[MIPS] Probe for usability of cp0 compare interrupt.

Some processors offer the option of using the interrupt on which
normally the count / compare interrupt would be signaled as a normal
interupt pin.  Previously this required some ugly hackery for each
system which is much easier done by a quick and simple probe.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 47 insertions, 0 deletions

diff --git a/arch/mips/kernel/time.c b/arch/mips/kernel/time.c
index 05b365167a09..e4b5e647b142 100644
--- a/arch/mips/kernel/time.c
+++ b/arch/mips/kernel/time.c
@@ -391,6 +391,50 @@ static void mips_event_handler(struct clock_event_device *dev)
 {
 }
 
+/*
+ * FIXME: This doesn't hold for the relocated E9000 compare interrupt.
+ */
+static int c0_compare_int_pending(void)
+{
+        return (read_c0_cause() >> cp0_compare_irq) & 0x100;
+}
+
+static int c0_compare_int_usable(void)
+{
+        const unsigned int delta = 0x300000;
+        unsigned int cnt;
+
+        /*
+         * IP7 already pending?  Try to clear it by acking the timer.
+         */
+        if (c0_compare_int_pending()) {
+                write_c0_compare(read_c0_compare());
+                irq_disable_hazard();
+                if (c0_compare_int_pending())
+                        return 0;
+        }
+
+        cnt = read_c0_count();
+        cnt += delta;
+        write_c0_compare(cnt);
+
+        while ((long)(read_c0_count() - cnt) <= 0)
+                ;       /* Wait for expiry  */
+
+        if (!c0_compare_int_pending())
+                return 0;
+
+        write_c0_compare(read_c0_compare());
+        irq_disable_hazard();
+        if (c0_compare_int_pending())
+                return 0;
+
+        /*
+         * Feels like a real count / compare timer.
+         */
+        return 1;
+}
+
 void __cpuinit mips_clockevent_init(void)
 {
         uint64_t mips_freq = mips_hpt_frequency;
@@ -412,6 +456,9 @@ void __cpuinit mips_clockevent_init(void)
                 return;
 #endif
 
+        if (!c0_compare_int_usable())
+                return;
+
         cd = &per_cpu(mips_clockevent_device, cpu);
 
         cd->name                = "MIPS";