[PATCH v4 0/2] arm64/cpufreq: report and track frequencies above 4.19 GHz

[Oleg Keri]

The Snapdragon X2 Elite (Glymur) is the first arm64 laptop part I have
seen whose boost OPP, 4723200 kHz, sits above 4194304 kHz.  Two
independent problems become visible there, both of which make the
kernel believe a boosted CPU is running slower than it is.

Patch 1 fixes an overflow in arch_freq_get_on_cpu(): the u64 product of
the frequency scale and the reference frequency is truncated to
unsigned int before being shifted back down, which wraps for any
reference frequency above 2^32 / SCHED_CAPACITY_SCALE = 4194304 kHz.

Patch 2 makes capacity_freq_ref include the boost frequencies.  For
frequency table drivers it is latched from policy->cpuinfo.max_freq on
CPUFREQ_CREATE_POLICY, and that value excludes the boost entries while
boost is off, so a machine that boots with boost disabled keeps the
sustained maximum as its reference forever.  On arm64 that saturates the
AMU frequency scale at SCHED_CAPACITY_SCALE once boost is enabled, so
the scheduler cannot distinguish a boosted CPU from one at the sustained
maximum, and arch_freq_get_on_cpu() cannot report above the reference.
With the boost maximum as the reference, a disabled boost is expressed
as cpufreq pressure instead, which is what CPPC based systems already
get from highest_perf.

The order matters: patch 2 puts the reference above 4194304 kHz on this
machine from boot, so patch 1 has to land with or before it.

Measured on a Lenovo Yoga Slim 7x Gen 11 (Glymur, scmi-cpufreq,
4032000 kHz sustained, 4723200 kHz boost, boost off at boot) with both
patches, pinning a big-core policy to a single OPP and sampling
cpuinfo_avg_freq under a single-threaded load:

  requested OPP    boost    cpuinfo_avg_freq
  --------------------------------------------
  4032000 kHz      off      4031325   (0.02% low)
  4723200 kHz      on       4709362   (0.29% low)

The reference of the big cores moves from 4032000 to 4723200 kHz, so
the little cores' cpu_capacity goes from 647 to 553 (647 * 4032000 /
4723200) while the big cores stay at 1024.  Without patch 1 the second
row wraps; v1 of this series measured 524283 kHz in that state.

Note that cpuinfo_cur_freq still reports 4032000 kHz at the boost OPP
on this machine.  That is a separate path -- scmi_dvfs_freq_get()
asking firmware for the current performance level -- with no clamp in
the kernel, and it is not addressed here.

Changes in v4:
- Patch 1: use cap_scale() and move its definition from
  kernel/sched/sched.h to include/linux/topology.h, as Dietmar
  proposed on v3.  Same semantics, one open-coded shift less.  Beata's
  Ack on v3 is not carried over since the diff changed.  The scheduler
  maintainers are on Cc for the header move.
- Patch 2: reworked after Dietmar's review of v3.  Instead of updating
  capacity_freq_ref from cpufreq when the boost state changes, take the
  highest frequency table entry, boost included, as the reference when
  the policy is created, and let cpufreq pressure express a disabled
  boost.  The cpufreq core hook and the arch_update_freq_ref() plumbing
  in the arm, arm64 and riscv headers are gone; the patch now touches
  drivers/base/arch_topology.c only.
- Link to v3: https://lore.kernel.org/all/20260910063440.4677-1-okerixx@xxxxxxxxx/

Changes in v3:
- Patch 1: drop the Suggested-by trailer.  It was not warranted for a
  review comment on the shape of an existing patch; my mistake.
- No code changes.
- Link to v2: https://lore.kernel.org/all/20260909192351.33910-1-okerixx@xxxxxxxxx/

Changes in v2:
- Patch 1: fold the multiply and the shift into a single expression, as
  suggested on v1, instead of reusing the u64 scale variable as scratch.
- Patch 2: unchanged.
- Link to v1: https://lore.kernel.org/all/cover.1788712186.git.okerixx@xxxxxxxxx/

Oleg Keri (2):
  arm64: topology: fix arch_freq_get_on_cpu() overflow above 4.19 GHz
  arch_topology: use the boost frequencies for capacity_freq_ref

 arch/arm64/kernel/topology.c |  5 +----
 drivers/base/arch_topology.c | 16 +++++++++++++++-
 include/linux/topology.h     |  2 ++
 kernel/sched/sched.h         |  2 --
 4 files changed, 18 insertions(+), 7 deletions(-)

-- 
2.55.0

base-commit: a7728f5e1fc3d472a314acdabca6039f71ec3a9d