[PATCH] clocksource: add enable() and disable() callbacks

[PATCH] clocksource: add enable() and disable() callbacks

[Magnus Damm]

From: Magnus Damm <damm@igel.co.jp>

Add enable() and disable() callbacks for clocksources. This allows
us to put unused clocksources in power save mode. The functions
clocksource_enable() and clocksource_disable() wrap the callbacks
and are inserted in the timekeeping code to enable before use and
disable after switching to a new clocksource.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: John Stultz <johnstul@us.ibm.com>
---

 Same as before, but applies on top of the new read() patch.

 include/linux/clocksource.h |   31 +++++++++++++++++++++++++++++++
 kernel/time/timekeeping.c   |   12 +++++++++---
 2 files changed, 40 insertions(+), 3 deletions(-)

--- 0005/include/linux/clocksource.h
+++ work/include/linux/clocksource.h	2008-12-11 19:52:25.000000000 +0900
@@ -43,6 +43,8 @@ struct clocksource;
  *				The ideal clocksource. A must-use where
  *				available.
  * @read:		returns a cycle value, passes clocksource as argument
+ * @enable:		optional function to enable the clocksource
+ * @disable:		optional function to disable the clocksource
  * @mask:		bitmask for two's complement
  *			subtraction of non 64 bit counters
  * @mult:		cycle to nanosecond multiplier (adjusted by NTP)
@@ -62,6 +64,8 @@ struct clocksource {
 	struct list_head list;
 	int rating;
 	cycle_t (*read)(struct clocksource *cs);
+	int (*enable)(struct clocksource *cs);
+	void (*disable)(struct clocksource *cs);
 	cycle_t mask;
 	u32 mult;
 	u32 mult_orig;
@@ -174,6 +178,33 @@ static inline cycle_t clocksource_read(s
 }
 
 /**
+ * clocksource_enable: - enable clocksource
+ * @cs:		pointer to clocksource
+ *
+ * Enables the specified clocksource. The clocksource callback
+ * function should start up the hardware and setup mult and field
+ * members of struct clocksource to reflect hardware capabilities.
+ */
+static inline int clocksource_enable(struct clocksource *cs)
+{
+	return cs->enable ? cs->enable(cs) : 0;
+}
+
+/**
+ * clocksource_disable: - disable clocksource
+ * @cs:		pointer to clocksource
+ *
+ * Disables the specified clocksource. The clocksource callback
+ * function should power down the now unused hardware block to
+ * save power.
+ */
+static inline void clocksource_disable(struct clocksource *cs)
+{
+	if (cs->disable)
+		cs->disable(cs);
+}
+
+/**
  * cyc2ns - converts clocksource cycles to nanoseconds
  * @cs:		Pointer to clocksource
  * @cycles:	Cycles
--- 0004/kernel/time/timekeeping.c
+++ work/kernel/time/timekeeping.c	2008-12-11 19:51:16.000000000 +0900
@@ -184,7 +184,7 @@ EXPORT_SYMBOL(do_settimeofday);
  */
 static void change_clocksource(void)
 {
-	struct clocksource *new;
+	struct clocksource *new, *old;
 
 	new = clocksource_get_next();
 
@@ -193,11 +193,16 @@ static void change_clocksource(void)
 
 	clocksource_forward_now();
 
-	new->raw_time = clock->raw_time;
+	if (clocksource_enable(new))
+		return;
 
+	new->raw_time = clock->raw_time;
+	old = clock;
 	clock = new;
+	clocksource_disable(old);
+
 	clock->cycle_last = 0;
-	clock->cycle_last = clocksource_read(new);
+	clock->cycle_last = clocksource_read(clock);
 	clock->error = 0;
 	clock->xtime_nsec = 0;
 	clocksource_calculate_interval(clock, NTP_INTERVAL_LENGTH);
@@ -293,6 +298,7 @@ void __init timekeeping_init(void)
 	ntp_init();
 
 	clock = clocksource_get_next();
+	clocksource_enable(clock);
 	clocksource_calculate_interval(clock, NTP_INTERVAL_LENGTH);
 	clock->cycle_last = clocksource_read(clock);
 

Re: [PATCH] clocksource: add enable() and disable() callbacks

[Andrew Morton]

On Thu, 11 Dec 2008 20:49:09 +0900 Magnus Damm <magnus.damm@gmail.com> wrote:

> From: Magnus Damm <damm@igel.co.jp>
> 
> Add enable() and disable() callbacks for clocksources. This allows
> us to put unused clocksources in power save mode. The functions
> clocksource_enable() and clocksource_disable() wrap the callbacks
> and are inserted in the timekeeping code to enable before use and
> disable after switching to a new clocksource.
> 
> Signed-off-by: Magnus Damm <damm@igel.co.jp>
> Acked-by: John Stultz <johnstul@us.ibm.com>
> ---
> 
>  Same as before, but applies on top of the new read() patch.

I asusme we'll be seeing some patches which _use_ these changes?

Re: [PATCH] clocksource: add enable() and disable() callbacks

[Magnus Damm]

On Fri, Dec 12, 2008 at 12:08 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 11 Dec 2008 20:49:09 +0900 Magnus Damm <magnus.damm@gmail.com> wrote:
>
>> From: Magnus Damm <damm@igel.co.jp>
>>
>> Add enable() and disable() callbacks for clocksources. This allows
>> us to put unused clocksources in power save mode. The functions
>> clocksource_enable() and clocksource_disable() wrap the callbacks
>> and are inserted in the timekeeping code to enable before use and
>> disable after switching to a new clocksource.
>>
>> Signed-off-by: Magnus Damm <damm@igel.co.jp>
>> Acked-by: John Stultz <johnstul@us.ibm.com>
>> ---
>>
>>  Same as before, but applies on top of the new read() patch.
>
> I asusme we'll be seeing some patches which _use_ these changes?

I've send a few iterations of my SuperH CMT timer patches to the
linux-sh list, but I'd be happy to post next version to lkml instead
if you prefer that!

The CMT driver also needs the following patches:

[PATCH] clocksource: pass clocksource to read() callback
[PATCH][RFC] early platform driver support

Any comments on the early platform driver code?

Thanks,

/ magnus

Re: [PATCH] clocksource: add enable() and disable() callbacks

[Ingo Molnar]

* Magnus Damm <magnus.damm@gmail.com> wrote:

> +static inline int clocksource_enable(struct clocksource *cs)
> +{
> +	return cs->enable ? cs->enable(cs) : 0;
> +}

> +static inline void clocksource_disable(struct clocksource *cs)
> +{
> +	if (cs->disable)
> +		cs->disable(cs);
> +}

why have the two different styles? The first one should be:

	if (cs->enable)
		return cs->enable(cs);
	return 0;

> @@ -193,11 +193,16 @@ static void change_clocksource(void)
>  
>  	clocksource_forward_now();
>  
> -	new->raw_time = clock->raw_time;
> +	if (clocksource_enable(new))
> +		return;

that looks fragile to me: if the enable fails we'll return silently, 
while change_clocksource() assumes that things went fine. At least put a 
WARN_ON_ONCE() in there.

also, why does it have to fail? If a clocksource cannot be enabled it 
should not be offered as a clocksource.

> +	clocksource_disable(old);

i do agree with the core purpose here, to allow lowlevel code to 
deactivate unused clocksources.

John, Thomas, what's your take on this?

	Ingo

Re: [PATCH] clocksource: add enable() and disable() callbacks

[Magnus Damm]

Hi Ingo,

On Fri, Dec 12, 2008 at 3:36 PM, Ingo Molnar <mingo@elte.hu> wrote:
> * Magnus Damm <magnus.damm@gmail.com> wrote:
>> +static inline int clocksource_enable(struct clocksource *cs)
>> +{
>> +     return cs->enable ? cs->enable(cs) : 0;
>> +}
>
>> +static inline void clocksource_disable(struct clocksource *cs)
>> +{
>> +     if (cs->disable)
>> +             cs->disable(cs);
>> +}
>
> why have the two different styles? The first one should be:
>
>        if (cs->enable)
>                return cs->enable(cs);
>        return 0;

Sure, that's fine too.

>> @@ -193,11 +193,16 @@ static void change_clocksource(void)
>>
>>       clocksource_forward_now();
>>
>> -     new->raw_time = clock->raw_time;
>> +     if (clocksource_enable(new))
>> +             return;
>
> that looks fragile to me: if the enable fails we'll return silently,
> while change_clocksource() assumes that things went fine. At least put a
> WARN_ON_ONCE() in there.

Yeah, John and I discussed this before. What we really want it to move
the failing clocksource out of the list of available clocksources.
That type of change is pretty intrusive though, and I rather see it as
a separate topic.

> also, why does it have to fail? If a clocksource cannot be enabled it
> should not be offered as a clocksource.

Right. I guess most clocksource drivers for embedded platforms will
tie in the clock framework and use clk_enable() and clk_disable().
clk_enable() returns an int.

>> +     clocksource_disable(old);
>
> i do agree with the core purpose here, to allow lowlevel code to
> deactivate unused clocksources.

That's good! I hope we can sort out the details then!

Cheers,

/ magnus