linux-kernel - Re: [PATCH v3] PM / devfreq: Restart previous governor if new governor fails to start

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Message-id: <5821948B.2010907@samsung.com>
Date:   Tue, 08 Nov 2016 18:02:03 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Saravana Kannan <skannan@...eaurora.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PM / devfreq: Restart previous governor if new governor
 fails to start

Hi,

On 2016년 11월 08일 03:57, Saravana Kannan wrote:
> On 10/26/2016 05:06 PM, Chanwoo Choi wrote:
>> Hi,
>>
>> On 2016년 10월 27일 04:17, Saravana Kannan wrote:
>>> If the new governor fails to start, switch back to old governor so that the
>>> devfreq state is not left in some weird limbo.
>>>
>>> Signed-off-by: Saravana Kannan <skannan@...eaurora.org>
>>> ---
>>> * Fix NULL deref for real this time.
>>> * Addressed some style preferences.
>>>
>>>   drivers/devfreq/devfreq.c | 13 +++++++++++--
>>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>> index bf3ea76..77c41a5 100644
>>> --- a/drivers/devfreq/devfreq.c
>>> +++ b/drivers/devfreq/devfreq.c
>>> @@ -919,7 +919,7 @@ 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];
>>> -    struct devfreq_governor *governor;
>>> +    const struct devfreq_governor *governor, *prev_governor;
>>>
>>>       ret = sscanf(buf, "%" __stringify(DEVFREQ_NAME_LEN) "s", str_governor);
>>>       if (ret != 1)
>>> @@ -944,12 +944,21 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr,
>>>               goto out;
>>>           }
>>>       }
>>> +    prev_governor = df->governor;
>>>       df->governor = governor;
>>>       strncpy(df->governor_name, governor->name, DEVFREQ_NAME_LEN);
>>>       ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
>>> -    if (ret)
>>> +    if (ret) {
>>>           dev_warn(dev, "%s: Governor %s not started(%d)\n",
>>>                __func__, df->governor->name, ret);
>>> +        if (prev_governor) {
>>
>> I think that prev_governor is always set. You don't need to check it.
>> Why do check it?
> 
> Not true. Even higher up in this function, we check if df->governor != NULL. Simple example would be that the default governor passed in while adding the device isn't compiled in.

I don't understand. If device is not registered by devfreq_add_device(),
you don't change the governor by using governor_store().

If you can change the governor through governor_store(),
it means that the devfreq instance was added without any problem.
The added devfreq instance must have the sepecific governor
on df->governor variable.

So, I think that df->governor is always set and then prev_governor is always set.

Best Regards,
Chanwoo Choi