linux-kernel - Re: [PATCH 3/3] thermal/sysfs: Remove cooling device sysfs statistics

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Message-ID: <eada018c-b857-7946-ae4b-2532e6866a45@arm.com>
Date:   Thu, 2 Jun 2022 10:16:23 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rafael@...nel.org, quic_manafm@...cinc.com,
        Amit Kucheria <amitk@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH 3/3] thermal/sysfs: Remove cooling device sysfs statistics



On 6/2/22 09:37, Daniel Lezcano wrote:
> 
> Hi Lukasz,
> 
> [Adding Todd]
> 
> On 01/06/2022 17:33, Lukasz Luba wrote:
>> Hi Daniel,
>>
>>
>> On 6/1/22 16:14, Daniel Lezcano wrote:
>>> The statistics are for debugging purpose and belong to debugfs rather
>>> than sysfs. As the previous changes introduced the same statistics in
>>> debugfs, those in sysfs are no longer needed and can be removed.
>>
>> I just want to let you know that in current Android kernels we cannot
>> even compile the kernel with CONFIG_DEBUG_FS.
> 
> Right, it makes sense. Precisely, with the sysfs stats they are always 
> compiled in for the Android kernel and is a problem for low memory 
> systems. While debugfs can fulfill its purpose in the developement and 
> will be removed in production systems.

True.

> 
>> I have this pain with
>> Energy Model there... Some vendors might see useful info via this
>> sysfs interface in bring-up of the SoC.
> 
> Well alternatively, information can be extracted from procfs in the 
> device-tree description.
> 
> What prevents to add energy information in sysfs now that the energy 
> model is per device ?

Probably nothing, but we need strong need. I have proposed this
a few times internally, but this must have a requirement.
If a user-space tool would ask for it, then I could send a patch
exposing the sysfs. So far we have only one user-space tool, which
suffers the missing debugfs EM dir: LISA (but we are working on a
workaround for it).
If you have a tool or plan to have such, which uses EM, please let
me know. I'm gathering the requirements.

> 
>> I don't know if there are user-space tools tracking this
>> information via sysfs. We probably should check that.
>>
>> I agree that these statistics look more like debug info, rather than
>> something useful for control.
>>
>> Furthermore, we have trace events for the cooling state changes, which
>> should be good enough for bring-up and experiments.
>>
>> I don't have strong preferences here. I tend to agree to remove this
>> interface if there are no user-space tools using it.
> 
> I agree userspace can also get information about the transition but the 
> goal of the debugfs is also add information about thermal internals like 
> average temperature at mitigation time, min and max, timings, etc ...
> 
> 

I see, it makes sense. Let's see if Todd and Android folks don't
use this thermal sysfs stats, so we could remove them.