[PATCH v2 1/3] arm/cpu/lowlevel: invalidate i-cache before enabling

[Uwe Kleine-König]

Architecturally the cache contents are undefined so it might well
contain stale data at reset. So better be save than sorry.

I verifyed that the added instructions are defined for both, ARMv6 and
ARMv7, using the ARM Architecture Reference Manual, ARMv7-A and ARMv7-R
edition (ARM DDI 0406C.c). For the already existing mcr instruction see
the newly added comment.

This patch also unifies handling of ARMv6 and ARMv7, the isb instruction
can also be done on the latter via mcr which simplifies the code a bit.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
---
 arch/arm/cpu/lowlevel.S | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/arm/cpu/lowlevel.S b/arch/arm/cpu/lowlevel.S
index c615d5b58160..e000cd8eae4c 100644
--- a/arch/arm/cpu/lowlevel.S
+++ b/arch/arm/cpu/lowlevel.S
@@ -11,9 +11,26 @@ ENTRY(arm_cpu_lowlevel_init)
 	orr	r12, r12, #0xd3
 	msr	cpsr, r12
 
-#if __LINUX_ARM_ARCH__ >= 7
-	isb
-#elif __LINUX_ARM_ARCH__ == 6
+#if __LINUX_ARM_ARCH__ >= 6
+	/*
+	 * ICIALLU: Invalidate all instruction caches to PoU,
+	 * includes flushing of branch predictors.
+	 * Even if the i-cache is off it might contain stale entries
+	 * that are better discarded before enabling the cache.
+	 * Architectually this is even possible after a cold reset.
+	 */
+	mcr	p15, 0, r12, c7, c5, 0
+	/* DSB, ensure completion of the invalidation */
+	mcr	p15, 0, r12, c7, c10, 4
+	/*
+	 * ISB, ensure instruction fetch path is in sync.
+	 * Note that the ARM Architecture Reference Manual, ARMv7-A and ARMv7-R
+	 * edition (ARM DDI 0406C.c) doesn't define this instruction in the
+	 * ARMv6 part (D12.7.10). It only has: "Support of additional
+	 * operations is IMPLEMENTATION DEFINED".
+	 * But an earlier version of the ARMARM (ARM DDI 0100I) does define it
+	 * as "Flush prefetch buffer (PrefetchFlush)".
+	 */
 	mcr	p15, 0, r12, c7, c5, 4
 #endif
 
-- 
2.1.3