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

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Message-id: <58228B9E.2090108@samsung.com>
Date:   Wed, 09 Nov 2016 11:36:14 +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 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().

I discussed this issue with Myungjoo Ham (devfreq maintainer).
It is bug of devfreq_add_device(). I'll fix this issue.

Best Regards,
Chanwoo Choi