arm64: mm: Preserve existing table mappings when mapping DRAM

Instead of blindly overwriting an existing table entry when mapping DRAM
regions, take care not to replace a pre-existing table entry with a
block entry. This permits the logic of mapping the kernel's linear alias
to be simplified in a subsequent patch.

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

Author: ardbiesheuvel

arch/arm64/mm/mmu.c

@@ -256,7 +256,8 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
 
 		/* try section mapping first */
 		if (((addr | next | phys) & ~PMD_MASK) == 0 &&
-		    (flags & NO_BLOCK_MAPPINGS) == 0) {
+		    (flags & NO_BLOCK_MAPPINGS) == 0 &&
+		    !pmd_table(old_pmd)) {
 			WARN_ON(!pmd_set_huge(pmdp, phys, prot));
 
 			/*
@@ -379,7 +380,8 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
 		 */
 		if (pud_sect_supported() &&
 		   ((addr | next | phys) & ~PUD_MASK) == 0 &&
-		    (flags & NO_BLOCK_MAPPINGS) == 0) {
+		    (flags & NO_BLOCK_MAPPINGS) == 0 &&
+		    !pud_table(old_pud)) {
 			WARN_ON(!pud_set_huge(pudp, phys, prot));
 
 			/*