[PATCH v6 3/8] arm: snapdragon: Disable MMU early before U-Boot reset vector

[Balaji Selvanathan via U-Boot <u-boot@lists.u-boot-project.org>]

When U-Boot is loaded by XBL in Snagboot mode, XBL leaves the MMU
enabled with its own page tables. This causes stale TLB entries and
incorrect memory mappings when U-Boot initializes its own MMU.

Disable the MMU and invalidate TLBs before branching to the
reset vector. This ensures a clean MMU state for U-Boot initialization
in Snagboot mode.

Signed-off-by: Balaji Selvanathan <balaji.selvanathan@oss.qualcomm.com>
---
Changes iv v5:
- Clear C bit (Dcache enable) also during MMU disable

Changes iv v4:
- Remove handling for EL1 or EL2 entry as for Snagboot we would
  only enter U-Boot in EL3 from previous bootloader

Changes in v3:
- Brought the MMU disable logic under CONFIG_QCOM_SNAGBOOT_SUPPORT
  ifdef condition
- Moved the MMU disable codes to a new seperate file

Changes in v2:
- Newly introduced in v2
---
 arch/arm/mach-snapdragon/include/mach/boot0.h      |  2 ++
 .../mach-snapdragon/include/mach/snagboot_boot0.h  | 27 ++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/arch/arm/mach-snapdragon/include/mach/boot0.h b/arch/arm/mach-snapdragon/include/mach/boot0.h
index b3c76d6d97d..a4eb5723d2e 100644
--- a/arch/arm/mach-snapdragon/include/mach/boot0.h
+++ b/arch/arm/mach-snapdragon/include/mach/boot0.h
@@ -4,6 +4,8 @@
 #else
 #if defined(CONFIG_BOOT0_MSM8916_PSCI_WORKAROUND)
 #include "msm8916_boot0.h"
+#elif defined(CONFIG_QCOM_BOOT0_SNAGBOOT_MODE)
+#include "snagboot_boot0.h"
 #else
 	b	reset
 #endif
diff --git a/arch/arm/mach-snapdragon/include/mach/snagboot_boot0.h b/arch/arm/mach-snapdragon/include/mach/snagboot_boot0.h
new file mode 100644
index 00000000000..f82ee87c9b9
--- /dev/null
+++ b/arch/arm/mach-snapdragon/include/mach/snagboot_boot0.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Early MMU Disable for Snagboot Mode
+ *
+ * Disable MMU at the earliest possible point for EL3 entry.
+ * Snagboot mode always enters U-Boot at EL3.
+ *
+ * After disabling MMU, invalidate TLB to clear any stale entries that might
+ * cause issues when MMU is re-enabled later.
+ */
+
+	mrs	x0, CurrentEL
+	cmp	x0, #(3 << 2)	/* Verify EL3 entry */
+	b.ne	reset		/* Unexpected: not at EL3 */
+
+	/* Disable MMU at EL3 */
+	mrs	x0, sctlr_el3
+	bic	x0, x0, #1	/* Clear M bit (MMU enable) */
+	bic	x0, x0, #(1 << 2)   /* Clear C bit (Dcache enable) */
+	bic	x0, x0, #(1 << 19)  /* Clear WXN bit (Write XOR Execute) */
+	msr	sctlr_el3, x0
+	isb
+	/* Invalidate entire TLB for all ELs */
+	tlbi	alle3
+	dsb	sy
+	isb
+	b	reset
\ No newline at end of file

-- 
2.34.1