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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     Linux ACPI <linux-acpi@vger.kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Zhang Rui <rui.zhang@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Len Brown <lenb@kernel.org>,
        Mario Limonciello <mario.limonciello@amd.com>
Subject: [PATCH] ACPI: PM: x86: Print messages regarding LPS0 idle support
Date:   Thu, 21 Jul 2022 18:13:30 +0200
Message-ID: <12039470.O9o76ZdvQC@kreacher>
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From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Because suspend-to-idle is always supported and on x86 it is the only
way to suspend the system if S3 is not supported by the platform, the
kernel attempts to enter low-power S0 idle in the suspend-to-idle flow
regardless of whether or not the ACPI_FADT_LOW_POWER_S0 flag is set in
the FADT.  However, if that flag is not set, residency counters
associated with low-power S0 idle may not count and the platform may
refuse to put the EC into a low-power mode, for example.

For this reason, print diagnostic messages when the platform should
achieve significant energy savings in low-power S0 idle (because the
ACPI_FADT_LOW_POWER_S0 flag is set in the FADT) and when
suspend-to-idle becomes the default suspend method (because low-power
S0 idle should be equally or more efficient than S3, if available).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/acpi/sleep.c      |    3 +++
 drivers/acpi/x86/s2idle.c |    4 +++-
 2 files changed, 6 insertions(+), 1 deletion(-)

Index: linux-pm/drivers/acpi/sleep.c
===================================================================
--- linux-pm.orig/drivers/acpi/sleep.c
+++ linux-pm/drivers/acpi/sleep.c
@@ -824,6 +824,9 @@ static const struct platform_s2idle_ops
 
 void __weak acpi_s2idle_setup(void)
 {
+	if (acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0)
+		pr_info("Efficient low-power S0 idle declared\n");
+
 	s2idle_set_ops(&acpi_s2idle_ops);
 }
 
Index: linux-pm/drivers/acpi/x86/s2idle.c
===================================================================
--- linux-pm.orig/drivers/acpi/x86/s2idle.c
+++ linux-pm/drivers/acpi/x86/s2idle.c
@@ -423,8 +423,10 @@ static int lps0_device_attach(struct acp
 	 * line.
 	 */
 	if ((acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0) &&
-	    mem_sleep_default > PM_SUSPEND_MEM && !acpi_sleep_default_s3)
+	    mem_sleep_default > PM_SUSPEND_MEM && !acpi_sleep_default_s3) {
 		mem_sleep_current = PM_SUSPEND_TO_IDLE;
+		pr_info("Low-power S0 idle used by default for system suspend\n");
+	}
 
 	/*
 	 * Some LPS0 systems, like ASUS Zenbook UX430UNR/i7-8550U, require the