Date:	Tue, 16 Dec 2014 10:23:50 +0900
From:	Chanwoo Choi <cw00.choi@...sung.com>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
Cc:	myungjoo.ham@...sung.com, kyungmin.park@...sung.com,
	kgene.kim@...sung.com, rafael.j.wysocki@...el.com,
	a.kesavan@...sung.com, tomasz.figa@...il.com,
	b.zolnierkie@...sung.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org
Subject: Re: [RFC PATCHv3 1/7] devfreq: event: Add new devfreq_event class to
 provide basic data for devfreq governor

Hi Krzysztof,

On 12/15/2014 11:53 PM, Krzysztof Kozlowski wrote:
> On pią, 2014-12-12 at 17:27 +0900, Chanwoo Choi wrote:
>> This patch add new devfreq_event class for devfreq_event device which provide
>> raw data (e.g., memory bus utilization/GPU utilization). This raw data from
>> devfreq_event data would be used for the governor of devfreq subsystem.
>> - devfreq_event device : Provide raw data for governor of existing devfreq device
>> - devfreq device       : Monitor device state and change frequency/voltage of device
>>                          using the raw data from devfreq_event device
>>
>> The devfreq subsystem support generic DVFS(Dynamic Voltage/Frequency Scaling)
>> for Non-CPU Devices. The devfreq device would dertermine current device state
>> using various governor (e.g., ondemand, performance, powersave). After completed
>> determination of system state, devfreq device would change the frequency/voltage
>> of devfreq device according to the result of governor.
>>
>> But, devfreq governor must need basic data which indicates current device state.
>> Existing devfreq subsystem only consider devfreq device which check current system
>> state and determine proper system state using basic data. There is no subsystem
>> for device providing basic data to devfreq device.
>>
>> The devfreq subsystem must need devfreq_event device(data-provider device) for
>> existing devfreq device. So, this patch add new devfreq_event class for
>> devfreq_event device which read various basic data(e.g, memory bus utilization,
>> GPU utilization) and provide measured data to existing devfreq device through
>> standard APIs of devfreq_event class.
>>
>> The following description explains the feature of two kind of devfreq class:
>> - devfreq class (existing)
>>  : devfreq consumer device use raw data from devfreq_event device for
>>    determining proper current system state and change voltage/frequency
>>    dynamically using various governors.
>>
>> - devfreq_event class (new)
>>  : Provide measured raw data to devfreq device for governor
>>
>> Cc: MyungJoo Ham <myungjoo.ham@...sung.com>
>> Cc: Kyungmin Park <kyungmin.park@...sung.com>
>> Signed-off-by: Chanwoo Choi <cw00.choi@...sung.com>
> 
> [...]
> 
>> +/**
>> + * devfreq_event_get_event() - Get event and total_event from devfreq-event dev.
>> + * @edev	: the devfreq-event device
>> + *
>> + * Note that this function get the calculated event data from devfreq-event dev
>> + * after stoping the progress of whole sequence of devfreq-event dev. Return
>> + * current event data and total_event should be stored in second parameter
>> + * (total_event).
>> + */
>> +u64 devfreq_event_get_event(struct devfreq_event_dev *edev, u64 *total_event)
>> +{
> 
> I think this function should return int (0 for success, negative
> otherwise) and store the "event" under pointer passed as argument.
> Why? Because:
> 1. error conditions are indicated with 'return 0' but should be 'return
> -EINVAL'
> 2. Exynos-ppmu driver returns -EINVAL.
> Checking for error conditions is in such case more complex than it
> should.

In this patchset,
the return value (event) and *total_event of devfreq_event_get_event() should
be used for busy_time/total_time of struct devfreq_dev_status (include/linux/devfreq.h).
The busy_time/total_time is 'unsigned long' type.

So, I'll modify the prototype of devfreq_event_get_event() as following by adding
new 'devfreq_event_data' structure.

struct devfreq_event_data {
	u64 event;
	u64 total_event;
};

int devfreq_event_get_event(struct devfreq_event_edev *edev, struct devfreq_event_data *edata);

Best Regards,
Chanwoo Choi



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