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 <skan...@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].
Best Regards,
Chanwoo Choi
