[PATCH] arm64/mm: Describe TTBR1_BADDR_4852_OFFSET

[Anshuman Khandual]

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: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
---
This applies on v7.0-rc1

Earlier context can be found here.

https://lore.kernel.org/linux-arm-kernel/aRb8ezhQd0c0jp9G@J2N7QTR9R3/

 arch/arm64/include/asm/pgtable-hwdef.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index 5e6809a462c7..72f31800c703 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -285,9 +285,12 @@
 #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
-- 
2.30.2