linux-kernel - Re: [PATCH v2 1/1] arm64: dts: rockchip: rk3528: Add CPU frequency scaling support

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Message-ID: <20250730133343.GC26787@1wt.eu>
Date: Wed, 30 Jul 2025 15:33:43 +0200
From: Willy Tarreau <w@....eu>
To: Chukun Pan <amadeus@....edu.cn>
Cc: alchark@...il.com, conor+dt@...nel.org, devicetree@...r.kernel.org,
        heiko@...ech.de, jonas@...boo.se, krzk+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, ziyao@...root.org
Subject: Re: [PATCH v2 1/1] arm64: dts: rockchip: rk3528: Add CPU frequency
 scaling support

Hi,

On Wed, Jul 30, 2025 at 09:20:26PM +0800, Chukun Pan wrote:
> Hi,
> 
> > My point is that if you disable cpufreq, the CPU is running at 1.2V, which
> > is even higher. I don't know why it's running at this voltage, maybe as
> > the result of initializing some regulators, but that's what we're getting.
> > So the question about safety of running between 1.13-1.15 resolves to
> > "it's at least safer than running without cpufreq" in the current state.
> >
> > And as I mentioned it's clearly linux and not u-boot that is setting 1.2V,
> > because under u-boot and during kernel selection and image loading, my
> > board is at 0.95V. It's only once the kernel starts to boot that it bumps
> > to 1.2V.
> 
> If opp-table is not configured, kernel will initialize the pwm-regulator
> to maximum microvolts. This can be solved by configuring the pwm-regulator
> in U-Boot (waiting for U-Boot to synchronize the DT of kernel 6.16):
> 
> ```
> &vdd_arm {
> 	regulator-init-microvolt = <953000>;
> };

OK thanks for the explanation, but will be stable at highest frequency, or
do we expect the TF-A or PVTM mechanism to automatically reduce the
frequency enough to keep everything stable ? I'm asking because right now
I'm booting at 2 GHz, and I don't imagine 2 GHz to work fine at 953mV.

Also, if we'd set this to a low enough voltage that it results in throttling
the CPU, it could significantly slow down the boot for no reason. If we
consider that the configured max-microvolt is valid, then better continue
to use it. And if it's not valid, then maybe just fix it without going
full throttle to a lower value either. Just my two cents.

Willy