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 10:33:15 +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

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].

Best Regards,
Chanwoo Choi