linux-kernel - Re: [PATCH 1/2] clk: mvebu: armada-37xx-periph: Fix switching CPU rate from 300Mhz to 1.2GHz

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Date:   Fri, 06 Jul 2018 16:44:44 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Gregory CLEMENT <gregory.clement@...tlin.com>,
        Mike Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>
Cc:     linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Jason Cooper <jason@...edaemon.net>,
        Hua Jing <jinghua@...vell.com>,
        Antoine Tenart <antoine.tenart@...tlin.com>,
        stable@...r.kernel.org,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Nadav Haklai <nadavh@...vell.com>, Ken Ma <make@...vell.com>,
        Victor Gu <xigu@...vell.com>,
        Neta Zur Hershkovits <neta@...vell.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Wilson Ding <dingwei@...vell.com>,
        linux-arm-kernel@...ts.infradead.org,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCH 1/2] clk: mvebu: armada-37xx-periph: Fix switching CPU rate from
 300Mhz to 1.2GHz

Quoting Gregory CLEMENT (2018-06-29 07:44:02)
> Hi,
>  
>  On mar., juin 19 2018, Gregory CLEMENT <gregory.clement@...tlin.com> wrote:
> 
> > Switching the CPU from the L2 or L3 frequencies (300 and 200 Mhz
> > respectively) to L0 frequency (1.2 Ghz) requires a significant amount
> > of time to let VDD stabilize to the appropriate voltage. This amount of
> > time is large enough that it cannot be covered by the hardware
> > countdown register. Due to this, the CPU might start operating at L0
> > before the voltage is stabilized, leading to CPU stalls.
> >
> > To work around this problem, we prevent switching directly from the
> > L2/L3 frequencies to the L0 frequency, and instead switch to the L1
> > frequency in-between. The sequence therefore becomes:
> >
> > 1. First switch from L2/L3(200/300MHz) to L1(600MHZ)
> > 2. Sleep 20ms for stabling VDD voltage
> > 3. Then switch from L1(600MHZ) to L0(1200Mhz).
> 
> Do you have any comment on this fix?
> 

Looks good. Is it crashing right now? I can throw it into clk-fixes if
it's fixing pain.