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

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Date:   Thu, 2 Jun 2022 12:02:53 -0700
From:   Todd Kjos <tkjos@...gle.com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        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>
Subject: Re: [PATCH 3/3] thermal/sysfs: Remove cooling device sysfs statistics

On Thu, Jun 2, 2022 at 2:16 AM Lukasz Luba <lukasz.luba@....com> wrote:
>
>
>
> 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.

Android HALs do use the thermal sysfs stats. debugfs isn't a viable
replacement since debugfs must not be mounted during normal operation.