[PATCH RFC v2 02/20] ARM: mm: make 2-level pgd_t a scalar

[Yeoreum Yun <yeoreum.yun@arm.com>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

We don't want pgd_t to be an array, as it prohibits returning it from a
function, like pgdp_get().

So let's just use an u64, and extract the right 32bit value in
pgd_val().

Leave the STRICT_MM_TYPECHECKS case alone for now.

As an alternative, we could use the STRICT_MM_TYPECHECKS approach here
as well, but using an u64 looks conceptually cleaner, even though
pgd_val() gets a bit more involved.

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 arch/arm/include/asm/pgtable-2level-types.h | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/arch/arm/include/asm/pgtable-2level-types.h b/arch/arm/include/asm/pgtable-2level-types.h
index 650e793f4142..7f85384ac386 100644
--- a/arch/arm/include/asm/pgtable-2level-types.h
+++ b/arch/arm/include/asm/pgtable-2level-types.h
@@ -25,7 +25,7 @@ typedef struct { pteval_t pgprot; } pgprot_t;
 
 #define pte_val(x)      ((x).pte)
 #define pmd_val(x)      ((x).pmd)
-#define pgd_val(x)	((x).pgd[0])
+#define pgd_val(x)      ((x).pgd[0])
 #define pgprot_val(x)   ((x).pgprot)
 
 #define __pte(x)        ((pte_t) { (x) } )
@@ -36,14 +36,32 @@ typedef struct { pteval_t pgprot; } pgprot_t;
 /*
  * .. while these make it easier on the compiler
  */
+typedef u64 pgdval_t;
+
 typedef pteval_t pte_t;
 typedef pmdval_t pmd_t;
-typedef pmdval_t pgd_t[2];
+typedef pgdval_t pgd_t;
 typedef pteval_t pgprot_t;
 
 #define pte_val(x)      (x)
 #define pmd_val(x)      (x)
-#define pgd_val(x)	((x)[0])
+
+static inline pmdval_t pgd_val(pgd_t pgd)
+{
+	/*
+	 * The lower and higher pmdval_t values point to the same page.
+	 * the only difference is their offset within that page.
+	 * Therefore, whichever value is returned, the behavior of pmd_pfn(),
+	 * pmd_page(), and other users of pgd_val() remains unchanged.
+	 *
+	 * However, pgd_val() has historically returned the lower pmdval_t, so
+	 * keep returning the lower one.
+	 *
+	 * See the comments in include/asm/pgtable-2level.
+	 */
+	return (*(pmdval_t (*)[2])&pgd)[0];
+}
+
 #define pgprot_val(x)   (x)
 
 #define __pte(x)        (x)

-- 
2.43.0