[PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[Sibi Sankar]

From: Saravana Kannan <skannan@codeaurora.org>

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: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
---

V4:
* Removed prev_governor check.

V3:
* Fix NULL deref for real this time.
* Addressed some style preferences.

V2:
* Fixed typo in commit text
* Fixed potential NULL deref

 drivers/devfreq/devfreq.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 141413067b5c..ba2875a0b90e 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -1024,7 +1024,7 @@ 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)
@@ -1053,12 +1053,19 @@ 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);
+		df->governor = prev_governor;
+		strncpy(df->governor_name, prev_governor->name,
+			DEVFREQ_NAME_LEN);
+		df->governor->event_handler(df, DEVFREQ_GOV_START,
+					    NULL);
+	}
 out:
 	mutex_unlock(&devfreq_list_lock);
 
-- 
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project

RE: [PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[MyungJoo Ham]

> From: Saravana Kannan <skannan@codeaurora.org>
> 
> 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: Sibi Sankar <sibis@codeaurora.org>
> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>

Hello,

In overall, the idea and the implementation looks good.

However, I have a question:

What if the following line fails?

+		df->governor->event_handler(df, DEVFREQ_GOV_START,
+					    NULL);

Don't we still need something to handle for such events?

Cheers,
MyungJoo

Re: [PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[Sibi Sankar]

Hey MyungJoo,

On 12/14/18 7:15 AM, MyungJoo Ham wrote:
>> From: Saravana Kannan <skannan@codeaurora.org>
>>
>> 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: Sibi Sankar <sibis@codeaurora.org>
>> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
> 
> Hello,
> 
> In overall, the idea and the implementation looks good.
> 
> However, I have a question:
> 
> What if the following line fails?
> 
> +		df->governor->event_handler(df, DEVFREQ_GOV_START,
> +					    NULL);
> 
> Don't we still need something to handle for such events?

The original discussion went as follows:
governor_store is expected to be used only on cases
where devfreq_add_device() succeeded i.e prev->governor
is expected to be present and DEVFREQ_GOV_START is
expected to succeed. Hence falling back to the previous
governor seems like a sensible idea.

This would also prevent DEVFREQ_GOV_STOP from being called
on a governor were DEVFREQ_GOV_START had failed which is
ideal.

That being said DEVFREQ_GOV_START can still fail for the
prev-governor due to some change in state of the system.
Do you want to handle this case by clearing the state of
governor rather than switching to previous governor?

> 
> Cheers,
> MyungJoo
> 
> 

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

Re: [PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[Sibi Sankar]

Hey MyungJoo, Kyungmin
Did you get a chance to think about how you
want this fix implemented?

On 2019-02-19 10:42, Sibi Sankar wrote:
> Hey MyungJoo,
> 
> On 12/14/18 7:15 AM, MyungJoo Ham wrote:
>>> From: Saravana Kannan <skannan@codeaurora.org>
>>> 
>>> 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: Sibi Sankar <sibis@codeaurora.org>
>>> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
>>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>> 
>> Hello,
>> 
>> In overall, the idea and the implementation looks good.
>> 
>> However, I have a question:
>> 
>> What if the following line fails?
>> 
>> +		df->governor->event_handler(df, DEVFREQ_GOV_START,
>> +					    NULL);
>> 
>> Don't we still need something to handle for such events?
> 
> The original discussion went as follows:
> governor_store is expected to be used only on cases
> where devfreq_add_device() succeeded i.e prev->governor
> is expected to be present and DEVFREQ_GOV_START is
> expected to succeed. Hence falling back to the previous
> governor seems like a sensible idea.
> 
> This would also prevent DEVFREQ_GOV_STOP from being called
> on a governor were DEVFREQ_GOV_START had failed which is
> ideal.
> 
> That being said DEVFREQ_GOV_START can still fail for the
> prev-governor due to some change in state of the system.
> Do you want to handle this case by clearing the state of
> governor rather than switching to previous governor?
> 
>> 
>> Cheers,
>> MyungJoo
>> 
>> 

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

RE: Re: [PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[MyungJoo Ham]

>Hey MyungJoo, Kyungmin
>Did you get a chance to think about how you
>want this fix implemented?
>
>On 2019-02-19 10:42, Sibi Sankar wrote:
>> Hey MyungJoo,
>> 
>> On 12/14/18 7:15 AM, MyungJoo Ham wrote:
>>>> From: Saravana Kannan <skannan@codeaurora.org>
>>>> 
>>>> 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: Sibi Sankar <sibis@codeaurora.org>
>>>> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
>>>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>>> 
>>> Hello,
>>> 
>>> In overall, the idea and the implementation looks good.
>>> 
>>> However, I have a question:
>>> 
>>> What if the following line fails?
>>> 
>>> +		df->governor->event_handler(df, DEVFREQ_GOV_START,
>>> +					    NULL);
>>> 
>>> Don't we still need something to handle for such events?
>> 
>> The original discussion went as follows:
>> governor_store is expected to be used only on cases
>> where devfreq_add_device() succeeded i.e prev->governor
>> is expected to be present and DEVFREQ_GOV_START is
>> expected to succeed. Hence falling back to the previous
>> governor seems like a sensible idea.
>> 
>> This would also prevent DEVFREQ_GOV_STOP from being called
>> on a governor were DEVFREQ_GOV_START had failed which is
>> ideal.
>> 
>> That being said DEVFREQ_GOV_START can still fail for the
>> prev-governor due to some change in state of the system.
>> Do you want to handle this case by clearing the state of
>> governor rather than switching to previous governor?
>> 

If moving back to previous governor fails after
failing for "next" governor, we may assume it's fatal
and stop the device; we can simply return errors.

In such a case, df->governor may need to be NULL as well.


Cheers,
MyungJoo

Re: [PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

[Sibi Sankar]

On 2019-03-05 12:48, MyungJoo Ham wrote:
>> Hey MyungJoo, Kyungmin
>> Did you get a chance to think about how you
>> want this fix implemented?
>> 
>> On 2019-02-19 10:42, Sibi Sankar wrote:
>>> Hey MyungJoo,
>>> 
>>> On 12/14/18 7:15 AM, MyungJoo Ham wrote:
>>>>> From: Saravana Kannan <skannan@codeaurora.org>
>>>>> 
>>>>> 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: Sibi Sankar <sibis@codeaurora.org>
>>>>> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
>>>>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>>>> 
>>>> Hello,
>>>> 
>>>> In overall, the idea and the implementation looks good.
>>>> 
>>>> However, I have a question:
>>>> 
>>>> What if the following line fails?
>>>> 
>>>> +		df->governor->event_handler(df, DEVFREQ_GOV_START,
>>>> +					    NULL);
>>>> 
>>>> Don't we still need something to handle for such events?
>>> 
>>> The original discussion went as follows:
>>> governor_store is expected to be used only on cases
>>> where devfreq_add_device() succeeded i.e prev->governor
>>> is expected to be present and DEVFREQ_GOV_START is
>>> expected to succeed. Hence falling back to the previous
>>> governor seems like a sensible idea.
>>> 
>>> This would also prevent DEVFREQ_GOV_STOP from being called
>>> on a governor were DEVFREQ_GOV_START had failed which is
>>> ideal.
>>> 
>>> That being said DEVFREQ_GOV_START can still fail for the
>>> prev-governor due to some change in state of the system.
>>> Do you want to handle this case by clearing the state of
>>> governor rather than switching to previous governor?
>>> 
> 
> If moving back to previous governor fails after
> failing for "next" governor, we may assume it's fatal
> and stop the device; we can simply return errors.
> 
> In such a case, df->governor may need to be NULL as well.

Thanks. Will make the necessary changes in the
next re-spin.

> 
> 
> Cheers,
> MyungJoo

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