ar71xx: fix occasional kernel hangs during boot on AR934x

The patch taken from the linux-mips mailing list.

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.

SVN-Revision: 29009

target/linux/ar71xx/patches-2.6.39/902-fix-occasional-kernel-hangs-during-boot.patch

@@ -0,0 +1,104 @@
+From patchwork Tue Nov  8 14:59:01 2011
+Content-Type: text/plain; charset="utf-8"
+MIME-Version: 1.0
+Content-Transfer-Encoding: 7bit
+Subject: MIPS: Kernel hangs occasionally during boot.
+Date: Tue, 08 Nov 2011 13:59:01 -0000
+From: Al Cooper <alcooperx@gmail.com>
+X-Patchwork-Id: 2911
+Message-Id: <1320764341-4275-1-git-send-email-alcooperx@gmail.com>
+To: ralf@linux-mips.org, linux-mips@linux-mips.org,
+	linux-kernel@vger.kernel.org
+Cc: "Al Cooper" <alcooperx@gmail.com>
+
+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>
+
+---
+arch/mips/kernel/cevt-r4k.c |   38 +++++++++++++++++++-------------------
+ 1 files changed, 19 insertions(+), 19 deletions(-)
+
+--- 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;
+