[PATCH 2/4] ARM64: mmu: document granule size calculation

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Copy a nice-looking ASCII table from U-Boot's cache_v8.c to help with
understanding the function.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 arch/arm/cpu/mmu_64.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/arm/cpu/mmu_64.c b/arch/arm/cpu/mmu_64.c
index bc1a44d0a7b8..d9b0b74d7314 100644
--- a/arch/arm/cpu/mmu_64.c
+++ b/arch/arm/cpu/mmu_64.c
@@ -176,6 +176,24 @@ static void create_sections(uint64_t virt, uint64_t phys, uint64_t size,
 
 static size_t granule_size(int level)
 {
+	/*
+	 *  With 4k page granule, a virtual address is split into 4 lookup parts
+	 *  spanning 9 bits each:
+	 *
+	 *    _______________________________________________
+	 *   |       |       |       |       |       |       |
+	 *   |   0   |  Lv0  |  Lv1  |  Lv2  |  Lv3  |  off  |
+	 *   |_______|_______|_______|_______|_______|_______|
+	 *     63-48   47-39   38-30   29-21   20-12   11-00
+	 *
+	 *             mask        page size
+	 *
+	 *    Lv0: FF8000000000       --
+	 *    Lv1:   7FC0000000       1G
+	 *    Lv2:     3FE00000       2M
+	 *    Lv3:       1FF000       4K
+	 *    off:          FFF
+	 */
 	switch (level) {
 	default:
 	case 0:
-- 
2.39.5