[PATCH v2] cpufreq: armada-37xx: forbid cpufreq for 1.2 GHz variant

[Pali Rohár]

Konstantin, Nadav, Ken, Victor, Jason: This issue is pretty serious,
CPU on 1.2GHz A3720 is crashing. Could you please look at it?

On Friday 02 July 2021 18:30:35 Pali Rohár wrote:
> +Jason from GlobalScale as this issue affects GlobalScale Espressobin Ultra and V7 1.2 GHz boards.
> 
> On Thursday 01 July 2021 00:56:01 Marek Behún wrote:
> > The 1.2 GHz variant of the Armada 3720 SOC is unstable with DVFS: when
> > the SOC boots, the WTMI firmware sets clocks and AVS values that work
> > correctly with 1.2 GHz CPU frequency, but random crashes occur once
> > cpufreq driver starts scaling.
> > 
> > We do not know currently what is the reason:
> > - it may be that the voltage value for L0 for 1.2 GHz variant provided
> >   by the vendor in the OTP is simply incorrect when scaling is used,
> > - it may be that some delay is needed somewhere,
> > - it may be something else.
> > 
> > The most sane solution now seems to be to simply forbid the cpufreq
> > driver on 1.2 GHz variant.
> > 
> > Signed-off-by: Marek Behún <kabel at kernel.org>
> > Fixes: 92ce45fb875d ("cpufreq: Add DVFS support for Armada 37xx")
> > ---
> > If someone from Marvell could look into this, it would be great since
> > basically 1.2 GHz variant cannot scale, which is a feature that was
> > claimed to be supported by the SOC.
> > 
> > Ken Ma / Victor Gu, you have worked on commit
> > https://github.com/MarvellEmbeddedProcessors/linux-marvell/commit/d6719fdc2b3cac58064f41b531f86993c919aa9a
> > in linux-marvell.
> > Your patch takes away the 1202 mV constant for 1.2 GHz base CPU
> > frequency and instead adds code that computes the voltages from the
> > voltage found in L0 AVS register (which is filled in by WTMI firmware).
> > 
> > Do you know why the code does not work correctly for some 1.2 GHz
> > boards? Do we need to force the L0 voltage to 1202 mV if it is lower,
> > or something?
> > ---
> >  drivers/cpufreq/armada-37xx-cpufreq.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cpufreq/armada-37xx-cpufreq.c b/drivers/cpufreq/armada-37xx-cpufreq.c
> > index 3fc98a3ffd91..c10fc33b29b1 100644
> > --- a/drivers/cpufreq/armada-37xx-cpufreq.c
> > +++ b/drivers/cpufreq/armada-37xx-cpufreq.c
> > @@ -104,7 +104,11 @@ struct armada_37xx_dvfs {
> >  };
> >  
> >  static struct armada_37xx_dvfs armada_37xx_dvfs[] = {
> > -	{.cpu_freq_max = 1200*1000*1000, .divider = {1, 2, 4, 6} },
> > +	/*
> > +	 * The cpufreq scaling for 1.2 GHz variant of the SOC is currently
> > +	 * unstable because we do not know how to configure it properly.
> > +	 */
> > +	/* {.cpu_freq_max = 1200*1000*1000, .divider = {1, 2, 4, 6} }, */
> >  	{.cpu_freq_max = 1000*1000*1000, .divider = {1, 2, 4, 5} },
> >  	{.cpu_freq_max = 800*1000*1000,  .divider = {1, 2, 3, 4} },
> >  	{.cpu_freq_max = 600*1000*1000,  .divider = {2, 4, 5, 6} },
> > -- 
> > 2.31.1
> >