arm64: Add mmu context save/restore APIs

New API required by sdm845 DDR init/training protocol

TEST=build & run

Change-Id: I8442442c0588dd6fb5e461b399e48a761f7bbf29
Signed-off-by: T Michael Turney <mturney@codeaurora.org>
Reviewed-on: https://review.coreboot.org/25818
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/arch/arm64/armv8/mmu.c

@@ -274,6 +274,40 @@ void mmu_init(void)
 			  TCR_TBI_USED);
 }
 
+/* Func : mmu_save_context
+ * Desc : Save mmu context (registers and ttbr base).
+ */
+void mmu_save_context(struct mmu_context *mmu_context)
+{
+	assert(mmu_context);
+
+	/* Back-up MAIR_ATTRIBUTES */
+	mmu_context->mair = raw_read_mair_el3();
+
+	/* Back-up TCR value */
+	mmu_context->tcr = raw_read_tcr_el3();
+}
+
+/* Func : mmu_restore_context
+ * Desc : Restore mmu context using input backed-up context
+ */
+void mmu_restore_context(const struct mmu_context *mmu_context)
+{
+	assert(mmu_context);
+
+	/* Restore TTBR */
+	raw_write_ttbr0_el3((uintptr_t)_ttb);
+
+	/* Restore MAIR indices */
+	raw_write_mair_el3(mmu_context->mair);
+
+	/* Restore TCR flags */
+	raw_write_tcr_el3(mmu_context->tcr);
+
+	/* invalidate tlb since ttbr is updated. */
+	tlb_invalidate_all();
+}
+
 void mmu_enable(void)
 {
 	if (((get_pte(_ttb) >> BLOCK_INDEX_SHIFT) & BLOCK_INDEX_MASK)

src/arch/arm64/include/armv8/arch/mmu.h

@@ -148,8 +148,17 @@
 #define TCR_TBI_USED               (0x0 << TCR_TBI_SHIFT)
 #define TCR_TBI_IGNORED            (0x1 << TCR_TBI_SHIFT)
 
+struct mmu_context {
+	uint64_t mair;         /* MAIR attributes */
+	uint64_t tcr;          /* TCR Attributes */
+};
+
 /* Initialize MMU registers and page table memory region. */
 void mmu_init(void);
+/* Desc : Save mmu context (registers and ttbr base/size). */
+void mmu_save_context(struct mmu_context *mmu_context);
+/* Desc : Restore mmu context using input backed-up context */
+void mmu_restore_context(const struct mmu_context *mmu_context);
 /* Change a memory type for a range of bytes at runtime. */
 void mmu_config_range(void *start, size_t size, uint64_t tag);
 /* Enable the MMU (need previous mmu_init() and configured ranges!). */