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

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Date:   Wed, 09 Nov 2016 12:34:08 -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 11/08/2016 06:38 PM, Chanwoo Choi wrote:
> On 2016년 11월 09일 11:36, Chanwoo Choi wrote:
>> Hi,
>>
>> On 2016년 11월 09일 10:33, Chanwoo Choi wrote:
>>> On 2016년 11월 09일 05:52, Saravana Kannan wrote:
>>>> On 11/08/2016 01:02 AM, Chanwoo Choi wrote:
>>>>> 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.
>>>>
>>>> Read the code more closely. add_device doesn't (and shouldn't) fail if the governor isn't present. After that, the governor could be changed from user space.
>>>
>>> If governor is not present during devfreq_add_device(),
>>> the devfreq instance is not able to register the devfreq framework.
>>> So, the user never touch the sysfs[1] to change the governor
>>> because the sysfs[1] is not created by devfreq framework.
>>> [1] /sys/class/devfreq/[device name]/governor
>>>
>>> In summary, if governor is not present during devfreq_add_device(),
>>> the devfreq framework doesn't create tye sysfs[1] for governor.
>>>
>>> The user-space never change the governor through sysfs[1]
>>> because there is no any sysfs entry[1].
>>
>> I checked the code of devfreq_add_device().
>> As you mentioned, if there is no governor,
>> devfreq_add_device() don't pass wihtout calling the devfreq->governor->even_handler().
>
> Wrong expression / don't pass -> pass
>

I think it's correct as is. Just because the governor isn't there (or 
hasn't registered yet) doesn't mean the device add should fail.

That aside, do you care to ack this patch for the way the code is currently?

-Saravana

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