[PATCH 0/1] cpufreq/amd-pstate: Prevent active mode on systems without EPP support

[Marco Scardovi]

Hi all,

Mario, I've made the actual changes you have requested me on bugzilla for
https://bugzilla.kernel.org/show_bug.cgi?id=221473 . Let me know if it sits
right with you.

Prevent the amd-pstate driver from loading in EPP-reliant "active" mode on
systems without Collaborative Processor Performance Control (CPPC) Energy
Performance Preference (EPP) support (such as Zen 2 and older processors).

Zen 2 and older processors do not support EPP. Loading the driver in active
mode on these systems pins the CPU frequency to the lowest non-linear
frequency (1.7GHz).

To resolve this:
- Unconditionally probe EPP support during driver initialization via a new
  amd_pstate_epp_supported() helper, which queries cppc_get_epp_perf() on
  the first online CPU.
- Lack of ENERGY_PERF support or any error from cppc_get_epp_perf() causes
  EPP to be treated as unsupported.
- Cache EPP support to avoid repeated capability checks during mode switches.
- Fall back to passive mode at boot if EPP is unsupported.
- Reject runtime switches to active mode with -ENODEV.

Please review and provide feedback.

Thanks,
Marco


Marco Scardovi (1):
  cpufreq/amd-pstate: Prevent active mode on systems without EPP support

 drivers/cpufreq/amd-pstate.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

-- 
2.54.0