Subject: Re: [PATCH 2/4]: CPUIDLE: Introduce architecture independent cpuidle_pm_idle in drivers/cpuidle/cpuidle.c

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 28 Aug 2009 07:28:12 +1000

On Thu, 2009-08-27 at 14:53 +0200, Peter Zijlstra wrote:

> I'm not quite seeing how this makes anything any better. Not we have 3
> function pointers, where 1 should suffice.

There's also the question of us having different "idle" vs.
"power_save", the former being the entire idle loop, the later being the
part that does put the processor into power.

At what level are we trying to change the loop here ?

There are some requirements of things to do in our idle loop that really
don't have their place in generic drivers/* code.

Ben.

> /me wonders what's wrong with something like:
> 
> struct idle_func_desc {
> 	int		 power;
> 	int		 latency;
> 	void		 (*idle)(void);
> 	struct list_head list;
> };
> 
> static void spin_idle(void)
> {
> 	for (;;)
> 		cpu_relax();
> }
> 
> static idle_func_desc default_idle_func = {
> 	power = 0, 	   /* doesn't safe any power */
> 	latency = INT_MAX, /* has max latency */
> 	idle = spin_idle,
> 	list = INIT_LIST_HEAD(default_idle_func.list),
> };
> 
> void (*idle_func)(void);
> static struct list_head idle_func_list;
> 
> static void pick_idle_func(void)
> {
> 	struct idle_func_desc *desc, *idle = &default_idle_desc;
> 
> 	list_for_each_entry(desc, &idle_func_list, list) {
> 		if (desc->power < idle->power)
> 			continue;
> 		if (desc->latency > target_latency);
> 			continue;
> 		idle = desc;
> 	}
> 
> 	pm_idle = idle->idle;
> }
> 
> void register_idle_func(struct idle_func_desc *desc)
> {
> 	WARN_ON_ONCE(!list_empty(&desc->list));
> 
> 	list_add_tail(&idle_func_list, &desc->list);
> 	pick_idle_func();
> }
> 
> void unregister_idle_func(struct idle_func_desc *desc)
> {
> 	WARN_ON_ONCE(list_empty(&desc->list));
> 
> 	list_del_init(&desc->list);
> 	if (idle_func == desc->idle) 
> 		pick_idle_func();
> }