linux-kernel - Re: [PATCH v2] ARM64: dts: meson-gxbb-odroidc2: Disable SCPI DVFS

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Date:   Thu, 12 Jan 2017 18:40:51 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Neil Armstrong <narmstrong@...libre.com>, brian.kim@...dkernel.com
Cc:     khilman@...libre.com, carlo@...one.org,
        linux-amlogic@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        moon.linux@...dkernel.com
Subject: Re: [PATCH v2] ARM64: dts: meson-gxbb-odroidc2: Disable SCPI DVFS

Hi Neil,

(adding Brian Kim, one of the Hardkernel developers to this conversation)

On Fri, Jan 6, 2017 at 9:04 AM, Neil Armstrong <narmstrong@...libre.com> wrote:
> The current hardware is not able to run with all cores enabled at a
> cluster frequency superior at 1536MHz.
> But the currently shipped u-boot for the platform still reports an OPP
> table with possible DVFS frequency up to 2GHz, and will not change since
> the off-tree linux tree supports limiting the OPPs with a kernel parameter.
> A recent u-boot change reports the boot-time DVFS around 100MHz and
> the default performance cpufreq governor sets the maximum frequency.
> Previous version of u-boot reported to be already at the max OPP and
> left the OPP as is.
> Nevertheless, other governors like ondemand could setup the max frequency
> and make the system crash.
>
> This patch disables the DVFS clock and disables cpufreq.
I don't have any Odroid-C2 board, but having to live without cpufreq
sounds bad for the Odroid-C2 users.
What would we expect from a kernel perspective (maybe the Hardkernel
guys would adjust their u-boot instead of us adjusting to the behavior
of one specific device? one solution that I could think of involves
the "maxcpus" kernel parameter (see [0]), if this is not set u-boot
should report a max CPU frequency of 1536MHz (= max frequency for 4
active cores). Based on the "maxcpus" value additional frequencies can
be unlocked (this could be step-by-step if there are different
frequencies for one core/two cores/etc.). However, I'd like to hear
other opinions as well.


Regards,
Martin


[0] http://lxr.free-electrons.com/source/Documentation/kernel-parameters.txt?v=4.8#L2163