arm64/mm: Describe TTBR1_BADDR_4852_OFFSET

TTBR1_BADDR_4852_OFFSET is a constant offset which gets added into kernel
page table physical address for TTBR1_EL1 when kernel is build for 52 bit
VA but found to be running on 48 bit VA capable system. Although there is
no explanation on how the macro is computed.

Describe TTBR1_BADDR_4852_OFFSET computation in detail via deriving from
all required parameters involved thus improving clarity and readability.

Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Anshuman Khandual

arch/arm64/include/asm/pgtable-hwdef.h

@@ -287,9 +287,12 @@
 #endif
 
 #ifdef CONFIG_ARM64_VA_BITS_52
+#define PTRS_PER_PGD_52_VA (UL(1) << (52 - PGDIR_SHIFT))
+#define PTRS_PER_PGD_48_VA (UL(1) << (48 - PGDIR_SHIFT))
+#define PTRS_PER_PGD_EXTRA (PTRS_PER_PGD_52_VA - PTRS_PER_PGD_48_VA)
+
 /* Must be at least 64-byte aligned to prevent corruption of the TTBR */
-#define TTBR1_BADDR_4852_OFFSET	(((UL(1) << (52 - PGDIR_SHIFT)) - \
-				 (UL(1) << (48 - PGDIR_SHIFT))) * 8)
+#define TTBR1_BADDR_4852_OFFSET (PTRS_PER_PGD_EXTRA << PTDESC_ORDER)
 #endif
 
 #endif