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Subject: Re: [PATCH v3] PM / devfreq: Fix devfreq_add_device() when drivers
 are built as modules.
To:     Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        linux-kernel@vger.kernel.org
Cc:     kernel@collabora.com, Chanwoo Choi <cw00.choi@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        linux-pm@vger.kernel.org
References: <20180621220430.25644-1-enric.balletbo@collabora.com>
 <0383b4a703a0b619023c0654a4a7637419c72ebd.camel@collabora.com>
 <addc4280-729f-85d7-a314-35afbb7c8bb1@codeaurora.org>
 <36c9720c-7428-0a17-5252-7afbd80bce6b@collabora.com>
From:   Akhil P Oommen <akhilpo@codeaurora.org>
Message-ID: <9ecb2d61-5713-c0dc-9d35-2c926372fd03@codeaurora.org>
Date:   Fri, 22 Jun 2018 14:36:31 +0530
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On 6/22/2018 1:52 PM, Enric Balletbo i Serra wrote:
> Hi Ezequiel and Akhil,
>
> On 22/06/18 09:03, Akhil P Oommen wrote:
>> On 6/22/2018 6:41 AM, Ezequiel Garcia wrote:
>>> 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@collabora.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.
>>>
> I think so too.
>
>>>>        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
>> Adding to Ezequiel's point, shouldn't we take more granular lock (devfreq->lock)
>> first and then call devfreq_list_lock at the time of adding to the list?
>>
> Yes, I think so. I think, though, that this should be a separate patch, not sure
> if a pre or post patch to this one, but for sure it's another topic. Current
> patch tries to solve different problem an only tries to follow the current
> locking/unlocking. Anyway this is a maintainer decision I guess.
>
> Thanks,
>   Enric
>
>> -Akhil.
>>
I agree.
-Akhil.