linux-kernel - Re: [PATCH v3 30/30] ARM: sun8i: a83t: full range OPP tables and CPUfreq

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Date:   Thu, 6 Sep 2018 13:42:41 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Philipp Rossak <embed3d@...il.com>
Cc:     Quentin Schulz <quentin.schulz@...tlin.com>, lee.jones@...aro.org,
        robh+dt@...nel.org, mark.rutland@....com, wens@...e.org,
        linux@...linux.org.uk, jic23@...nel.org, knaack.h@....de,
        lars@...afoo.de, pmeerw@...erw.net, eugen.hristev@...rochip.com,
        rdunlap@...radead.org, vilhelm.gray@...il.com,
        clabbe.montjoie@...il.com, geert+renesas@...der.be,
        lukas@...ner.de, icenowy@...c.io, arnd@...db.de,
        broonie@...nel.org, arnaud.pouliquen@...com,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com
Subject: Re: [PATCH v3 30/30] ARM: sun8i: a83t: full range OPP tables and
 CPUfreq

On Thu, Sep 06, 2018 at 01:39:43PM +0200, Philipp Rossak wrote:
> On 06.09.2018 09:24, Quentin Schulz wrote:
> > Hi Philipp,
> > 
> > On Thu, Aug 30, 2018 at 05:45:18PM +0200, Philipp Rossak wrote:
> > > Since we have now thermal trotteling enabeled we can now add the full
> > > range of the OPP table.
> > > 
> > That's not the reason why they were not added.
> > 
> > Please see commit 2db639d8c1663d7543c9ab5323383d94c8a76c63[1].
> > 
> > Basically, you only want the OPPs which can work below or at the default
> > voltage of the CPU supply, because the CPU supply is specific to each
> > board.
> > 
> > If you set your CPU to work at a given frequency and the voltage isn't
> > updated (saying opp-microvolt = <x>; in DT isn't enough, you need
> > cpu-supply to be provided and functional), the CPU might just crash.
> > 
> > Without cpu-supply property, underclocking isn't effective in term of
> > thermal cooling or power saving. Overclocking is very, very, very likely
> > to make the CPU crash.
> > 
> > It's not a very difficult thing to do to test if a given frequency work
> > well but it needs a specific test environment and it's a lengthy test,
> > you can have a look at those tools here[3] if you like. It's not because
> > it works in a given test case that'll work on the long term under heavy
> > load and constant frequency changes.
> > 
> > For A83T, I already did it and the outcome is the patch in [1]. Same for
> > A33.
> > 
> > So, if you want to use these three higher OPPs, you need to define them
> > in your board DTS and add the cpu-supply property. See what's done for
> > the A33 and more specifically the Sinlinx SinA33[2] as an example.
> > 
> > [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2db639d8c1663d7543c9ab5323383d94c8a76c63
> > [2]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/sun8i-a33-sinlinx-sina33.dts
> > [3]http://linux-sunxi.org/Hardware_Reliability_Tests#CPU
> > 
> > Quentin
> > 
> 
> Hey Quentin,
> 
> thanks for your feedback!
> 
> Sounds like we will never be able to run the A83T on its maximum frequency
> in mainline.
> 
> I will do some testing, during the next weeks/months when I have time.
> With the old Allwinner kernel I was able to run the A83T with its maximum
> frequency without any problems since my board is very good cooled.

You definitely can, but I think Quentin's point was to do it on a
per-board basis, not for all of the A83t boards at once.

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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