Subject: [PATCH 3/7] ARM: tegra30: cpuidle: add LP2 driver for secondary CPUs

From: [email-masked] (Stephen Warren)
Date: Thu, 11 Oct 2012 10:24:14 -0600

On 10/11/2012 03:15 AM, Joseph Lo wrote:
> On Wed, 2012-10-10 at 06:38 +0800, Stephen Warren wrote:
>> On 10/08/2012 04:26 AM, Joseph Lo wrote:
>>> This supports power-gated (LP2) idle on secondary CPUs for Tegra30.
>>> The secondary CPUs can go into LP2 state independently. When CPU goes
>>> into LP2 state, it saves it's state and puts itself to flow controlled
>>> WFI state. After that, it will been power gated.
>>
>>> diff --git a/arch/arm/mach-tegra/cpuidle-tegra30.c b/arch/arm/mach-tegra/cpuidle-tegra30.c
>>
>>>  static struct cpuidle_driver tegra_idle_driver = {
>>>  	.name = "tegra_idle",
>>>  	.owner = THIS_MODULE,
>>>  	.en_core_tk_irqen = 1,
>>> -	.state_count = 1,
>>> +	.state_count = 2,
>>
>> Doesn't that assignment need to be ifdef'd just like the array entry
>> setup below:
>>
>>>  	.states = {
>>>  		[0] = ARM_CPUIDLE_WFI_STATE_PWR(600),
>>> +#ifdef CONFIG_PM_SLEEP
>>> +		[1] = {
>>> +			.enter			= tegra30_idle_lp2,
>>> +			.exit_latency		= 2000,
>>> +			.target_residency	= 2200,
>>> +			.power_usage		= 0,
>>> +			.flags			= CPUIDLE_FLAG_TIME_VALID,
>>> +			.name			= "LP2",
>>> +			.desc			= "CPU power-gate",
>>> +		},
>>> +#endif
>>>  	},
>>>  };
>>
>>> @@ -41,6 +114,10 @@ int __init tegra30_cpuidle_init(void)
>>>  	struct cpuidle_device *dev;
>>>  	struct cpuidle_driver *drv = &tegra_idle_driver;
>>>  
>>> +#ifndef CONFIG_PM_SLEEP
>>> +	drv->state_count = 1;	/* support clockgating only */
>>> +#endif
>>
>> Oh, I see it's done here. Just fixing the static initialization seems a
>> lot simpler?
>>
> OK. Will do.
> 
>>> diff --git a/arch/arm/mach-tegra/pm.c b/arch/arm/mach-tegra/pm.c
>>
>>> +void __cpuinit tegra_clear_cpu_in_lp2(int cpu)
>>> +{
>>> +	spin_lock(&tegra_lp2_lock);
>>> +	BUG_ON(!cpumask_test_cpu(cpu, &tegra_in_lp2));
>>> +	cpumask_clear_cpu(cpu, &tegra_in_lp2);
>>> +
>>> +	/*
>>> +	 * Update the IRAM copy used by the reset handler. The IRAM copy
>>> +	 * can't use used directly by cpumask_clear_cpu() because it uses
>>> +	 * LDREX/STREX which requires the addressed location to be inner
>>> +	 * cacheable and sharable which IRAM isn't.
>>> +	 */
>>> +	writel(tegra_in_lp2.bits[0], tegra_cpu_lp2_mask);
>>> +	dsb();
>>
>> Why not /just/ store the data in IRAM, and read/write directly to it,
>> rather than maintaining an SDRAM-based copy of it?
>>
>> Then, wouldn't the body of this function be simply:
>>
>> spin_lock();
>> BUG_ON(!(tegra_cpu_lp2_mask & BIT(cpu)));
>> tegra_cpu_lp2_mask |= BIT(cpu);
>> spin_unlock();
>>
> 
> It may not simple like this. To maintain it identical to a cpumask. It
> may look likes below. Because I need to compare it with cpu_online_mask.

Oh, the comparison against cpu_online_mask() is what I was missing. I
guess that offline CPUs don't go into LP2, so you can't just check that
tegra_cpu_lp2_mask == (1 << num_cpus()) - 1.

One way to avoid that might be to maintain a cpu_in_lp2_count variable
alongside the mask, and simply compare that against num_online_cpus()
rather than comparing the two masks. At least that would avoid the
following line:

>>> +	writel(tegra_in_lp2.bits[0], tegra_cpu_lp2_mask);

... making use of knowledge of the internal structure of the struct
cpumask type.

However, given the comparison requirement, either way is probably fine.