cppc_set_perf_limited(9)

✨ cppc_set_perf_limited — the kernel API manual, bestie edition ✨

Okay so we're in cppc_set_perf_limited(9) territory, which is a page from the Kernel API Manual, and honestly? It's a little niche but it's giving "quietly important" 💅 Let's get into it.

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🌸 NAME 🌸

cppc_set_perf_limited — clears bits in the Performance Limited register. That's literally the whole job. She clears. She does not set. Respect her lane 🎀

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🦋 SYNOPSIS 🦋

int cppc_set_perf_limited (int cpu , u64 bits_to_clear );

That's the whole call, bestie. A cpu and a bits_to_clear, and it hands you back an int. Clean. Minimal. Iconic.

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💖 ARGUMENTS 💖

• cpu — the CPU on which to write the register. So like, you gotta tell her which CPU you're talking about, she's not a mind reader 😭
• bits_to_clear — a bitmask of the bits you want cleared in the perf_limited register. That's it, that's the vibe 🫶

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🔥 DESCRIPTION 🔥

So the Performance Limited register has two sticky bits that the platform sets. Not you. The platform. And they stay set until OSPM explicitly clears them — which is where this function comes in.

Here's the two of them, no cap:

• Bit 0 (Desired_Excursion) — set when delivered performance is constrained below desired performance. ⚠️ Not used when Autonomous Selection is enabled. Just so you know.
• Bit 1 (Minimum_Excursion) — set when delivered performance is constrained below minimum performance.

These bits are sticky and remain set until OSPM explicitly clears them. And this function only allows clearing bits — the platform is the one who sets them. You do not get to set them. That's not your job. Periodt 💅

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⭐ RETURN ⭐

Here's the scorecard:

• 0 for success — slay ✨
• -EINVAL for invalid bits — you passed something weird, bestie 🙈
• -EIO on register access failure — the register didn't pick up, tragic 💀
• -EOPNOTSUPP if not supported — she's just not available rn 😭

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👑 SEE ALSO 👑

Kernel file ./drivers/acpi/cppc_acpi.c, plus the whole friend group:

acpi_get_psd_map(9), cpc_ffh_supported(9), cpc_supported_by_cpu(9), pcc_data_alloc(9), acpi_cppc_processor_probe(9), acpi_cppc_processor_exit(9), cpc_read_ffh(9), cpc_read_ffh_fb_ctrs(9), cpc_write_ffh(9), cppc_get_desired_perf(9), cppc_get_nominal_perf(9), cppc_get_highest_perf(9), cppc_get_epp_perf(9), cppc_get_perf_caps(9), cppc_perf_ctrs_in_pcc_cpu(9), cppc_perf_ctrs_in_pcc(9), cppc_get_perf_ctrs(9), cppc_set_epp(9), cppc_get_auto_act_window(9), cppc_set_auto_act_window(9), cppc_get_auto_sel(9), cppc_set_auto_sel(9), cppc_set_enable(9), cppc_get_perf(9), cppc_set_perf(9), cppc_get_perf_limited(9), cppc_get_transition_latency(9) 🧚

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Fr fr, if you're poking at sticky platform bits, read the register docs first. She bites. 💀

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Okay and this whole masterpiece lives in ./drivers/acpi, dated October 2026 — yeah, from the future, main character energy only 🌈⭐ Thanks for reading my little manual page TED talk, mwah 💜🎀