[Bug 220715] [REGRESSION] AMD P-State fails with "_CPC object is not present" on AMD Zen 4 between 6.10 and 6.11

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=220715

--- Comment #8 from Chris Harris (chris.harris79@gmail.com) ---
Thank you for the continued engagement

1) Good suggestion to test removal of 'nosmt' in the presence of
'amd_pstate=passive' on the problem kernel versions, results:
In the absence of 'nosmt=force' *I am* successfully getting the "amd-pstate"
scaling driver.  In the presence of 'nosmt=force' I fail to get the
"amd-pstate" driver, and instead receive "acpi-cpufreq", along with the error
"amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled".  This
behavior of 'amd_pstate=passive' failing in the presence of 'nosmt' did not
occur on the kernels prior to the commit identified. In my view, it does seem
to be a bug, as one should be able to achieve both 'amd_pstate=passive' and
'nosmt' (as I have been on kernels prior to the affected kernels).

2) I can confirm the Ryzen 7950X is affected.  I primarily tested against
4484PX on seven identical servers, all which behave similarly -- the logs
shared are from one of these servers.  I did a sanity-check on a single 7950X
server, which as I said was also similarly affected, but I did not share logs
from that server.

3) LLM: I'll test this patch shortly. Based on the findings from question 1, 
this makes perfect sense - the issue is specifically the interaction between 
the commit and nosmt=force. Your patch changes the loop to check only enabled 
CPUs rather than all present CPUs, which should skip the disabled SMT threads 
that don't have CPC structures initialized.

I'll apply this to a current kernel (6.11+) and test with both:
- amd_pstate=passive + nosmt=force (currently broken)
- amd_pstate=passive without nosmt (currently works)

Will report back with results tomorrow.

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