ARC: SMP support

ARC common code to enable a SMP system + ISS provided SMP extensions.

ARC700 natively lacks SMP support, hence some of the core features are
are only enabled if SoCs have the necessary h/w pixie-dust. This
includes:
-Inter Processor Interrupts (IPI)
-Cache coherency
-load-locked/store-conditional
...

The low level exception handling would be completely broken in SMP
because we don't have hardware assisted stack switching. Thus a fair bit
of this code is repurposing the MMU_SCRATCH reg for event handler
prologues to keep them re-entrant.

Many thanks to Rajeshwar Ranga for his initial "major" contributions to
SMP Port (back in 2008), and to Noam Camus and Gilad Ben-Yossef for help
with resurrecting that in 3.2 kernel (2012).

Note that this platform code is again singleton design pattern - so
multiple SMP platforms won't build at the moment - this deficiency is
addressed in subsequent patches within this series.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rajeshwar Ranga <rajeshwar.ranga@gmail.com>
Cc: Noam Camus <noamc@ezchip.com>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/arc/plat-arcfpga/irq.c b/arch/arc/plat-arcfpga/irq.c
index ed726360b9f6..590edd174c47 100644
--- a/arch/arc/plat-arcfpga/irq.c
+++ b/arch/arc/plat-arcfpga/irq.c
@@ -9,7 +9,17 @@
  */
 
 #include <linux/interrupt.h>
+#include <asm/irq.h>
 
 void __init plat_init_IRQ(void)
 {
+        /*
+         * SMP Hack because UART IRQ hardwired to cpu0 (boot-cpu) but if the
+         * request_irq() comes from any other CPU, the low level IRQ unamsking
+         * essential for getting Interrupts won't be enabled on cpu0, locking
+         * up the UART state machine.
+         */
+#ifdef CONFIG_SMP
+        arch_unmask_irq(UART0_IRQ);
+#endif
 }