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>

1 files changed, 9 insertions, 2 deletions

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 344e52b16c8c..0ffcf5c0da43 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -361,8 +361,7 @@ 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.
@@ -370,6 +369,14 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
                 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.
                  */