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

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Submitter Hiroshi Doyu
Date Jan. 10, 2013, 12:58 p.m.
Message ID <20130110.145813.1159140334089730421.hdoyu@nvidia.com>
Download mbox | patch
Permalink /patch/210999/
State Not Applicable, archived
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Comments

Hiroshi Doyu - Jan. 10, 2013, 12:58 p.m.
Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Wed, 9 Jan 2013 16:17:00 +0100:

....

> > diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
> > index 68e76ef..51d24ae 100644
> > --- a/arch/arm/mach-tegra/platsmp.c
> > +++ b/arch/arm/mach-tegra/platsmp.c
> > @@ -152,7 +152,7 @@ done:
> >   * 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)
> > +static void __init tegra_smp_detect_cores(void)
> >  {
> >  	unsigned int i, cpu_id, ncores;
> >  	u32 l2ctlr;
> > @@ -183,6 +183,12 @@ static void __init tegra_smp_init_cpus(void)
> >  
> >  	for (i = 0; i < ncores; i++)
> >  		set_cpu_possible(i, true);
> > +}
> > +
> > +static void __init tegra_smp_init_cpus(void)
> > +{
> > +	if (!num_possible_cpus())
> > +		tegra_smp_detect_cores();
> 
> Thought about this, I would prefer writing an inline function that
> provides this information (DT missing /cpu information or parsing failure)
> instead of relying on num_possible_cpus to be == 0 as a way to check that.

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@nvidia.com>
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.

Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
---
 arch/arm/mach-tegra/platsmp.c |   54 ++++++++++++++++++++++++++++++++---------
 1 file changed, 43 insertions(+), 11 deletions(-)
Lorenzo Pieralisi - Jan. 10, 2013, 1:47 p.m.
On Thu, Jan 10, 2013 at 12:58:13PM +0000, Hiroshi Doyu wrote:
> Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Wed, 9 Jan 2013 16:17:00 +0100:
> 
> ....
> 
> > > diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
> > > index 68e76ef..51d24ae 100644
> > > --- a/arch/arm/mach-tegra/platsmp.c
> > > +++ b/arch/arm/mach-tegra/platsmp.c
> > > @@ -152,7 +152,7 @@ done:
> > >   * 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)
> > > +static void __init tegra_smp_detect_cores(void)
> > >  {
> > >  	unsigned int i, cpu_id, ncores;
> > >  	u32 l2ctlr;
> > > @@ -183,6 +183,12 @@ static void __init tegra_smp_init_cpus(void)
> > >  
> > >  	for (i = 0; i < ncores; i++)
> > >  		set_cpu_possible(i, true);
> > > +}
> > > +
> > > +static void __init tegra_smp_init_cpus(void)
> > > +{
> > > +	if (!num_possible_cpus())
> > > +		tegra_smp_detect_cores();
> > 
> > Thought about this, I would prefer writing an inline function that
> > provides this information (DT missing /cpu information or parsing failure)
> > instead of relying on num_possible_cpus to be == 0 as a way to check that.
> 
> 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?

Possibly, but I have still some comments to add.

> 
> From f5f2a43952ce75fe061b3808b994c3ceb07f1af1 Mon Sep 17 00:00:00 2001
> From: Hiroshi Doyu <hdoyu@nvidia.com>
> 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.
> 
> Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
> ---
>  arch/arm/mach-tegra/platsmp.c |   54 ++++++++++++++++++++++++++++++++---------
>  1 file changed, 43 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
> index 6867030..9a1324a 100644
> --- a/arch/arm/mach-tegra/platsmp.c
> +++ b/arch/arm/mach-tegra/platsmp.c
> @@ -24,6 +24,8 @@
>  #include <asm/mach-types.h>
>  #include <asm/smp_scu.h>
>  #include <asm/smp_plat.h>
> +#include <asm/cputype.h>
> +#include <asm/prom.h>
>  
>  #include <mach/powergate.h>
>  
> @@ -38,7 +40,7 @@
>  extern void tegra_secondary_startup(void);
>  
>  static cpumask_t tegra_cpu_init_mask;
> -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)
> @@ -177,23 +179,52 @@ done:
>  	return status;
>  }
>  
> +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;

I do not want to see the case above merged. We keep the existing legacy
methods there for legacy reasons and as a fall-back mechanism but I am not
keen on adding new HW probing code to detect the number of cores.

From A15/A7 onwards DT-only cpu map initialization is the way to go.

Lorenzo

> +	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;
> +}
> +
>  /*
>   * 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();
> +		if (ncores > nr_cpu_ids) {
> +			pr_warn("SMP: %u cores greater than maximum (%u), clipping\n",
> +				ncores, nr_cpu_ids);
> +			ncores = nr_cpu_ids;
> +		}
> +		for (i = 0; i < ncores; i++)
> +			set_cpu_possible(i, true);
>  	}
>  
> -	for (i = 0; i < ncores; i++)
> -		set_cpu_possible(i, true);
> -
>  	set_smp_cross_call(gic_raise_softirq);
>  }
>  
> @@ -202,7 +233,8 @@ static void __init tegra_smp_prepare_cpus(unsigned int max_cpus)
>  	/* Always mark the boot CPU (CPU0) as initialized. */
>  	cpumask_set_cpu(0, &tegra_cpu_init_mask);
>  
> -	scu_enable(scu_base);
> +	if (scu_base)
> +		scu_enable(scu_base);
>  }
>  
>  struct smp_operations tegra_smp_ops __initdata = {
> -- 
> 1.7.9.5
> 
> 
> 
> 

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Hiroshi Doyu - Jan. 10, 2013, 2:03 p.m.
Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Thu, 10 Jan 2013 14:47:23 +0100:

> On Thu, Jan 10, 2013 at 12:58:13PM +0000, Hiroshi Doyu wrote:
> > Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Wed, 9 Jan 2013 16:17:00 +0100:
> > 
> > ....
> > 
> > > > diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
> > > > index 68e76ef..51d24ae 100644
> > > > --- a/arch/arm/mach-tegra/platsmp.c
> > > > +++ b/arch/arm/mach-tegra/platsmp.c
> > > > @@ -152,7 +152,7 @@ done:
> > > >   * 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)
> > > > +static void __init tegra_smp_detect_cores(void)
> > > >  {
> > > >  	unsigned int i, cpu_id, ncores;
> > > >  	u32 l2ctlr;
> > > > @@ -183,6 +183,12 @@ static void __init tegra_smp_init_cpus(void)
> > > >  
> > > >  	for (i = 0; i < ncores; i++)
> > > >  		set_cpu_possible(i, true);
> > > > +}
> > > > +
> > > > +static void __init tegra_smp_init_cpus(void)
> > > > +{
> > > > +	if (!num_possible_cpus())
> > > > +		tegra_smp_detect_cores();
> > > 
> > > Thought about this, I would prefer writing an inline function that
> > > provides this information (DT missing /cpu information or parsing failure)
> > > instead of relying on num_possible_cpus to be == 0 as a way to check that.
> > 
> > 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?
> 
> Possibly, but I have still some comments to add.
> 
> > 
> > From f5f2a43952ce75fe061b3808b994c3ceb07f1af1 Mon Sep 17 00:00:00 2001
> > From: Hiroshi Doyu <hdoyu@nvidia.com>
> > 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.
> > 
> > Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
> > ---
> >  arch/arm/mach-tegra/platsmp.c |   54 ++++++++++++++++++++++++++++++++---------
> >  1 file changed, 43 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
> > index 6867030..9a1324a 100644
> > --- a/arch/arm/mach-tegra/platsmp.c
> > +++ b/arch/arm/mach-tegra/platsmp.c
> > @@ -24,6 +24,8 @@
> >  #include <asm/mach-types.h>
> >  #include <asm/smp_scu.h>
> >  #include <asm/smp_plat.h>
> > +#include <asm/cputype.h>
> > +#include <asm/prom.h>
> >  
> >  #include <mach/powergate.h>
> >  
> > @@ -38,7 +40,7 @@
> >  extern void tegra_secondary_startup(void);
> >  
> >  static cpumask_t tegra_cpu_init_mask;
> > -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)
> > @@ -177,23 +179,52 @@ done:
> >  	return status;
> >  }
> >  
> > +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;
> 
> I do not want to see the case above merged. We keep the existing legacy
> methods there for legacy reasons and as a fall-back mechanism but I am not
> keen on adding new HW probing code to detect the number of cores.
> 
> From A15/A7 onwards DT-only cpu map initialization is the way to go.

Ok, then, smp_detect_ncores(void) could be...

static int __init smp_detect_ncores(void)
{
	unsigned int ncores, mpidr;
	phys_addr_t pa;

	mpidr = read_cpuid_mpidr();
	switch (mpidr) {
	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;
	case ARM_CPU_PART_CORTEX_A15:
	default:
		pr_warn("Unsupported MPIDR %x\n", mpidr);
		ncores = 1;
		break;
	}
	return ncores;
}
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Lorenzo Pieralisi - Jan. 10, 2013, 2:33 p.m.
On Thu, Jan 10, 2013 at 02:03:50PM +0000, Hiroshi Doyu wrote:
> Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Thu, 10 Jan 2013 14:47:23 +0100:

[...]

> > > +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;
> > 
> > I do not want to see the case above merged. We keep the existing legacy
> > methods there for legacy reasons and as a fall-back mechanism but I am not
> > keen on adding new HW probing code to detect the number of cores.
> > 
> > From A15/A7 onwards DT-only cpu map initialization is the way to go.
> 
> Ok, then, smp_detect_ncores(void) could be...
> 
> static int __init smp_detect_ncores(void)
> {
> 	unsigned int ncores, mpidr;
> 	phys_addr_t pa;
> 
> 	mpidr = read_cpuid_mpidr();

Careful, MPIDR is not at all what you want. MIDR is.

> 	switch (mpidr) {
> 	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;
> 	case ARM_CPU_PART_CORTEX_A15:

Remove the ARM_CPU_PART_CORTEX_A15 case, I do not see how it helps.

And be careful with the case matching, if I am not mistaken read_cpuid_id()
(which is the function you have to use) will return the full MIDR, you need
to mask the return value.

> 	default:
> 		pr_warn("Unsupported MPIDR %x\n", mpidr);
> 		ncores = 1;
> 		break;

This is reasonable.

Lorenzo

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Hiroshi Doyu - Jan. 10, 2013, 2:59 p.m.
Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Thu, 10 Jan 2013 15:33:34 +0100:

> On Thu, Jan 10, 2013 at 02:03:50PM +0000, Hiroshi Doyu wrote:
> > Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Thu, 10 Jan 2013 14:47:23 +0100:
> 
> [...]
> 
> > > > +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;
> > > 
> > > I do not want to see the case above merged. We keep the existing legacy
> > > methods there for legacy reasons and as a fall-back mechanism but I am not
> > > keen on adding new HW probing code to detect the number of cores.
> > > 
> > > From A15/A7 onwards DT-only cpu map initialization is the way to go.
> > 
> > Ok, then, smp_detect_ncores(void) could be...
> > 
> > static int __init smp_detect_ncores(void)
> > {
> > 	unsigned int ncores, mpidr;
> > 	phys_addr_t pa;
> > 
> > 	mpidr = read_cpuid_mpidr();
> 
> Careful, MPIDR is not at all what you want. MIDR is.
> 
> > 	switch (mpidr) {
> > 	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;
> > 	case ARM_CPU_PART_CORTEX_A15:
> 
> Remove the ARM_CPU_PART_CORTEX_A15 case, I do not see how it helps.
> 
> And be careful with the case matching, if I am not mistaken read_cpuid_id()
> (which is the function you have to use) will return the full MIDR, you need
> to mask the return value.

Oops, I'll fix.

> > 	default:
> > 		pr_warn("Unsupported MPIDR %x\n", mpidr);
> > 		ncores = 1;
> > 		break;
> 
> This is reasonable.

Ok, thank you for your reivew, I'll keep this func,
"smp_detect_ncores()" in this file since it's mostly for the backward
compatibility. Not necessary to be relocated in common place newly..
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Stephen Warren - Jan. 10, 2013, 4:54 p.m.
On 01/10/2013 05:58 AM, Hiroshi Doyu wrote:
> Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> 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@nvidia.com>
> 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.
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Lorenzo Pieralisi - Jan. 11, 2013, 10:11 a.m.
On Thu, Jan 10, 2013 at 04:54:13PM +0000, Stephen Warren wrote:
> On 01/10/2013 05:58 AM, Hiroshi Doyu wrote:
> > Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> 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@nvidia.com>
> > 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.

If you are willing to accept that your code proposal is perfectly fine by me.

Thanks,
Lorenzo

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Hiroshi Doyu - Jan. 11, 2013, 11:56 a.m.
Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote @ Fri, 11 Jan 2013 11:11:34 +0100:

> On Thu, Jan 10, 2013 at 04:54:13PM +0000, Stephen Warren wrote:
> > On 01/10/2013 05:58 AM, Hiroshi Doyu wrote:
> > > Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> 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@nvidia.com>
> > > 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.
> 
> If you are willing to accept that your code proposal is perfectly fine by me.

That's simple. I'll send ones to add cpu nodes, and then this without detection.
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Patch

diff --git a/arch/arm/mach-tegra/platsmp.c b/arch/arm/mach-tegra/platsmp.c
index 6867030..9a1324a 100644
--- a/arch/arm/mach-tegra/platsmp.c
+++ b/arch/arm/mach-tegra/platsmp.c
@@ -24,6 +24,8 @@ 
 #include <asm/mach-types.h>
 #include <asm/smp_scu.h>
 #include <asm/smp_plat.h>
+#include <asm/cputype.h>
+#include <asm/prom.h>
 
 #include <mach/powergate.h>
 
@@ -38,7 +40,7 @@ 
 extern void tegra_secondary_startup(void);
 
 static cpumask_t tegra_cpu_init_mask;
-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)
@@ -177,23 +179,52 @@  done:
 	return status;
 }
 
+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;
+}
+
 /*
  * 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();
+		if (ncores > nr_cpu_ids) {
+			pr_warn("SMP: %u cores greater than maximum (%u), clipping\n",
+				ncores, nr_cpu_ids);
+			ncores = nr_cpu_ids;
+		}
+		for (i = 0; i < ncores; i++)
+			set_cpu_possible(i, true);
 	}
 
-	for (i = 0; i < ncores; i++)
-		set_cpu_possible(i, true);
-
 	set_smp_cross_call(gic_raise_softirq);
 }
 
@@ -202,7 +233,8 @@  static void __init tegra_smp_prepare_cpus(unsigned int max_cpus)
 	/* Always mark the boot CPU (CPU0) as initialized. */
 	cpumask_set_cpu(0, &tegra_cpu_init_mask);
 
-	scu_enable(scu_base);
+	if (scu_base)
+		scu_enable(scu_base);
 }
 
 struct smp_operations tegra_smp_ops __initdata = {