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Subject: Re: [PATCH 0/9] MIPS: CI20: Add WiFi / Bluetooth support
From:   Paul Cercueil <paul@crapouillou.net>
To:     "H. Nikolaus Schaller" <hns@goldelico.com>
Cc:     list@opendingux.net, Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
        Conor Dooley <conor+dt@kernel.org>, linux-mips@vger.kernel.org,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Paul Boddie <paul@boddie.org.uk>,
        Paul Burton <paulburton@kernel.org>,
        Christophe Branchereau <cbranchereau@gmail.com>
Date:   Sun, 18 Jun 2023 15:58:01 +0200
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Hi All,

[...]

> Looking at the JZ4780_DS.PDF file, the SoC actually wants 1.1V so the
> DT is not wrong - in theory. But in practice it does not work, as you
> experienced yourself. However, if the ACT8600 defaults to 1.2V, or if
> the bootloader configures it to 1.2V, I would think that this is
> actually a voltage that the SoC can handle - otherwise the SoC would
> be
> overvolted until the kernel starts, and the board design would be
> flawed.
>=20
> I measured that the old 3.x kernel keeps the SoC voltage at 1.2V, so
> it
> sounds like a better default. Therefore the fix here would be to
> raise
> the DCDC1 regulator to 1.2V.
>=20
> I'll send a patch later today.

After a talk with Christophe (Cc'd), I changed my mind.

A +100 mV overvolt is a *huge* step up, and although the SoC doesn't
burst into flames, it could very well reduce its life span.

I used to have huge stability issues (kernel not booting to userspace
half the times, or just plain reboots after a few minutes of uptime)
and I now realize it's because I was running the core at 1.1V, because
these issues disappeared the moment I switched to 1.2V.

However, I am now running at 1.125 volts, which is just 25mV above the
nominal voltage - and it's been extremely stable so far.

Nikolaus: could you test at 1.125 volts? If it's stable for you as
well, I'd suggest to use this as the new default.

Paul (Burton): As you wrote most of the drivers (and uboot?) for the
board, do you know why VDDCORE was set to 1.2V?

Cheers,
-Paul