ARC: SMP resurrect foss-for-synopsys-dwc-arc-processors#29: [plat-arc…

…fpga] userspace prints locking bootup

This happens occasionally, when init task starts to run on CPU1, making
it's STDOUT open call land in ARC serial driver. The request_irq() call
is thus made from CPU1, leading to low level IRQ unmasking on CPU1 only,
whereas the UART IRQ is hardwired to CPU0. Thus despite the driver being
SMP re-entrant, the driver Tx locks up because TX_DONE interrupts are
asserted for CPU0, which is just not listening for them.

The workaround for now, is to unconditionally enable UART IRQ on it's
"home-cpu". Need to find the "right" way.

Signed-off-by: Vineet Gupta <[email protected]>

arch/arc/plat-arcfpga/irq.c

@@ -39,4 +39,14 @@ void __init plat_init_IRQ()
 	for (i = 0; i < NR_IRQS; i++) {
 		irq_set_chip_and_handler(i, &fpga_chip, handle_level_irq);
 	}
+
+	/*
+	 * 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
 }