CPU max boost clamped to ~2.4 GHz after resume from sleep until reboot (SteamOS 3.8.14, Steam Deck LCD)

System

• Steam Deck LCD, BIOS F7A0133
• SteamOS 3.8.14 (build 20260703.1), kernel 6.16.12-valve24.4-1-neptune-616
• amd-pstate-epp, active mode (default)

Symptom

After every resume from S3 sleep ("deep"), the whole system feels laggy until reboot.
Cause: CPU max boost is clamped. A pinned single-core busy loop
(taskset -c 2 sh -c 'while :; do :; done') reaches 3.5 GHz before sleep but only
~2.42 GHz after resume — reproducible on every sleep cycle, including
rtcwake -m mem -s 45 with the machine otherwise idle.

Not thermal or power related: 46 °C, plugged in and charging, GPU idle.

Software state after resume looks normal

• scaling_max_freq = 3501250, boost = 1
• governor powersave, EPP balance_performance on all 8 CPUs
  (i.e. #2383 does NOT reproduce here — the EPP fix from
  torvalds/linux@85d7dda5a9f6 appears present and working)
• ACPI CPPC still advertises highest_perf = 255, nominal_perf = 204

Things that do NOT restore boost (only reboot does)

• toggling /sys/devices/system/cpu/cpufreq/boost 0 → 1
• writing EPP performance to all CPUs
• performance governor
• re-writing scaling_max_freq
• amd_pstate mode switches, including a full disable → active driver reset
  (passive mode then caps at 2.8 GHz nominal, so the clamp appears to live in
  SMU/firmware state rather than driver sysfs state)

A second suspend/resume cycle occasionally restores full boost, but it is not reliable.

Related (but distinct symptom, already fixed): #2383

Start by reproducing the issue with the pinned busy loop and rtcwake -m mem -s 45, comparing boost before and after resume. Inspect the amd-pstate and suspend/resume paths associated with the reported kernel version; no source file or test is named in the issue. Done means identifying and correcting the post-resume boost clamp without relying on a reboot, with the resume cycle reliably reaching the pre-sleep boost frequency.