Commit a755d0e2d4 ("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. Also
update the related Rust binding.
Fixes: a755d0e2d4 ("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@kernel.org>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Reviewed-by: Jie Zhan <zhanjie9@hisilicon.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: 6.6+ <stable@vger.kernel.org> # 6.6+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2264949.irdbgypaU6@rafael.j.wysocki
[ rjw: Fix typo in new symbol name, drop redundant type cast from Rust binding ]
Tested-by: Shawn Guo <shawnguo@kernel.org> # with cpufreq-dt driver
Reviewed-by: Qais Yousef <qyousef@layalina.io>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
f97aef092e