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

From: Andrew Lunn <andrew@lunn.ch>
To: "Marek Behún" <kabel@kernel.org>
Cc: "Viresh Kumar" <viresh.kumar@linaro.org>,
	"Gregory CLEMENT" <gregory.clement@bootlin.com>,
	"Robert Marko" <robert.marko@sartura.hr>,
	"Pali Rohár" <pali@kernel.org>,
	"Tomasz Maciej Nowak" <tmn505@gmail.com>,
	"Anders Trier Olesen" <anders.trier.olesen@gmail.com>,
	"Philip Soares" <philips@netisense.com>,
	linux-pm@vger.kernel.org,
	"Sebastian Hesselbarth" <sebastian.hesselbarth@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	"Konstantin Porotchkin" <kostap@marvell.com>,
	nnet <nnet@fastmail.fm>, "Nadav Haklai" <nadavh@marvell.com>,
	"Marek Behún" <kabel@kernel.iorg>
Subject: Re: [PATCH] cpufreq: armada-37xx: forbid cpufreq for 1.2 GHz variant
Date: Wed, 30 Jun 2021 17:22:27 +0200	[thread overview]
Message-ID: <YNyMM20/nO3kQDVY@lunn.ch> (raw)
In-Reply-To: <20210630135942.29730-1-kabel@kernel.org>

On Wed, Jun 30, 2021 at 03:59:42PM +0200, 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@kernel.iorg>
> 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.
> ---
>  drivers/cpufreq/armada-37xx-cpufreq.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/cpufreq/armada-37xx-cpufreq.c b/drivers/cpufreq/armada-37xx-cpufreq.c
> index 3fc98a3ffd91..1ef3dde9a40b 100644
> --- a/drivers/cpufreq/armada-37xx-cpufreq.c
> +++ b/drivers/cpufreq/armada-37xx-cpufreq.c
> @@ -104,7 +104,13 @@ struct armada_37xx_dvfs {
>  };
>  
>  static struct armada_37xx_dvfs armada_37xx_dvfs[] = {
> +#if 0
> +	/*
> +	 * 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} },
> +#endif

I suspect you will get some drive by patches from bot handlers
removing the #if 0, since such code is not liked within the kernel,
and bot handlers blindly do what the bot tells them.

So i would suggest you avoid #if 0, and move the .cpu_req_max entry
into the comment.

     Andrew