linux-kernel - Re: [PATCH v2] cpuidle: Do not use CPUIDLE_DRIVER_STATE

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Message-ID: <55667640.6090605@linux.vnet.ibm.com>
Date:	Thu, 28 May 2015 07:28:24 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH v2] cpuidle: Do not use CPUIDLE_DRIVER_STATE_START in
 cpuidle.c

On 05/28/2015 07:39 AM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The CPUIDLE_DRIVER_STATE_START symbol is defined as 1 only if
> CONFIG_ARCH_HAS_CPU_RELAX is set, otherwise it is defined as 0.
> However, if CONFIG_ARCH_HAS_CPU_RELAX is set, the first (index 0)
> entry in the cpuidle driver's table of states is overwritten with
> the default "poll" entry by the core.  The "state" defined by the
> "poll" entry doesn't provide ->enter_dead and ->enter_freeze
> callbacks and its exit_latency is 0.
> 
> For this reason, it is not necessary to use CPUIDLE_DRIVER_STATE_START
> in cpuidle_play_dead() (->enter_dead is NULL, so the "poll state"
> will be skipped by the loop).
> 
> It also is arguably unuseful to return states with exit_latency
> equal to 0 from find_deepest_state(), so the function can be modified
> to start the loop from index 0 and the "poll state" will be skipped by
> it as a result of the check against latency_req.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpuidle/cpuidle.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> Index: linux-pm/drivers/cpuidle/cpuidle.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/cpuidle.c
> +++ linux-pm/drivers/cpuidle/cpuidle.c
> @@ -65,7 +65,7 @@ int cpuidle_play_dead(void)
>  		return -ENODEV;
> 
>  	/* Find lowest-power state that supports long-term idle */
> -	for (i = drv->state_count - 1; i >= CPUIDLE_DRIVER_STATE_START; i--)
> +	for (i = drv->state_count - 1; i >= 0; i--)
>  		if (drv->states[i].enter_dead)
>  			return drv->states[i].enter_dead(dev, i);
> 
> @@ -79,9 +79,9 @@ static int find_deepest_state(struct cpu
>  			      bool freeze)
>  {
>  	unsigned int latency_req = 0;
> -	int i, ret = freeze ? -1 : CPUIDLE_DRIVER_STATE_START - 1;
> +	int i, ret = -ENXIO;
> 
> -	for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
> +	for (i = 0; i < drv->state_count; i++) {
>  		struct cpuidle_state *s = &drv->states[i];
>  		struct cpuidle_state_usage *su = &dev->states_usage[i];
> 

Reviewed-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> 
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