linux-kernel - Re: [PATCH V3] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

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Message-ID: <ZnJfmUXmU_tsb9pV@kf-XE>
Date: Tue, 18 Jun 2024 23:33:29 -0500
From: Aaron Rainbolt <arainbolt@...cus.org>
To: Mario Limonciello <mario.limonciello@....com>,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: rafael@...nel.org, lenb@...nel.org, mmikowski@...cus.org,
	Perry.Yuan@....com
Subject: Re: [PATCH V3] acpi: Allow ignoring _OSC CPPC v2 bit via kernel
 parameter

acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

The _OSC is supposed to contain a bit indicating whether the hardware
supports CPPC v2 or not. This bit is not always set, causing CPPC v2 to
be considered absent. This results in severe single-core performance
issues with the EEVDF scheduler on heterogenous-core Intel processors.

To work around this, provide a new kernel parameter, "ignore_osc_cppc_bit",
which may be used to ignore the _OSC CPPC v2 bit and act as if the bit was
enabled. This allows CPPC to be properly detected even if not "enabled" by
_OSC, allowing users with problematic hardware to obtain decent single-core
performance.

Tested-by: Michael Mikowski <mmikowski@...cus.org>
Signed-off-by: Aaron Rainbolt <arainbolt@...cus.org>

---

V2 -> V3: Move bit ignore to before switch.
V1 -> V2: Rewrite to work in cpc_supported_by_cpu.

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b600df82669d..af2d8973ba3a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2063,6 +2063,12 @@
                        could change it dynamically, usually by
                        /sys/module/printk/parameters/ignore_loglevel.

+       ignore_osc_cppc_bit
+                       Assume CPPC is present and ignore the CPPC v2 bit from
+                       the ACPI _OSC method. This is useful for working
+                       around buggy firmware where CPPC is supported, but
+                       _OSC incorrectly reports it as being absent.
+
        ignore_rlimit_data
                        Ignore RLIMIT_DATA setting for data mappings,
                        print warning at first misuse.  Can be changed via
diff --git a/arch/x86/kernel/acpi/cppc.c b/arch/x86/kernel/acpi/cppc.c
index ff8f25faca3d..0ca1eac826af 100644
--- a/arch/x86/kernel/acpi/cppc.c
+++ b/arch/x86/kernel/acpi/cppc.c
@@ -11,8 +11,20 @@

 /* Refer to drivers/acpi/cppc_acpi.c for the description of functions */

+static bool ignore_osc_cppc_bit;
+static int __init parse_ignore_osc_cppc_bit(char *arg)
+{
+       ignore_osc_cppc_bit = true;
+       return 0;
+}
+early_param("ignore_osc_cppc_bit", parse_ignore_osc_cppc_bit);
+
 bool cpc_supported_by_cpu(void)
 {
+       if (ignore_osc_cppc_bit) {
+               return true;
+       }
+
        switch (boot_cpu_data.x86_vendor) {
        case X86_VENDOR_AMD:
        case X86_VENDOR_HYGON: