[U-Boot] [PATCH] arm: start.S: Fix startup of dragonboard410c

[Mateusz Kulikowski <mateusz.kulikowski@gmail.com>]

Commit d73718f3 breaks devices where:
- There is EL2/EL3 firmware and
- U-Boot starts in NS EL1 and
- EL2/EL3 firmware didn't unlocked write-access to CPUECTLR_EL1.

This patch makes that change opt-out configuration option,
and disables it for dragonboard410c.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
---
I make it opt-out so nobody will complain that I broke someones board.

If you prefer opt-in - just let me know, I'll flip the switch
in Kconfig.

BTW. I wonder if this register should be written at all on devices
that implement ARMv8 but are *not* ARM Cortex.

Mateusz

 arch/arm/cpu/armv8/Kconfig        | 13 +++++++++++++
 arch/arm/cpu/armv8/start.S        |  7 +++++--
 configs/dragonboard410c_defconfig |  1 +
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
index 3d19bbf..33af0a2 100644
--- a/arch/arm/cpu/armv8/Kconfig
+++ b/arch/arm/cpu/armv8/Kconfig
@@ -3,4 +3,17 @@ if ARM64
 config ARMV8_MULTIENTRY
         boolean "Enable multiple CPUs to enter into U-Boot"
 
+config ARMV8_SET_SMPEN
+        boolean "Enable data coherency with other cores in cluster"
+        default y if ARM64
+        help
+          Cortex A53/57/72 cores require CPUECTLR_EL1.SMPEN set even
+          for single core systems. Unfortunately write access to this
+          register may be controlled by EL3/EL2 firmware. To be more
+          precise, by default (if there is EL2/EL3 firmware running)
+          this register is RO for NS EL1.
+          This switch can be used to avoid writing to CPUECTLR_EL1,
+          it can be safely enabled when El2/EL3 initialized SMPEN bit
+          or when CPU implementation doesn't include that register.
+
 endif
diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
index dfce469..777cad3 100644
--- a/arch/arm/cpu/armv8/start.S
+++ b/arch/arm/cpu/armv8/start.S
@@ -81,13 +81,16 @@ reset:
 	msr	cpacr_el1, x0			/* Enable FP/SIMD */
 0:
 
-	/* Enalbe SMPEN bit for coherency.
+	/* Enable SMPEN bit for coherency.
 	 * This register is not architectural but at the moment
 	 * this bit should be set for A53/A57/A72.
+	 *
 	 */
-	mrs     x0, S3_1_c15_c2_1               /* cpuactlr_el1 */
+#ifdef CONFIG_ARMV8_SET_SMPEN
+	mrs     x0, S3_1_c15_c2_1               /* cpuectlr_el1 */
 	orr     x0, x0, #0x40
 	msr     S3_1_c15_c2_1, x0
+#endif
 
 	/* Apply ARM core specific erratas */
 	bl	apply_core_errata
diff --git a/configs/dragonboard410c_defconfig b/configs/dragonboard410c_defconfig
index ad2e8b8..d3cfa69 100644
--- a/configs/dragonboard410c_defconfig
+++ b/configs/dragonboard410c_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_ARCH_SNAPDRAGON=y
+CONFIG_ARMV8_SET_SMPEN=n
 CONFIG_DEFAULT_DEVICE_TREE="dragonboard410c"
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="dragonboard410c => "
-- 
2.7.4