linux-kernel - RE: Re: [RESEND] devfreq: governor: Save void *data in the governor userspace

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Message-ID: <20220914094355epcms1p8d6eeb0834e2d7c782d49558fac0ce6c4@epcms1p8>
Date:   Wed, 14 Sep 2022 18:43:55 +0900
From:   MyungJoo Ham <myungjoo.ham@...sung.com>
To:     Kant Fan <kant@...winnertech.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>
CC:     "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "allwinner-opensource-support@...winnertech.com" 
        <allwinner-opensource-support@...winnertech.com>
Subject: RE: Re: [RESEND] devfreq: governor: Save void *data in the governor
 userspace

>On 3/29/2022 5:14 PM, Kant Fan wrote:
>> The member void *data in the structure devfreq can be overwrite
>> by governor_userspace. 
>> Signed-off-by: Kant Fan <kant@...winnertech.com>
>> ---
>>   drivers/devfreq/governor_userspace.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/devfreq/governor_userspace.c b/drivers/devfreq/governor_userspace.c
>> index ab9db7adb3ad..dbbb448dcbcf 100644
>> --- a/drivers/devfreq/governor_userspace.c
>> +++ b/drivers/devfreq/governor_userspace.c
>> @@ -17,6 +17,7 @@
>>   struct userspace_data {
>>           unsigned long user_frequency;
>>           bool valid;
>> +        void *saved_data;
>>   };
>>   
>>   static int devfreq_userspace_func(struct devfreq *df, unsigned long *freq)
>> @@ -91,6 +92,7 @@ static int userspace_init(struct devfreq *devfreq)
>>                   goto out;
>>           }
>>           data->valid = false;
>> +        data->saved_data = devfreq->data;
>>           devfreq->data = data;
>>   
>>           err = sysfs_create_group(&devfreq->dev.kobj, &dev_attr_group);
>> @@ -100,6 +102,8 @@ static int userspace_init(struct devfreq *devfreq)
>>   
>>   static void userspace_exit(struct devfreq *devfreq)
>>   {
>> +        struct userspace_data *data = devfreq->data;
>> +        void *saved_data = data->saved_data;
>>           /*
>>            * Remove the sysfs entry, unless this is being called after
>>            * device_del(), which should have done this already via kobject_del().
>> @@ -108,7 +112,7 @@ static void userspace_exit(struct devfreq *devfreq)
>>                   sysfs_remove_group(&devfreq->dev.kobj, &dev_attr_group);
>>   
>>           kfree(devfreq->data);
>> -        devfreq->data = NULL;
>> +        devfreq->data = saved_data;
>>   }
>>   
>>   static int devfreq_userspace_handler(struct devfreq *devfreq,
>
>Dear MyungJoo, Kyungmin & Chanwoo,
>Gently ping this issue... Does this patch has a chance to be accepted?
>This seems to be a bug in devfreq userspace governor, which affects the 
>switching between governors -- When switching from userspace to 
>ondemand, the ondemand governor would be invalid.
>If there's any question, please let me know.
>Thank you.

Yes, indeed. This is a bug.

Actually, it appears that allocating a new memory buffer for
devfreq->data itself is a bug for a governor, this is supposed
to be allocated by a device driver. Thus, the comment of
"void *data" of "struct devfreq" should be updated:
"/* private data for governors given by device drivers */"

It'd be better to have something like, "void *internal_data"
for governors to freely handle within its context of init-exit,
which is not touched by its users (device drivers).

@Chanwoo: what's your opinion on this?

Cheers,
MyungJoo.