ARM: 6993/1: platsmp: Allow secondary cpu hotplug with maxcpus=1

If an ARM system has multiple cpus in the same socket and the
kernel is booted with maxcpus=1, secondary cpus are possible but
not present due to how platform_smp_prepare_cpus() is called.
Since most typical ARM processors don't actually support physical
hotplug, initialize the present map to be equal to the possible
map in generic ARM SMP code. Also, always call
platform_smp_prepare_cpus() as long as max_cpus is non-zero (0
means no SMP) to allow platform code to do any SMP setup.

After applying this patch it's possible to boot an ARM system
with maxcpus=1 on the command line and then hotplug in secondary
cpus via sysfs. This is more in line with how x86 does things.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: David Brown <davidb@codeaurora.org>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/smp.c

@@ -361,14 +361,21 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 	 */
 	if (max_cpus > ncores)
 		max_cpus = ncores;
-
-	if (max_cpus > 1) {
+	if (ncores > 1 && max_cpus) {
 		/*
 		 * Enable the local timer or broadcast device for the
 		 * boot CPU, but only if we have more than one CPU.
 		 */
 		percpu_timer_setup();
 
+		/*
+		 * Initialise the present map, which describes the set of CPUs
+		 * actually populated at the present time. A platform should
+		 * re-initialize the map in platform_smp_prepare_cpus() if
+		 * present != possible (e.g. physical hotplug).
+		 */
+		init_cpu_present(&cpu_possible_map);
+
 		/*
 		 * Initialise the SCU if there are more than one CPU
 		 * and let them know where to start.

arch/arm/mach-exynos4/platsmp.c

@@ -154,14 +154,6 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	/*
-	 * Initialise the present map, which describes the set of CPUs
-	 * actually populated at the present time.
-	 */
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
 
 	scu_enable(scu_base_addr());
 

arch/arm/mach-msm/platsmp.c

@@ -157,12 +157,4 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	/*
-	 * Initialise the present map, which describes the set of CPUs
-	 * actually populated at the present time.
-	 */
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
 }

arch/arm/mach-omap2/omap-smp.c

@@ -125,14 +125,6 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	/*
-	 * Initialise the present map, which describes the set of CPUs
-	 * actually populated at the present time.
-	 */
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
 
 	/*
 	 * Initialise the SCU and wake up the secondary core using

arch/arm/mach-realview/platsmp.c

@@ -68,14 +68,6 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	/*
-	 * Initialise the present map, which describes the set of CPUs
-	 * actually populated at the present time.
-	 */
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
 
 	scu_enable(scu_base_addr());
 

arch/arm/mach-shmobile/platsmp.c

@@ -64,10 +64,5 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
-
 	shmobile_smp_prepare_cpus();
 }

arch/arm/mach-tegra/platsmp.c

@@ -129,14 +129,6 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	/*
-	 * Initialise the present map, which describes the set of CPUs
-	 * actually populated at the present time.
-	 */
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
 
 	scu_enable(scu_base);
 }

arch/arm/mach-ux500/platsmp.c

@@ -172,14 +172,6 @@ void __init smp_init_cpus(void)
 
 void __init platform_smp_prepare_cpus(unsigned int max_cpus)
 {
-	int i;
-
-	/*
-	 * Initialise the present map, which describes the set of CPUs
-	 * actually populated at the present time.
-	 */
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
 
 	scu_enable(scu_base_addr());
 	wakeup_secondary();

arch/arm/mach-vexpress/ct-ca9x4.c

@@ -229,10 +229,6 @@ static void ct_ca9x4_init_cpu_map(void)
 
 static void ct_ca9x4_smp_enable(unsigned int max_cpus)
 {
-	int i;
-	for (i = 0; i < max_cpus; i++)
-		set_cpu_present(i, true);
-
 	scu_enable(MMIO_P2V(A9_MPCORE_SCU));
 }
 #endif