Re: [v2 3/9] ARM: tegra: # of CPU cores detection w/ & w/o HAVE_ARM_SCU

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 01/10/2013 05:58 AM, Hiroshi Doyu wrote:
> Lorenzo Pieralisi <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org> wrote @ Wed, 9 Jan 2013 16:17:00 +0100:
...
> With your "[PATCH] ARM: kernel: DT cpu map validity check helper
> function", my original patch gets as below. I think that the following
> "smp_detect_ncores()" function may be so generic that it could be
> moved in some common place than platform?
> 
> From f5f2a43952ce75fe061b3808b994c3ceb07f1af1 Mon Sep 17 00:00:00 2001
> From: Hiroshi Doyu <hdoyu-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> Date: Mon, 26 Nov 2012 12:25:14 +0200
> Subject: [PATCH 1/1] ARM: tegra: # of CPU cores detection w/ & w/o
>  HAVE_ARM_SCU
> 
> The method to detect the number of CPU cores on Cortex-A9 MPCore and
> Cortex-A15 MPCore is different. On Cortex-A9 MPCore we can get this
> information from the Snoop Control Unit(SCU). On Cortex-A15 MPCore we
> have to read it from the system coprocessor(CP15), because the SCU on
> Cortex-A15 MPCore does not have software readable registers. This
> patch selects the correct method at runtime based on the CPU ID.

> diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c

> +static int __init smp_detect_ncores(void)
> +{
> +	unsigned int ncores, mpidr;
> +	u32 l2ctlr;
> +	phys_addr_t pa;
> +
> +	mpidr = read_cpuid_mpidr();
> +	switch (mpidr) {
> +	case ARM_CPU_PART_CORTEX_A15:
> +		asm("mrc p15, 1, %0, c9, c0, 2\n" : "=r" (l2ctlr));
> +		ncores = ((l2ctlr >> 24) & 3) + 1;
> +		break;
> +	case ARM_CPU_PART_CORTEX_A9:
> +		/* Get SCU physical base */
> +		asm("mrc p15, 4, %0, c15, c0, 0" : "=r" (pa));
> +		scu_base = IO_ADDRESS(pa);
> +		ncores = scu_get_core_count(scu_base);
> +		break;
> +	default:
> +		pr_warn("Unsupported mpidr\n");
> +		ncores = 1;
> +		break;
> +	}
> +
> +	return ncores;
> +}

Why not just remove that function...

>  /*
>   * Initialise the CPU possible map early - this describes the CPUs
>   * which may be present or become present in the system.
>   */
>  static void __init tegra_smp_init_cpus(void)
>  {
> -	unsigned int i, ncores = scu_get_core_count(scu_base);
> -
> -	if (ncores > nr_cpu_ids) {
> -		pr_warn("SMP: %u cores greater than maximum (%u), clipping\n",
> -			ncores, nr_cpu_ids);
> -		ncores = nr_cpu_ids;
> +	if (!arm_dt_cpu_map_valid()) {
> +		unsigned int i, ncores;
> +
> +		ncores = smp_detect_ncores();

And just say "ncores = 1" here?

For Tegra, it's trivial to simply put the patch adding the required DT
nodes before this patch, and then there's no need for any
backwards-compatibility, since the nodes are guaranteed to be there.