linux-kernel - Re: [PATCH mvebu v2 00/10] Armada 37xx: Fix cpufreq changing base CPU speed to 800 MHz from 1000 MHz

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Date:   Fri, 19 Feb 2021 20:33:02 +0100
From:   Pali Rohár <pali@...nel.org>
To:     nnet <nnet@...tmail.fm>
Cc:     Marek Behún <kabel@...nel.org>, a.heider@...il.com,
        andrew@...n.ch, gerald@....net, gregory.clement@...tlin.com,
        kostap@...vell.com, linux-arm-kernel@...ts.infradead.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        luka.perkov@...tura.hr, miquel.raynal@...tlin.com,
        mturquette@...libre.com, rmk+kernel@...linux.org.uk,
        sboyd@...nel.org, tmn505@...il.com, vladimir.vid@...tura.hr
Subject: Re: [PATCH mvebu v2 00/10] Armada 37xx: Fix cpufreq changing base
 CPU speed to 800 MHz from 1000 MHz

On Tuesday 16 February 2021 08:27:10 nnet wrote:
> > Therefore I'm thinking if the correct way is instead to use L1 := L0 voltage value for 1/1.2 GHz mode.
> 
> This latest 04/10 works fine for me going 600MHz <-> 1.2GHz under with and without load.

Ok, thanks for testing! Just to note that typical documented value for
1.2GHz mode is 1.155V, so it would be useful to know if this value could
be stable for L1 with 1.2GHz mode.

I'm thinking that for 1GHz variant it would be better to rather use
1.108V like in my original patch as this is already tested by lot of
people, nobody complained yet and it can be lower value as L0 (so there
is benefit to decrease CPU frequency when CPU is idle).

For 1.2GHz variant I still do not know. You wrote that 1.132V is
unstable, so it cannot be used for sure. Documented typical value 1.155V
is bigger, so maybe it can be stable but needs testing. And stable seems
to be L0 value... But then I do not see a benefit for downclocking CPU
from 1.2 GHz frequency in L0 to 600 MHz freq. in L1 if it use same CPU
voltage... But it is still better than unstable CPU with crashes!

Could you test if 1.155V voltage for L1 is stable on 1.2 GHz variant?