linux-kernel - Re: [PATCH v2 2/2] PM / devfreq: Add governor flags to clarify the features

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Date:   Mon, 13 Jul 2020 21:26:25 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Dmitry Osipenko <digetx@...il.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     leonard.crestez@....com, lukasz.luba@....com,
        enric.balletbo@...labora.com, hl@...k-chips.com,
        thierry.reding@...il.com, jonathanh@...dia.com, abel.vesa@....com,
        chanwoo@...nel.org, myungjoo.ham@...sung.com,
        kyungmin.park@...sung.com,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] PM / devfreq: Add governor flags to clarify the
 features

On 7/13/20 7:37 PM, Dmitry Osipenko wrote:
> 13.07.2020 11:31, Chanwoo Choi пишет:
>> DEVFREQ supports the default governors like performance, powersave and also
>> allows the devfreq driver to add their own governor like tegra30-devfreq.c
>> according to their requirement. In result, some sysfs attributes are
>> useful or not useful. Prior to that the user can access all sysfs attributes
>> regardless of availability.
>>
>> So, clarify the access permission of sysfs attributes according to governor.
>> When adding the devfreq governor, can specify the available attribute
>> information by using DEVFREQ_GOV_ATTR_* constant variable. The user can
>> read or write the sysfs attributes in accordance to the specified attributes.
>>
>> /* Devfreq governor flags for attributes and features */
>> [Definition for sysfs attributes]
>> - DEVFREQ_GOV_ATTR_GOVERNOR
>> - DEVFREQ_GOV_ATTR_AVAIL_GOVERNORS
>> - DEVFREQ_GOV_ATTR_AVAIL_FREQUENCIES
>> - DEVFREQ_GOV_ATTR_CUR_FREQ
>> - DEVFREQ_GOV_ATTR_TARGET_FREQ
>> - DEVFREQ_GOV_ATTR_MIN_FREQ
>> - DEVFREQ_GOV_ATTR_MAX_FREQ
>> - DEVFREQ_GOV_ATTR_TRANS_STAT
>> - DEVFREQ_GOV_ATTR_POLLING_INTERVAL
>> - DEVFREQ_GOV_ATTR_TIMER
>>
>> Also, the devfreq governor is able to have the specific flag as follows
>> in order to implement the specific feature. For example, Devfreq allows
>> user to change the governors on runtime via sysfs interface.
>> But, if devfreq device uses 'passive' governor, don't allow user to change
>> the governor. For this case, define the DEVFREQ_GOV_FLAT_IMMUTABLE
>> and set it to flag of passive governor.
>>
>> [Definition for governor flag]
>> - DEVFREQ_GOV_FLAG_IMMUTABLE
>> : If immutable flag is set, governor is never changeable to other governors.
>> - DEVFREQ_GOV_FLAG_IRQ_DRIVEN
>> : Devfreq core won't schedule polling work for this governor if value is set.
>>
>> [Table of governor flag for devfreq governors]
>> ------------------------------------------------------------------------------
>>                       | simple    | perfor | power | user | passive | tegra30
>> 		      | ondemand  | mance  | save  | space|         |
>> ------------------------------------------------------------------------------
>> governor              | O         | O      | O     | O    | O       | O
>> available_governors   | O         | O      | O     | O    | O       | O
>> available_frequencies | O         | O      | O     | O    | O       | O
>> cur_freq              | O         | O      | O     | O    | O       | O
>> target_freq           | O         | O      | O     | O    | O       | O
>> min_freq              | O         | O      | O     | O    | O       | O
>> max_freq              | O         | O      | O     | O    | O       | O
>> trans_stat            | O         | O      | O     | O    | O       | O
>>                       --------------------------------------------------------
>> polling_interval      | O         | X      | X     | X    | X       | O
>> timer                 | O         | X      | X     | X    | X       | X
>> ------------------------------------------------------------------------------
>> immutable             | X         | X      | X     | X    | O       | O
>> interrupt_driven      | X(polling)| X      | X     | X    | X       | O (irq)
>> ------------------------------------------------------------------------------
>>
>> Signed-off-by: Chanwoo Choi <cw00.choi@...sung.com>
>> ---
> 
> Hello, Chanwoo! I tested this series on NVIDIA Tegra30 and everything
> working fine!
> 
> Reviewed-by: Dmitry Osipenko <digetx@...il.com>
> Tested-by: Dmitry Osipenko <digetx@...il.com>
> 
> 
> BTW, I'm curious what do you think about hiding the unsupported debugfs

Do you mean that sysfs?

> attributes per-device instead of returning the -EACCES?

I considered the hiding of sysfs node too instead of -EACCES.


But,
For a long time, devfreq showed the sysfs interface of all devfreq devices
regardless of the kind of devfreq governor. It means that devfreq keeps
the ABI interface. If devfreq hides the unsupported sysfs node
according to the type of governor, it will break the ABI.

Although I knew that maybe performance/powersave/userspace didn't use
the 'polling_interval' node, I just returned -EACCESS.

Thanks for suggesting your opinion. We can discuss it more.

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics