[01/10] clocksource: add generic dummy timer driver

Commit Message

Stephen Boyd March 7, 2013, 12:03 a.m. UTC

From: Mark Rutland <[e-mail]>

Several architectures have a dummy timer driver tightly coupled with
their broadcast code to support machines without cpu-local timers (or
where there is a lack of driver support).

Since 12ad100046: "clockevents: Add generic timer broadcast function"
it's been possible to write broadcast-capable timer drivers decoupled
from the broadcast mechanism. We can use this functionality to implement
a generic dummy timer driver that can be shared by all architectures
with generic tick broadcast (ARCH_HAS_TICK_BROADCAST).

This patch implements a generic dummy timer using this facility.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
[sboyd: Make percpu data static, use __this_cpu_ptr(), s/dummy/dummy_timer/]
Signed-off-by: Stephen Boyd <[e-mail]>
---
 drivers/clocksource/Makefile      |  1 +
 drivers/clocksource/dummy_timer.c | 67 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)
 create mode 100644 drivers/clocksource/dummy_timer.c

Comments

Thomas Gleixner March 7, 2013, 2 p.m. UTC | #1

On Wed, 6 Mar 2013, Stephen Boyd wrote:

> +static int __init dummy_timer_register(void)
> +{
> +	int err = register_cpu_notifier(&dummy_timer_cpu_nb);
> +	if (err)
> +		return err;
> +
> +	/* We won't get a call on the boot CPU, so register immediately */
> +	dummy_timer_setup();
> +
> +	return 0;
> +}
> +device_initcall(dummy_timer_register);

Are you sure that you want that to be installed unconditionally? What
about multi-platform kernels?

Thanks,

	tglx

Stephen Boyd March 7, 2013, 9:41 p.m. UTC | #2

On 03/07/13 06:00, Thomas Gleixner wrote:
> On Wed, 6 Mar 2013, Stephen Boyd wrote:
>
>> +static int __init dummy_timer_register(void)
>> +{
>> +	int err = register_cpu_notifier(&dummy_timer_cpu_nb);
>> +	if (err)
>> +		return err;
>> +
>> +	/* We won't get a call on the boot CPU, so register immediately */
>> +	dummy_timer_setup();
>> +
>> +	return 0;
>> +}
>> +device_initcall(dummy_timer_register);
> Are you sure that you want that to be installed unconditionally? What
> about multi-platform kernels?
>

A dummy timer is already installed unconditionally on devices with an
arm architected timer, so if this is breaking something then those
devices have been broken in the 3.9 merge window. I suppose we may need
to make the rating even lower if there is somebody using a clockevent
with a rating below 100. My grep of the tree shows 125 as the lowest,
but perhaps we want to set the rating to 1 to be safe?

Patch

diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
index 4d8283a..655d718 100644
--- a/drivers/clocksource/Makefile
+++ b/drivers/clocksource/Makefile
@@ -22,3 +22,4 @@  obj-$(CONFIG_VT8500_TIMER)	+= vt8500_timer.o
 
 obj-$(CONFIG_ARM_ARCH_TIMER)		+= arm_arch_timer.o
 obj-$(CONFIG_CLKSRC_METAG_GENERIC)	+= metag_generic.o
+obj-$(CONFIG_ARCH_HAS_TICK_BROADCAST)	+= dummy_timer.o
diff --git a/drivers/clocksource/dummy_timer.c b/drivers/clocksource/dummy_timer.c
new file mode 100644
index 0000000..7e1ce45
--- /dev/null
+++ b/drivers/clocksource/dummy_timer.c
@@ -0,0 +1,67 @@ 
+/*
+ *  linux/drivers/clocksource/dummy_timer.c
+ *
+ *  Copyright (C) 2013 ARM Ltd.
+ *  All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+#include <linux/clockchips.h>
+#include <linux/cpu.h>
+#include <linux/init.h>
+#include <linux/percpu.h>
+
+static DEFINE_PER_CPU(struct clock_event_device, dummy_timer_evt);
+
+static void dummy_timer_set_mode(enum clock_event_mode mode,
+			   struct clock_event_device *evt)
+{
+	/*
+	 * Core clockevents code will call this when exchanging timer devices.
+	 * We don't need to do anything here.
+	 */
+}
+
+static void __cpuinit dummy_timer_setup(void)
+{
+	int cpu = smp_processor_id();
+	struct clock_event_device *evt = __this_cpu_ptr(&dummy_timer_evt);
+
+	evt->name	= "dummy_timer";
+	evt->features	= CLOCK_EVT_FEAT_PERIODIC |
+			  CLOCK_EVT_FEAT_ONESHOT |
+			  CLOCK_EVT_FEAT_DUMMY;
+	evt->rating	= 100;
+	evt->set_mode	= dummy_timer_set_mode;
+	evt->cpumask	= cpumask_of(cpu);
+
+	clockevents_register_device(evt);
+}
+
+static int __cpuinit dummy_timer_cpu_notify(struct notifier_block *self,
+				      unsigned long action, void *hcpu)
+{
+	if ((action & ~CPU_TASKS_FROZEN) == CPU_STARTING)
+		dummy_timer_setup();
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block dummy_timer_cpu_nb __cpuinitdata = {
+	.notifier_call = dummy_timer_cpu_notify,
+};
+
+static int __init dummy_timer_register(void)
+{
+	int err = register_cpu_notifier(&dummy_timer_cpu_nb);
+	if (err)
+		return err;
+
+	/* We won't get a call on the boot CPU, so register immediately */
+	dummy_timer_setup();
+
+	return 0;
+}
+device_initcall(dummy_timer_register);