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Message-id: <5B3C1DA6.7040507@samsung.com>
Date:   Wed, 04 Jul 2018 10:06:46 +0900
From:   Chanwoo Choi <cw00.choi@samsung.com>
Organization: Samsung Electronics
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To:     Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        linux-kernel@vger.kernel.org
Cc:     kernel@collabora.com, Kyungmin Park <kyungmin.park@samsung.com>,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        linux-pm@vger.kernel.org
Subject: Re: [PATCH v4] PM / devfreq: Fix devfreq_add_device() when drivers
 are built as modules.
In-reply-to: <20180703132931.14389-1-enric.balletbo@collabora.com>
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References: <CGME20180703132943epcas1p25b68c00ca1143d5eabcc2285c2ce27a0@epcas1p2.samsung.com>
        <20180703132931.14389-1-enric.balletbo@collabora.com>
Sender: linux-kernel-owner@vger.kernel.org
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Hi Enric,

On 2018년 07월 03일 22:29, Enric Balletbo i Serra wrote:
> When the devfreq driver and the governor driver are built as modules,
> the call to devfreq_add_device() or governor_store() fails because the
> governor driver is not loaded at the time the devfreq driver loads. The
> devfreq driver has a build dependency on the governor but also should
> have a runtime dependency. We need to make sure that the governor driver
> is loaded before the devfreq driver.
> 
> This patch fixes this bug by adding a try_then_request_governor()
> function. First tries to find the governor, and then, if it is not found,
> it requests the module and tries again.
> 
> Fixes: 1b5c1be2c88e (PM / devfreq: map devfreq drivers to governor using name)
> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
> ---
> 
> Changes in v4:
> - Kept "locked" devfreq_list from the return of find_devfreq_governor() to
>   the unlock of governor_store(). Requested by MyungJoo Ham.
> 
> Changes in v3:
> - Remove unneded change in dev_err message.
> - Fix err returned value in case to not find the governor.
> 
> Changes in v2:
> - Add a new function to request the module and call that function from
>   devfreq_add_device and governor_store.
> 
>  drivers/devfreq/devfreq.c | 53 ++++++++++++++++++++++++++++++++++-----
>  1 file changed, 47 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 0b5b3abe054e..4ea6b19879a1 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -11,6 +11,7 @@
>   */
>  
>  #include <linux/kernel.h>
> +#include <linux/kmod.h>
>  #include <linux/sched.h>
>  #include <linux/errno.h>
>  #include <linux/err.h>
> @@ -221,6 +222,46 @@ static struct devfreq_governor *find_devfreq_governor(const char *name)
>  	return ERR_PTR(-ENODEV);
>  }
>  
> +/**
> + * try_then_request_governor() - Try to find the governor and request the
> + *                               module if is not found.
> + * @name:	name of the governor

Usually, devfreq used 'governor_name' indicating the name of governor.
you better to use 'governor_name' instead of 'name' for more readability.

> + *
> + * Search the list of devfreq governors and request the module and try again
> + * if is not found. This can happen when both drivers (the governor driver
> + * and the driver that call devfreq_add_device) are built as modules.
> + * devfreq_list_lock should be held by the caller.
> + *
> + * Return: The matched governor's pointer.

Usually, devfreq.c didn;t use the 'Return: ...'. So, you better to explain
what is returned from this function with function description.

> + */
> +static struct devfreq_governor *try_then_request_governor(const char *name)

ditto. (name -> governor_name)

> +{
> +	struct devfreq_governor *governor;
> +	int err = 0;

You have to check whether governor name is NULL or not.

	if (IS_ERR_OR_NULL(name)) {                                             
		pr_err("DEVFREQ: %s: Invalid parameters\n", __func__);          
		return ERR_PTR(-EINVAL);
	}

> +
> +	WARN(!mutex_is_locked(&devfreq_list_lock),
> +	     "devfreq_list_lock must be locked.");
> +
> +	governor = find_devfreq_governor(name);
> +	if (IS_ERR(governor)) {
> +		mutex_unlock(&devfreq_list_lock);
> +
> +		if (!strncmp(name, DEVFREQ_GOV_SIMPLE_ONDEMAND,
> +			     DEVFREQ_NAME_LEN))
> +			err = request_module("governor_%s", "simpleondemand");
> +		else
> +			err = request_module("governor_%s", name);
> +		if (err)
> +			return NULL;

When error happen, you unlock the mutex. If failed to request module,
you should restore the previous state. Please mutex_lock(&devfreq_list_lock)
before return.

> +
> +		mutex_lock(&devfreq_list_lock);
> +
> +		governor = find_devfreq_governor(name);
> +	}
> +
> +	return governor;
> +}
> +
>  static int devfreq_notify_transition(struct devfreq *devfreq,
>  		struct devfreq_freqs *freqs, unsigned int state)
>  {
> @@ -643,11 +684,9 @@ struct devfreq *devfreq_add_device(struct device *dev,
>  	srcu_init_notifier_head(&devfreq->transition_notifier_list);
>  
>  	mutex_unlock(&devfreq->lock);
> -

This change is not related to this patch.

>  	mutex_lock(&devfreq_list_lock);
> -	list_add(&devfreq->node, &devfreq_list);
>  
> -	governor = find_devfreq_governor(devfreq->governor_name);
> +	governor = try_then_request_governor(devfreq->governor_name);
>  	if (IS_ERR(governor)) {
>  		dev_err(dev, "%s: Unable to find governor for the device\n",
>  			__func__);
> @@ -663,14 +702,15 @@ struct devfreq *devfreq_add_device(struct device *dev,
>  			__func__);
>  		goto err_init;
>  	}
> +
> +	list_add(&devfreq->node, &devfreq_list);
> +
>  	mutex_unlock(&devfreq_list_lock);
>  
>  	return devfreq;
>  
>  err_init:
> -	list_del(&devfreq->node);
>  	mutex_unlock(&devfreq_list_lock);
> -

This change is not related to this patch.

>  	device_unregister(&devfreq->dev);
>  err_dev:
>  	if (devfreq)
> @@ -989,7 +1029,8 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
>  		return -EINVAL;
>  
>  	mutex_lock(&devfreq_list_lock);
> -	governor = find_devfreq_governor(str_governor);
> +

Don't need to add the blank line. It is enough to change the function
from find_devfreq_governor to try_then_request_governor.

> +	governor = try_then_request_governor(str_governor);
>  	if (IS_ERR(governor)) {
>  		ret = PTR_ERR(governor);
>  		goto out;
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics