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

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Date:   Tue, 08 Nov 2016 12:52:01 -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 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.

-Saravana

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