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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Linux PM <linux-pm@vger.kernel.org>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Doug Smythies <dsmythies@telus.net>,
        Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Subject: [PATCH v3 3/5] cpufreq: intel_pstate: Tweak the EPP sysfs interface
Date:   Thu, 27 Aug 2020 17:14:09 +0200
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From: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>

Modify the EPP sysfs interface to reject attempts to change the EPP
to values different from 0 ("performance") in the active mode with
the "performance" policy (ie. scaling_governor set to "performance"),
to avoid situations in which the kernel appears to discard data
passed to it via the EPP sysfs attribute.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

v2 -> v3: New patch

---
 Documentation/admin-guide/pm/intel_pstate.rst | 4 +++-
 drivers/cpufreq/intel_pstate.c                | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/pm/intel_pstate.rst b/Documentation/admin-guide/pm/intel_pstate.rst
index cdd1a9a7f9a2..5072e7064d13 100644
--- a/Documentation/admin-guide/pm/intel_pstate.rst
+++ b/Documentation/admin-guide/pm/intel_pstate.rst
@@ -123,7 +123,9 @@ Energy-Performance Bias (EPB) knob (otherwise), which means that the processor's
 internal P-state selection logic is expected to focus entirely on performance.
 
 This will override the EPP/EPB setting coming from the ``sysfs`` interface
-(see `Energy vs Performance Hints`_ below).
+(see `Energy vs Performance Hints`_ below).  Moreover, any attempts to change
+the EPP/EPB to a value different from 0 ("performance") via ``sysfs`` in this
+configuration will be rejected.
 
 Also, in this configuration the range of P-states available to the processor's
 internal P-state selection logic is always restricted to the upper boundary
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index e540448e0bd0..b308c39b6204 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -684,6 +684,14 @@ static int intel_pstate_set_energy_pref_index(struct cpudata *cpu_data,
 		else if (epp == -EINVAL)
 			epp = epp_values[pref_index - 1];
 
+		/*
+		 * To avoid confusion, refuse to set EPP to any values different
+		 * from 0 (performance) if the current policy is "performance",
+		 * because those values would be overridden.
+		 */
+		if (epp > 0 && cpu_data->policy == CPUFREQ_POLICY_PERFORMANCE)
+			return -EBUSY;
+
 		ret = intel_pstate_set_epp(cpu_data, epp);
 	} else {
 		if (epp == -EINVAL)
-- 
2.26.2