ARCv2: intc: default all interrupts to priority 1

ARC HS Cores support configurable multiple interrupt priorities of upto
16 levels. In commit dec2b284 ("ARCv2: intc: Allow interruption by
lowest priority interrupt") we switched to 15 which seems a bit
excessive given that there would be rare hardware implementing so many
preemption levels AND running Linux. It would seem that 2 levels will be
more common so switch to 1 as the default priority level. This will be
the "lower" priority level saving 0 for implementing NMI style support.

This scheme also works in systems with more than 2 prioity levels as
well.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/irqflags-arcv2.h

@@ -38,10 +38,10 @@
 #define AUX_IRQ_ACT_BIT_U	31
 
 /*
- * User space should be interruptable even by lowest prio interrupt
- * Safe even if actual interrupt priorities is fewer or even one
+ * Hardware supports 16 priorities (0 highest, 15 lowest)
+ * Linux by default runs at 1, priority 0 reserved for NMI style interrupts
  */
-#define ARCV2_IRQ_DEF_PRIO	15
+#define ARCV2_IRQ_DEF_PRIO	1
 
 /* seed value for status register */
 #define ISA_INIT_STATUS_BITS	(STATUS_IE_MASK | STATUS_AD_MASK | \

arch/arc/kernel/intc-arcv2.c

@@ -14,8 +14,6 @@
 #include <linux/irqchip.h>
 #include <asm/irq.h>
 
-static int irq_prio;
-
 /*
  * Early Hardware specific Interrupt setup
  * -Called very early (start_kernel -> setup_arch -> setup_processor)
@@ -24,7 +22,7 @@ static int irq_prio;
  */
 void arc_init_IRQ(void)
 {
-	unsigned int tmp;
+	unsigned int tmp, irq_prio;
 
 	struct irq_build {
 #ifdef CONFIG_CPU_BIG_ENDIAN
@@ -67,12 +65,12 @@ void arc_init_IRQ(void)
 
 	irq_prio = irq_bcr.prio;	/* Encoded as N-1 for N levels */
 	pr_info("archs-intc\t: %d priority levels (default %d)%s\n",
-		irq_prio + 1, irq_prio,
+		irq_prio + 1, ARCV2_IRQ_DEF_PRIO,
 		irq_bcr.firq ? " FIRQ (not used)":"");
 
 	/* setup status32, don't enable intr yet as kernel doesn't want */
 	tmp = read_aux_reg(0xa);
-	tmp |= STATUS_AD_MASK | (irq_prio << 1);
+	tmp |= STATUS_AD_MASK | (ARCV2_IRQ_DEF_PRIO << 1);
 	tmp &= ~STATUS_IE_MASK;
 	asm volatile("kflag %0	\n"::"r"(tmp));
 }
@@ -93,7 +91,7 @@ void arcv2_irq_enable(struct irq_data *data)
 {
 	/* set default priority */
 	write_aux_reg(AUX_IRQ_SELECT, data->irq);
-	write_aux_reg(AUX_IRQ_PRIORITY, irq_prio);
+	write_aux_reg(AUX_IRQ_PRIORITY, ARCV2_IRQ_DEF_PRIO);
 
 	/*
 	 * hw auto enables (linux unmask) all by default