[PATCH 2/2] ARM: optee-early: invalidate caches before jump to OP-TEE

[Fabian Pflug <f.pflug@pengutronix.de>]

The optee-early code was initially added for i.MX6UL. Trying to naively
enable it on an i.MX6Q boards was observed to cause spurious hangs on
return from OP-TEE to barebox.

The root cause seems to be inadequate cache handling by OP-TEE: OP-TEE
enables the MMU and caches with it, but didn't take care to invalidate
all cache lines before enabling the MMU, which triggered the
aforementioned hangs.

To paper over this issue, let's just invalidate the cache lines on the
barebox side instead before jumping to OP-TEE. This issue did likely not
affect the original i.MX6UL, because its Cortex-A7 has an architected L2
cache that's guaranteed zeroed (no dirty cache lines) on power-on reset,
unlike the i.MX6Q's Cortex-A9, where the external L2 cache powers on
with unpredictable content including the dirty bits.

This means on e.g. the i.MX6UL, we will now do one extra cache invalidation
that's not needed. This should be negligible and we are already had an
unconditional invalidation in __barebox_arm_entry.

Note that this is a different implementation than what we do on ARM64,
there we load TF-A before it jumps to OP-TEE and assuming
non-architected caches or caches with uninitialized content on power-on
to be a dying breed, our ARM64 implementation is likely not affected.

Co-authored-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Fabian Pflug <f.pflug@pengutronix.de>
---
 arch/arm/lib32/optee-early.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm/lib32/optee-early.c b/arch/arm/lib32/optee-early.c
index 0cda0ab163..b1dba67d42 100644
--- a/arch/arm/lib32/optee-early.c
+++ b/arch/arm/lib32/optee-early.c
@@ -35,6 +35,19 @@ int start_optee_early(void *fdt, void *tee)
 	/* We use setjmp/longjmp here because OP-TEE clobbers most registers */
 	ret = setjmp(tee_buf);
 	if (ret == 0) {
+		/*
+		 * At least OP-TEE v4.1.0 seems to not invalidate all dirty cache
+		 * lines before enabling the MMU. This can lead to spurious hangs
+		 * on return to barebox on systems where there might be left-over
+		 * dirty cache lines, whether from BootROM or because L2 cache
+		 * is non-architected and powers on with unpredictable content
+		 * like is the case with PL310 on i.MX6Q.
+		 *
+		 * Let's invalidate the caches here, so board entry points need
+		 * not bother.
+		 */
+		arm_early_mmu_cache_invalidate();
+
 		tee_start(0, 0, fdt);
 		longjmp(tee_buf, 1);
 	}
-- 
2.39.5