Re: [PATCH V2 RFC] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

[Aaron Rainbolt <arainbolt@kfocus.org>]

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.

Signed-off-by: Aaron Rainbolt <arainbolt@kfocus.org>

---

V1 -> V2: Rewrite to work in cpc_supported_by_cpu.

RFC: I have not yet tested this patch to ensure it functions properly,
 nor have I attempted to compile it against mainline. My system takes
 a couple of hours or so to build a kernel, and I'd like to submit this
 for feedback now and test once it's sent.

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..7346a25e68ce 100644
--- a/arch/x86/kernel/acpi/cppc.c
+++ b/arch/x86/kernel/acpi/cppc.c
@@ -11,6 +11,14 @@
 
 /* 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)
 {
 	switch (boot_cpu_data.x86_vendor) {
@@ -24,6 +32,10 @@ bool cpc_supported_by_cpu(void)
 			return true;
 		return boot_cpu_has(X86_FEATURE_CPPC);
 	}
+
+	if (ignore_osc_cppc_bit) {
+		return true;
+	}
 	return false;
 }