MIPS: Kernel hangs occasionally during boot.

The Kernel hangs occasionally during boot after "Calibrating delay loop..".
This is caused by the c0_compare_int_usable() routine in cevt-r4k.c
returning false which causes the system to disable the timer and hang later.
The false return happens because the routine is using a series of four calls
to irq_disable_hazard() as a delay while it waits for the timer changes to
propagate to the cp0 cause register. On newer MIPS cores, like the 74K, the
series of irq_disable_hazard() calls turn into ehb instructions and can take
as little as a few clock ticks for all 4 instructions. This is not enough of
a delay, so the routine thinks the timer is not working.  This fix uses up
to a max number of cycle counter ticks for the delay and uses
back_to_back_c0_hazard() instead of irq_disable_hazard() to handle the
hazard condition between cp0 writes and cp0 reads.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2911/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c
index 98c5a9737c14d2c200a161640122f9a8af053e88..e2d8e199be323345a6feaf3eb03a738e515dd5f5 100644 (file)
--- a/arch/mips/kernel/cevt-r4k.c
+++ b/arch/mips/kernel/cevt-r4k.c
@@ -103,19 +103,10 @@ static int c0_compare_int_pending(void)
 
 /*
  * Compare interrupt can be routed and latched outside the core,
- * so a single execution hazard barrier may not be enough to give
- * it time to clear as seen in the Cause register.  4 time the
- * pipeline depth seems reasonably conservative, and empirically
- * works better in configurations with high CPU/bus clock ratios.
+ * so wait up to worst case number of cycle counter ticks for timer interrupt
+ * changes to propagate to the cause register.
  */
-
-#define compare_change_hazard() \
-       do { \
-               irq_disable_hazard(); \
-               irq_disable_hazard(); \
-               irq_disable_hazard(); \
-               irq_disable_hazard(); \
-       } while (0)
+#define COMPARE_INT_SEEN_TICKS 50
 
 int c0_compare_int_usable(void)
 {
@@ -126,8 +117,12 @@ int c0_compare_int_usable(void)
         * IP7 already pending?  Try to clear it by acking the timer.
         */
        if (c0_compare_int_pending()) {
-               write_c0_compare(read_c0_count());
-               compare_change_hazard();
+               cnt = read_c0_count();
+               write_c0_compare(cnt);
+               back_to_back_c0_hazard();
+               while (read_c0_count() < (cnt  + COMPARE_INT_SEEN_TICKS))
+                       if (!c0_compare_int_pending())
+                               break;
                if (c0_compare_int_pending())
                        return 0;
        }
@@ -136,7 +131,7 @@ int c0_compare_int_usable(void)
                cnt = read_c0_count();
                cnt += delta;
                write_c0_compare(cnt);
-               compare_change_hazard();
+               back_to_back_c0_hazard();
                if ((int)(read_c0_count() - cnt) < 0)
                    break;
                /* increase delta if the timer was already expired */
@@ -145,12 +140,17 @@ int c0_compare_int_usable(void)
        while ((int)(read_c0_count() - cnt) <= 0)
                ;       /* Wait for expiry  */
 
-       compare_change_hazard();
+       while (read_c0_count() < (cnt + COMPARE_INT_SEEN_TICKS))
+               if (c0_compare_int_pending())
+                       break;
        if (!c0_compare_int_pending())
                return 0;
-
-       write_c0_compare(read_c0_count());
-       compare_change_hazard();
+       cnt = read_c0_count();
+       write_c0_compare(cnt);
+       back_to_back_c0_hazard();
+       while (read_c0_count() < (cnt + COMPARE_INT_SEEN_TICKS))
+               if (!c0_compare_int_pending())
+                       break;
        if (c0_compare_int_pending())
                return 0;