clocksource: Load the ACPI PM clocksource asynchronously

The ACPI clocksource takes quite some time to initialize,
and this increases the boot time of the kernel for a
double digit percentage. This while almost all modern
systems will be using the HPET already anyway.

This patch turns the clocksource loading into an asynchronous
operation; which means it won't hold up the boot while
still becoming available normally.

To make this work well, an udelay() had to be turned into an
usleep_range() so that on UP systems, we yield the CPU to
regular boot tasks instead of spinning.

CC: John Stultz <johnstul@us.ibm.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Len Brown <lenb@kernel.org>

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 16 insertions, 8 deletions

diff --git a/drivers/clocksource/acpi_pm.c b/drivers/clocksource/acpi_pm.c
index 6b5cf02c35c8..82e882028fcf 100644
--- a/drivers/clocksource/acpi_pm.c
+++ b/drivers/clocksource/acpi_pm.c
@@ -23,6 +23,7 @@
 #include <linux/init.h>
 #include <linux/pci.h>
 #include <linux/delay.h>
+#include <linux/async.h>
 #include <asm/io.h>
 
 /*
@@ -179,17 +180,15 @@ static int verify_pmtmr_rate(void)
 /* Number of reads we try to get two different values */
 #define ACPI_PM_READ_CHECKS 10000
 
-static int __init init_acpi_pm_clocksource(void)
+static void __init acpi_pm_clocksource_async(void *unused, async_cookie_t cookie)
 {
         cycle_t value1, value2;
         unsigned int i, j = 0;
 
-        if (!pmtmr_ioport)
-                return -ENODEV;
 
         /* "verify" this timing source: */
         for (j = 0; j < ACPI_PM_MONOTONICITY_CHECKS; j++) {
-                udelay(100 * j);
+                usleep_range(100 * j, 100 * j + 100);
                 value1 = clocksource_acpi_pm.read(&clocksource_acpi_pm);
                 for (i = 0; i < ACPI_PM_READ_CHECKS; i++) {
                         value2 = clocksource_acpi_pm.read(&clocksource_acpi_pm);
@@ -203,25 +202,34 @@ static int __init init_acpi_pm_clocksource(void)
                                " 0x%#llx, 0x%#llx - aborting.\n",
                                value1, value2);
                         pmtmr_ioport = 0;
-                        return -EINVAL;
+                        return;
                 }
                 if (i == ACPI_PM_READ_CHECKS) {
                         printk(KERN_INFO "PM-Timer failed consistency check "
                                " (0x%#llx) - aborting.\n", value1);
                         pmtmr_ioport = 0;
-                        return -ENODEV;
+                        return;
                 }
         }
 
         if (verify_pmtmr_rate() != 0){
                 pmtmr_ioport = 0;
-                return -ENODEV;
+                return;
         }
 
-        return clocksource_register_hz(&clocksource_acpi_pm,
+        clocksource_register_hz(&clocksource_acpi_pm,
                                                 PMTMR_TICKS_PER_SEC);
 }
 
+static int __init init_acpi_pm_clocksource(void)
+{
+        if (!pmtmr_ioport)
+                return -ENODEV;
+
+        async_schedule(acpi_pm_clocksource_async, NULL);
+        return 0;
+}
+
 /* We use fs_initcall because we want the PCI fixups to have run
  * but we still need to load before device_initcall
  */