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

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Date:   Mon, 07 Nov 2016 10:57:02 -0800
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Chanwoo Choi <cw00.choi@...sung.com>
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

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.


Thanks,
Saravana

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