MIPS: Read CPU IRQ line that FDC to routed to

Read the CPU IRQ line reportedly used for the Fast Debug Channel (FDC)
interrupt from the IntCtl register and store it in cp0_fdc_irq where
platform implementations of the new weak platform function
get_c0_fdc_int() can refer to it.

[ralf@linux-mips.org: Fixed conflict.]
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: James Hogan <james.hogan@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/9140/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/irq.h

@@ -47,6 +47,9 @@ extern void free_irqno(unsigned int irq);
 extern int cp0_compare_irq;
 extern int cp0_compare_irq_shift;
 extern int cp0_perfcount_irq;
+extern int cp0_fdc_irq;
+
+extern int __weak get_c0_fdc_int(void);
 
 void arch_trigger_all_cpu_backtrace(bool);
 #define arch_trigger_all_cpu_backtrace arch_trigger_all_cpu_backtrace

arch/mips/kernel/traps.c

@@ -1969,6 +1969,12 @@ int cp0_compare_irq_shift;
 int cp0_perfcount_irq;
 EXPORT_SYMBOL_GPL(cp0_perfcount_irq);
 
+/*
+ * Fast debug channel IRQ or -1 if not present
+ */
+int cp0_fdc_irq;
+EXPORT_SYMBOL_GPL(cp0_fdc_irq);
+
 static int noulri;
 
 static int __init ulri_disable(char *s)
@@ -2050,15 +2056,21 @@ void per_cpu_trap_init(bool is_boot_cpu)
 	 *
 	 *  o read IntCtl.IPTI to determine the timer interrupt
 	 *  o read IntCtl.IPPCI to determine the performance counter interrupt
+	 *  o read IntCtl.IPFDC to determine the fast debug channel interrupt
 	 */
 	if (cpu_has_mips_r2_r6) {
 		cp0_compare_irq_shift = CAUSEB_TI - CAUSEB_IP;
 		cp0_compare_irq = (read_c0_intctl() >> INTCTLB_IPTI) & 7;
 		cp0_perfcount_irq = (read_c0_intctl() >> INTCTLB_IPPCI) & 7;
+		cp0_fdc_irq = (read_c0_intctl() >> INTCTLB_IPFDC) & 7;
+		if (!cp0_fdc_irq)
+			cp0_fdc_irq = -1;
+
 	} else {
 		cp0_compare_irq = CP0_LEGACY_COMPARE_IRQ;
 		cp0_compare_irq_shift = CP0_LEGACY_PERFCNT_IRQ;
 		cp0_perfcount_irq = -1;
+		cp0_fdc_irq = -1;
 	}
 
 	if (!cpu_data[cpu].asid_cache)