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SubjectRe: [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@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.

-Saravana

--
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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