linux-kernel - Re: [PATCH v3 0/3] thermal: introduce by-name softlink

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Date:   Thu, 12 Dec 2019 08:34:43 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Wei Wang <wvw@...gle.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Wei Wang <wei.vince.wang@...il.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Amit Kucheria <amit.kucheria@...durent.com>,
        Linux PM list <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v3 0/3] thermal: introduce by-name softlink

On Wed, Dec 11, 2019 at 12:11:53PM -0800, Wei Wang wrote:
> On Wed, Dec 11, 2019 at 12:54 AM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > On Tue, Dec 10, 2019 at 09:54:11PM +0100, Daniel Lezcano wrote:
> > > On 10/12/2019 21:01, Wei Wang wrote:
> > > > On Tue, Dec 10, 2019 at 6:36 AM Daniel Lezcano
> > > > <daniel.lezcano@...aro.org> wrote:
> > > >>
> > > >> On 05/12/2019 08:19, Wei Wang wrote:
> > > >>> The paths thermal_zone%d and cooling_device%d are not intuitive and the
> > > >>> numbers are subject to change due to device tree change. This usually
> > > >>> leads to tree traversal in userspace code.
> >
> > tree traversal is supposed to be done in userspace code :)
> >
> Yes, that can be done in userspace, but given the amount of thermal
> zones we have in some mobile devices, this will bring a lot of
> convenience.

Userspace is easier to write than kernel code.  Kernel code and apis you
have to retain for the next 20+ years, are you willing to do that?

> e.g. this is on Pixel 4 XL:
> coral:/ # ls  /sys/devices/virtual/thermal/
> cdev-by-name      cooling_device15  cooling_device22  cooling_device3
>  cooling_device9  thermal_zone15  thermal_zone22  thermal_zone3
> thermal_zone37  thermal_zone44  thermal_zone51  thermal_zone59
> thermal_zone66  thermal_zone73  thermal_zone80  thermal_zone88
> cooling_device0   cooling_device16  cooling_device23  cooling_device30
>  thermal_zone0    thermal_zone16  thermal_zone23  thermal_zone30
> thermal_zone38  thermal_zone45  thermal_zone52  thermal_zone6
> thermal_zone67  thermal_zone74  thermal_zone81  thermal_zone9
> cooling_device1   cooling_device17  cooling_device24  cooling_device31
>  thermal_zone1    thermal_zone17  thermal_zone24  thermal_zone31
> thermal_zone39  thermal_zone46  thermal_zone53  thermal_zone60
> thermal_zone68  thermal_zone75  thermal_zone82  tz-by-name
> cooling_device10  cooling_device18  cooling_device25  cooling_device4
>  thermal_zone10   thermal_zone18  thermal_zone25  thermal_zone32
> thermal_zone4   thermal_zone47  thermal_zone54  thermal_zone61
> thermal_zone69  thermal_zone76  thermal_zone83
> cooling_device11  cooling_device19  cooling_device26  cooling_device5
>  thermal_zone11   thermal_zone19  thermal_zone26  thermal_zone33
> thermal_zone40  thermal_zone48  thermal_zone55  thermal_zone62
> thermal_zone7   thermal_zone77  thermal_zone84
> cooling_device12  cooling_device2   cooling_device27  cooling_device6
>  thermal_zone12   thermal_zone2   thermal_zone27  thermal_zone34
> thermal_zone41  thermal_zone49  thermal_zone56  thermal_zone63
> thermal_zone70  thermal_zone78  thermal_zone85
> cooling_device13  cooling_device20  cooling_device28  cooling_device7
>  thermal_zone13   thermal_zone20  thermal_zone28  thermal_zone35
> thermal_zone42  thermal_zone5   thermal_zone57  thermal_zone64
> thermal_zone71  thermal_zone79  thermal_zone86
> cooling_device14  cooling_device21  cooling_device29  cooling_device8
>  thermal_zone14   thermal_zone21  thermal_zone29  thermal_zone36
> thermal_zone43  thermal_zone50  thermal_zone58  thermal_zone65
> thermal_zone72  thermal_zone8   thermal_zone87

Nice, you have a crazy SoC here, good luck!  :)

> > But what userspace code needs to do this, and for what?
> In Android, thermal daemon and thermal HAL as well as some init.rc
> script would use those thermal paths for managing and monitoring
> thermal. The daemon/HAL could have logic pipled in, however Android's
> init.rc script would be really tricky.

Then put that logic in your userspace code.

> On a related note, we also create /dev/block/by-name links from userspace.

Yes we do, from userspace, not from within the kernel itself, so I think
you have answered the question :)

thanks,

greg k-h