are fully test for all governor
and various case to switch governor(stop/start/switch). It makes the problem.
Also, there is an AI report indicating that this patch has a problem related to passive govenor.

We must guarantee the safe operation without any side-effect.
Don't update devfreq_add_device and better to make the separate function 'start_new_governor'
and then do next update separately.

-- 
Best Regards,
Chanwoo Choi
Principal Engineer, Samsung Electronics