[PATCH] clocksource: Add `nopmtmr` to disable ACPI PM Timer at run-time

[Paul Menzel]

Initializing the ACPI Power Management Timer takes 33 ms on the ASUS
F2A85-M PRO.

     [    0.248373] calling  init_acpi_pm_clocksource+0x0/0x197 @ 1
     [    0.282913] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 2085701024 ns
     [    0.282916] initcall init_acpi_pm_clocksource+0x0/0x197 returned 0 after 33731 usecs

Currently, itâs only possible to disable that clocksource with the
Kconfig option `X86_PM_TIMER`, so add the option `nopmtmr` to disable
that clocksource at run-time.

     [    0.248381] calling  init_acpi_pm_clocksource+0x0/0x197 @ 1
     [    0.248383] initcall init_acpi_pm_clocksource+0x0/0x197 returned -19 after 0 usecs

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>
---
Please consider this a request for comments.

1.  What would be the correct return value? Can ENODEV be used?
2.  Should the attribute `__read_mostly` be added?
---
 Documentation/admin-guide/kernel-parameters.txt |  2 ++
 drivers/clocksource/acpi_pm.c                   | 15 +++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7bc83f3d9bdf..4ea7fa94b015 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3246,6 +3246,8 @@
 
 	nopcid		[X86-64] Disable the PCID cpu feature.
 
+	nopmtmr		[X86] Disable the ACPI Power Management Timer.
+
 	norandmaps	Don't use address space randomization.  Equivalent to
 			echo 0 > /proc/sys/kernel/randomize_va_space
 
diff --git a/drivers/clocksource/acpi_pm.c b/drivers/clocksource/acpi_pm.c
index eb596ff9e7bb..c1249f9c9acf 100644
--- a/drivers/clocksource/acpi_pm.c
+++ b/drivers/clocksource/acpi_pm.c
@@ -30,6 +30,7 @@
  * in arch/i386/kernel/acpi/boot.c
  */
 u32 pmtmr_ioport __read_mostly;
+static bool nopmtmr;
 
 static inline u32 read_pmtmr(void)
 {
@@ -177,6 +178,9 @@ static int __init init_acpi_pm_clocksource(void)
 	u64 value1, value2;
 	unsigned int i, j = 0;
 
+	if (nopmtmr)
+		return -ENODEV;
+
 	if (!pmtmr_ioport)
 		return -ENODEV;
 
@@ -239,3 +243,14 @@ static int __init parse_pmtmr(char *arg)
 	return 1;
 }
 __setup("pmtmr=", parse_pmtmr);
+
+/*
+ * Allow to disable PMTimer at run-time.
+ */
+static int __init parse_nopmtmr(char *arg)
+{
+	nopmtmr = true;
+
+	return 0;
+}
+__setup("nopmtmr", parse_nopmtmr);
-- 
2.26.2