linux-kernel - Re: [PATCH 3/9] ARM: tegra: # of CPU cores detection w/ & w/o HAVE_ARM

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Date:	Thu, 20 Dec 2012 12:26:33 +0100
From:	Hiroshi Doyu <hdoyu@...dia.com>
To:	"marc.zyngier@....com" <marc.zyngier@....com>
CC:	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"andrew@...n.ch" <andrew@...n.ch>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"johnstul@...ibm.com" <johnstul@...ibm.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/9] ARM: tegra: # of CPU cores detection w/ & w/o
 HAVE_ARM_SCU

Marc Zyngier <marc.zyngier@....com> wrote @ Thu, 20 Dec 2012 12:17:08 +0100:

> On 20/12/12 09:44, Hiroshi Doyu wrote:
> > 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.
> > 
> > Signed-off-by: Hiroshi Doyu <hdoyu@...dia.com>
> > ---
> >  arch/arm/mach-tegra/platsmp.c |   31 ++++++++++++++++++++++++++++---
> >  1 file changed, 28 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
> > index 1b926df..68e76ef 100644
> > --- a/arch/arm/mach-tegra/platsmp.c
> > +++ b/arch/arm/mach-tegra/platsmp.c
> > @@ -23,6 +23,7 @@
> >  #include <asm/hardware/gic.h>
> >  #include <asm/mach-types.h>
> >  #include <asm/smp_scu.h>
> > +#include <asm/cputype.h>
> >  
> >  #include <mach/powergate.h>
> >  
> > @@ -34,9 +35,13 @@
> >  #include "common.h"
> >  #include "iomap.h"
> >  
> > +#define CPU_MASK		0xff0ffff0
> > +#define CPU_CORTEX_A9		0x410fc090
> > +#define CPU_CORTEX_A15		0x410fc0f0
> > +
> >  extern void tegra_secondary_startup(void);
> >  
> > -static void __iomem *scu_base = IO_ADDRESS(TEGRA_ARM_PERIF_BASE);
> > +static void __iomem *scu_base;
> >  
> >  #define EVP_CPU_RESET_VECTOR \
> >  	(IO_ADDRESS(TEGRA_EXCEPTION_VECTORS_BASE) + 0x100)
> > @@ -149,7 +154,26 @@ done:
> >   */
> >  static void __init tegra_smp_init_cpus(void)
> >  {
> > -	unsigned int i, ncores = scu_get_core_count(scu_base);
> > +	unsigned int i, cpu_id, ncores;
> > +	u32 l2ctlr;
> > +	phys_addr_t pa;
> > +
> > +	cpu_id = read_cpuid(CPUID_ID) & CPU_MASK;
> > +	switch (cpu_id) {
> > +	case CPU_CORTEX_A15:
> > +		asm("mrc p15, 1, %0, c9, c0, 2\n" : "=r" (l2ctlr));
> > +		ncores = ((l2ctlr >> 24) & 3) + 1;
> 
> Please, do not do that. It doesn't scale to multiple clusters. Instead,
> you can now rely on arm_dt_init_cpu_maps() to do the right thing as long
> as your device tree exposes all the cpu nodes.

Ok, this could be the answer why DT for cpu is necessary even when
it's got from CP15?

http://lists.infradead.org/pipermail/linux-arm-kernel/2012-December/137989.html
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