linux-kernel - Re: [PATCH v1 1/4] cpufreq: Make drivers using CPUFREQ

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Message-ID: <4f5e0e2e-7754-4d29-9eba-c671f084fc45@hisilicon.com>
Date: Fri, 26 Sep 2025 17:46:25 +0800
From: Jie Zhan <zhanjie9@...ilicon.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>, Linux PM
	<linux-pm@...r.kernel.org>
CC: Shawn Guo <shawnguo@...nel.org>, Qais Yousef <qyousef@...alina.io>, LKML
	<linux-kernel@...r.kernel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
	Prashanth Prakash <pprakash@...eaurora.org>, Pierre Gondois
	<pierre.gondois@....com>, Mario Limonciello <mario.limonciello@....com>,
	Linux ACPI <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v1 1/4] cpufreq: Make drivers using CPUFREQ_ETERNAL
 specify transition latency



On 9/25/2025 11:44 PM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Commit a755d0e2d41b ("cpufreq: Honour transition_latency over
> transition_delay_us") caused platforms where cpuinfo.transition_latency
> is CPUFREQ_ETERNAL to get a very large transition latency whereas
> previously it had been capped at 10 ms (and later at 2 ms).
> 
> This led to a user-observable regression between 6.6 and 6.12 as
> described by Shawn:
> 
> "The dbs sampling_rate was 10000 us on 6.6 and suddently becomes
>  6442450 us (4294967295 / 1000 * 1.5) on 6.12 for these platforms
>  because the default transition delay was dropped [...].
> 
>  It slows down dbs governor's reacting to CPU loading change
>  dramatically.  Also, as transition_delay_us is used by schedutil
>  governor as rate_limit_us, it shows a negative impact on device
>  idle power consumption, because the device gets slightly less time
>  in the lowest OPP."
> 
> Evidently, the expectation of the drivers using CPUFREQ_ETERNAL as
> cpuinfo.transition_latency was that it would be capped by the core,
> but they may as well return a default transition latency value instead
> of CPUFREQ_ETERNAL and the core need not do anything with it.
> 
> Accordingly, introduce CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS and make
> all of the drivers in question use it instead of CPUFREQ_ETERNAL.
> 
> Fixes: a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us")
> Closes: https://lore.kernel.org/linux-pm/20250922125929.453444-1-shawnguo2@yeah.net/
> Reported-by: Shawn Guo <shawnguo@...nel.org>
> Cc: 6.6+ <stable@...r.kernel.org> # 6.6+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Thanks, we've seen similar issues.

Reviewed-by: Jie Zhan <zhanjie9@...ilicon.com>
> ---
>  drivers/cpufreq/cpufreq-dt.c          |    2 +-
>  drivers/cpufreq/imx6q-cpufreq.c       |    2 +-
>  drivers/cpufreq/mediatek-cpufreq-hw.c |    2 +-
>  drivers/cpufreq/scmi-cpufreq.c        |    2 +-
>  drivers/cpufreq/scpi-cpufreq.c        |    2 +-
>  drivers/cpufreq/spear-cpufreq.c       |    2 +-
>  include/linux/cpufreq.h               |    3 +++
>  7 files changed, 9 insertions(+), 6 deletions(-)