linux-kernel - Re: [PATCH v3] PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.

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Message-ID: <0383b4a703a0b619023c0654a4a7637419c72ebd.camel@collabora.com>
Date:   Thu, 21 Jun 2018 22:11:36 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel@...r.kernel.org
Cc:     kernel@...labora.com, Chanwoo Choi <cw00.choi@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3] PM / devfreq: Fix devfreq_add_device() when drivers
 are built as modules.

Hey Enric,

On Fri, 2018-06-22 at 00:04 +0200, 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@...labora.com>
> ---
> 
> 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 | 65 ++++++++++++++++++++++++++++++++-----
> --
[snip snip]
> -	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__);
>  		err = PTR_ERR(governor);
> -		goto err_init;
> +		goto err_unregister;
>  	}
>  
> +	mutex_lock(&devfreq_list_lock);
> +

I know it's not something we are introducing in this patch,
but still... calling a hook with a mutex held looks
fishy to me.

This lock should only protect the list, unless I am missing
something.

>  	devfreq->governor = governor;
>  	err = devfreq->governor->event_handler(devfreq,
> DEVFREQ_GOV_START,
>  						NULL);
> @@ -663,14 +703,16 @@ 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);
> -
> +err_unregister:
>  	device_unregister(&devfreq->dev);
>  err_dev:
>  	if (devfreq)
> @@ -988,12 +1030,13 @@ static ssize_t governor_store(struct device
> *dev, struct device_attribute *attr,
>  	if (ret != 1)
>  		return -EINVAL;
>  
> -	mutex_lock(&devfreq_list_lock);
> -	governor = find_devfreq_governor(str_governor);
> +	governor = try_then_request_governor(str_governor);
>  	if (IS_ERR(governor)) {
> -		ret = PTR_ERR(governor);
> -		goto out;
> +		return PTR_ERR(governor);
>  	}
> +
> +	mutex_lock(&devfreq_list_lock);
> +
>  	if (df->governor == governor) {
>  		ret = 0;
>  		goto out;
> -- 
> 2.17.1
> 
> 


Regards,
Eze