Re: [PATCH] ARM: smp: allow get the core count from L2 control on A15

[Kukjin Kim <kgene.kim@samsung.com>]

On 01/31/12 23:13, Will Deacon wrote:
> Hi Kukjin,
>
> On Tue, Jan 31, 2012 at 12:11:10PM +0000, Kukjin Kim wrote:
>>
>> Actually, the number of A15 CPU core gets from L2 control
>> register not SCU configuration.
>>
>> Suggested-by: Changhwan Youn<chaos.youn@samsung.com>
>> Signed-off-by: Kukjin Kim<kgene.kim@samsung.com>
>> Cc: Russell King<rmk+kernel@arm.linux.org.uk>
>> ---
>
> NAK.
>
>> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
>> index 1e5717a..b67084f 100644
>> --- a/arch/arm/include/asm/smp.h
>> +++ b/arch/arm/include/asm/smp.h
>> @@ -71,6 +71,11 @@ extern void platform_secondary_init(unsigned int cpu);
>>   extern void platform_smp_prepare_cpus(unsigned int);
>>
>>   /*
>> + * Get the number of CPU cores from the L2 control register on A15
>> + */
>> +extern unsigned long a15_get_core_count(void);
>> +
>> +/*
>>    * Logical CPU mapping.
>>    */
>>   extern int __cpu_logical_map[NR_CPUS];
>> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
>> index 57db122..be4d31d 100644
>> --- a/arch/arm/kernel/smp.c
>> +++ b/arch/arm/kernel/smp.c
>> @@ -233,6 +233,22 @@ void __ref cpu_die(void)
>>   }
>>   #endif /* CONFIG_HOTPLUG_CPU */
>>
>> +/*
>> + * Get the number of CPU cores from the L2 control register on A15
>> + */
>> +unsigned long a15_get_core_count(void)
>> +{
>> +	unsigned long val;
>> +
>> +	/* Read L2 control register */
>> +	asm volatile("mrc p15, 1, %0, c9, c0, 2" : "=r"(val));
>> +
>> +	/* [25:24] of L2 control register means core count - 1 */
>> +	val = ((val>>  24)&  0x3) + 1;
>> +
>> +	return val;
>> +}
>
> This doesn't belong in smp.c but, more importantly, this doesn't work for
> multi-cluster configurations at all. Since all A15 implementations will be
> on new platforms, the code will be device-tree only and so we should use

Why not? As I know, current arm kernel ARMv7 arch can support A15 
without device-tree. And you know, the core number should be counted by 
L2 control register. no?

> that to determine the CPU topology as, unfortunately, there is no architected
> way of doing this.
>
> I believe Lorenzo posted some patches which you could look at.
>
OK, Would be better.

Thanks.

Best regards,
Kgene.
--
Kukjin Kim <kgene.kim@samsung.com>, Senior Engineer,
SW Solution Development Team, Samsung Electronics Co., Ltd.