intel_idle: introduce 'no_native' module parameter

Since commit 1873495 ("intel_idle: Use ACPI _CST for processor models
without C-state tables") the intel_idle driver has had the ability to use
the ACPI _CST to populate C-states when the processor model is not
recognized.

However, even when the processor model is recognized (native mode) there
are cases where it is useful to make the driver ignore the per-CPU idle
states in lieu of ACPI C-states (such as specific application performance).

Add a new 'no_native' module parameter to provide this functionality.

Signed-off-by: David Arcari <darcari@redhat.com>
Link: https://patch.msgid.link/20250220151120.1131122-1-darcari@redhat.com
Reviewed-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
[ rjw: Spell CPU in capitals ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: darcari

Documentation/admin-guide/pm/intel_idle.rst

@@ -192,11 +192,19 @@ even if they have been enumerated (see :ref:`cpu-pm-qos` in
 Documentation/admin-guide/pm/cpuidle.rst).
 Setting ``max_cstate`` to 0 causes the ``intel_idle`` initialization to fail.
 
-The ``no_acpi`` and ``use_acpi`` module parameters (recognized by ``intel_idle``
-if the kernel has been configured with ACPI support) can be set to make the
-driver ignore the system's ACPI tables entirely or use them for all of the
-recognized processor models, respectively (they both are unset by default and
-``use_acpi`` has no effect if ``no_acpi`` is set).
+The ``no_acpi``, ``use_acpi`` and ``no_native`` module parameters are
+recognized by ``intel_idle`` if the kernel has been configured with ACPI
+support.  In the case that ACPI is not configured these flags have no impact
+on functionality.
+
+``no_acpi`` - Do not use ACPI at all.  Only native mode is available, no
+ACPI mode.
+
+``use_acpi`` - No-op in ACPI mode, the driver will consult ACPI tables for
+C-states on/off status in native mode.
+
+``no_native`` - Work only in ACPI mode, no native mode available (ignore
+all custom tables).
 
 The value of the ``states_off`` module parameter (0 by default) represents a
 list of idle states to be disabled by default in the form of a bitmask.

drivers/idle/intel_idle.c

@@ -1692,6 +1692,10 @@ static bool force_use_acpi __read_mostly; /* No effect if no_acpi is set. */
 module_param_named(use_acpi, force_use_acpi, bool, 0444);
 MODULE_PARM_DESC(use_acpi, "Use ACPI _CST for building the idle states list");
 
+static bool no_native __read_mostly; /* No effect if no_acpi is set. */
+module_param_named(no_native, no_native, bool, 0444);
+MODULE_PARM_DESC(no_native, "Ignore cpu specific (native) idle states in lieu of ACPI idle states");
+
 static struct acpi_processor_power acpi_state_table __initdata;
 
 /**
@@ -1831,6 +1835,11 @@ static bool __init intel_idle_off_by_default(unsigned int flags, u32 mwait_hint)
 	}
 	return true;
 }
+
+static inline bool ignore_native(void)
+{
+	return no_native && !no_acpi;
+}
 #else /* !CONFIG_ACPI_PROCESSOR_CSTATE */
 #define force_use_acpi	(false)
 
@@ -1840,6 +1849,7 @@ static inline bool intel_idle_off_by_default(unsigned int flags, u32 mwait_hint)
 {
 	return false;
 }
+static inline bool ignore_native(void) { return false; }
 #endif /* !CONFIG_ACPI_PROCESSOR_CSTATE */
 
 /**
@@ -2333,6 +2343,10 @@ static int __init intel_idle_init(void)
 	pr_debug("MWAIT substates: 0x%x\n", mwait_substates);
 
 	icpu = (const struct idle_cpu *)id->driver_data;
+	if (icpu && ignore_native()) {
+		pr_debug("ignoring native CPU idle states\n");
+		icpu = NULL;
+	}
 	if (icpu) {
 		if (icpu->state_table)
 			cpuidle_state_table = icpu->state_table;