linux-kernel - Re: [PATCH 4/4] arm64: dts: rockchip: Add OPP data for CPU cores on RK3588

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Date: Fri, 26 Jan 2024 13:56:48 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Dragan Simic <dsimic@...jaro.org>, Alexey Charkov <alchark@...il.com>
Cc: Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Heiko Stuebner <heiko@...ech.de>,
 devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 4/4] arm64: dts: rockchip: Add OPP data for CPU cores on
 RK3588

On 26/01/2024 08:49, Dragan Simic wrote:
> On 2024-01-26 08:30, Alexey Charkov wrote:
>> On Fri, Jan 26, 2024 at 11:05 AM Dragan Simic <dsimic@...jaro.org> wrote:
>>> On 2024-01-26 07:44, Alexey Charkov wrote:
>>> > On Fri, Jan 26, 2024 at 10:32 AM Dragan Simic <dsimic@...jaro.org>
>>> > wrote:
>>> >> On 2024-01-25 10:30, Daniel Lezcano wrote:
>>> >> > On 24/01/2024 21:30, Alexey Charkov wrote:
>>> >> >> By default the CPUs on RK3588 start up in a conservative 
>>> performance
>>> >> >> mode. Add frequency and voltage mappings to the device tree to 
>>> enable

[ ... ]

>> Throttling would also lower the voltage at some point, which cools it
>> down much faster!
> 
> Of course, but the key is not to cool (and slow down) the CPU cores too
> much, but just enough to stay within the available thermal envelope,
> which is where the same-voltage, lower-frequency OPPs should shine.

That implies the resulting power is sustainable which I doubt it is the 
case.

The voltage scaling makes the cooling effect efficient not the frequency.

For example:
	opp5 = opp(2GHz, 1V) => 2 BogoWatt
	opp4 = opp(1.9GHz, 1V) => 1.9 BogoWatt
	opp3 = opp(1.8GHz, 0.9V) => 1.458 BogoWatt
	[ other states but we focus on these 3 ]

opp5->opp4 => -5% compute capacity, -5% power, ratio=1
opp4->opp3 => -5% compute capacity, -23.1% power, ratio=21,6

opp5->opp3 => -10% compute capacity, -27.1% power, ratio=36.9

In burst operation (no thermal throttling), opp4 is pointless we agree 
on that.

IMO the following will happen: in burst operation with thermal 
throttling we hit the trip point and then the step wise governor reduces 
opp5 -> opp4. We have slight power reduction but the temperature does 
not decrease, so at the next iteration, it is throttle at opp3. And at 
the end we have opp4 <-> opp3 back and forth instead of opp5 <-> opp3.

It is probable we end up with an equivalent frequency average (or 
compute capacity avg).

opp4 <-> opp3 (longer duration in states, less transitions)
opp5 <-> opp3 (shorter duration in states, more transitions)

Some platforms had their higher OPPs with the same voltage and they 
failed to cool down the CPU in the long run.

Anyway, there is only one way to check it out :)

Alexey, is it possible to compare the compute duration for 'dhrystone' 
with these voltage OPP and without ? (with a period of cool down between 
the test in order to start at the same thermal condition) ?



> When the CPU load isn't bursty but steady and high, we don't race to
> idle, but run a marathon instead, so to speak. :)

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