[PATCH] arc: use hardware ARCNUM in smp_processor_id()

[Alexey Brodkin]

We used to think that ARC cores in SMP SoC start
consequentially, i.e. core0 -> core1 -> core2 -> core4.

Moreover we treat core0 as a master core which does some
low-level initialization before allowing other cores to
start doing real stuff.

In that case everything works as expected - smp_processor_id()
returns expected values for all cores, i.e.:
 0 for core0
 1 for core1 etc.

But what if instead of core0 we want core1 to be the master?
That might be the case if we want to use only one core out of
SMP setup and that core is not core0 or for some other reason
modify core start-up sequence to say: core3 -> core1 > ...

In that case smp_processor_id() returns values that differ
from real hardware core index. For the first/master core that
we'l get 0, the next one will be 1 etc.

The problem here is we'll use improper cpu indexes in MCIP commands
and inevitably commands will be sent to unexpected cores causing all
sorts of unexpected behavior.

But if we use hardware core index out out IDENTITY AUX reg that problem
won't happen because cpu value will match its hardware index.

Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/arc/include/asm/smp.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arc/include/asm/smp.h b/arch/arc/include/asm/smp.h
index 0861007d9ef3..5aad65d3defd 100644
--- a/arch/arc/include/asm/smp.h
+++ b/arch/arc/include/asm/smp.h
@@ -14,8 +14,9 @@
 #include <linux/types.h>
 #include <linux/init.h>
 #include <linux/threads.h>
+#include <asm/arcregs.h>
 
-#define raw_smp_processor_id() (current_thread_info()->cpu)
+#define raw_smp_processor_id() ((int)(read_aux_reg(AUX_IDENTITY) >> 8) & 0xFF)
 
 /* including cpumask.h leads to cyclic deps hence this Forward declaration */
 struct cpumask;
-- 
2.7.4