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>

1 files changed, 19 insertions, 19 deletions

diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c
index 98c5a9737c14..e2d8e199be32 100644
--- 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;