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Subject: Re: [PATCH v3] PM / devfreq: Restart previous governor if new
 governor fails to start
To:     Chanwoo Choi <cw00.choi@samsung.com>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org
References: <CGME20161026191715epcas1p311c2c626563a736e14c83fb0504c099e@epcas1p3.samsung.com>
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From:   Saravana Kannan <skannan@codeaurora.org>
Message-ID: <5164dea6-ad0c-d817-eb0d-36d9f4a718d4@codeaurora.org>
Date:   Fri, 7 Dec 2018 12:29:17 -0800
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On 11/9/16 4:10 PM, Chanwoo Choi wrote:
> Hi,
>
> On 2016년 11월 10일 05:34, Saravana Kannan wrote:
>> 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@codeaurora.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?
> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>
> After fixing the bug of devfreq_add_device(),
> I'll remove the unneeded 'if statement' of prev_governor in governor_store.

It's been 2 years and this patch still hasn't been picked up. Can we 
please pick this up and get this into the next kernel release?

Thanks,

Saravana

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