arm64/mm: Simplify and document pte_to_phys() for 52 bit addresses

pte_to_phys() assembly definition does multiple bits field transformations
to derive physical address, embedded inside a page table entry. Unlike its
C counter part i.e __pte_to_phys(), pte_to_phys() is not very apparent. It
simplifies these operations via a new macro PTE_ADDR_HIGH_SHIFT indicating
how far the pte encoded higher address bits need to be left shifted. While
here, this also updates __pte_to_phys() and __phys_to_pte_val().

Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Ard Biesheuvel <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Reviewed-by: Ard Biesheuvel <[e-mail]>
Suggested-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Anshuman Khandual <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/assembler.h

···
 	.endm
 
 	.macro	pte_to_phys, phys, pte
-#ifdef CONFIG_ARM64_PA_BITS_52
-	ubfiz	\phys, \pte, #(48 - 16 - 12), #16
-	bfxil	\phys, \pte, #16, #32
-	lsl	\phys, \phys, #16
-#else
 	and	\phys, \pte, #PTE_ADDR_MASK
+#ifdef CONFIG_ARM64_PA_BITS_52
+	orr	\phys, \phys, \phys, lsl #PTE_ADDR_HIGH_SHIFT
+	and	\phys, \phys, GENMASK_ULL(PHYS_MASK_SHIFT - 1, PAGE_SHIFT)
 #endif
 	.endm
 

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

···
 #ifdef CONFIG_ARM64_PA_BITS_52
 #define PTE_ADDR_HIGH		(_AT(pteval_t, 0xf) << 12)
 #define PTE_ADDR_MASK		(PTE_ADDR_LOW | PTE_ADDR_HIGH)
+#define PTE_ADDR_HIGH_SHIFT	36
 #else
 #define PTE_ADDR_MASK		PTE_ADDR_LOW
 #endif

arch/arm64/include/asm/pgtable.h

···
 static inline phys_addr_t __pte_to_phys(pte_t pte)
 {
 	return (pte_val(pte) & PTE_ADDR_LOW) |
-		((pte_val(pte) & PTE_ADDR_HIGH) << 36);
+		((pte_val(pte) & PTE_ADDR_HIGH) << PTE_ADDR_HIGH_SHIFT);
 }
 static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 {
-	return (phys | (phys >> 36)) & PTE_ADDR_MASK;
+	return (phys | (phys >> PTE_ADDR_HIGH_SHIFT)) & PTE_ADDR_MASK;
 }
 #else
 #define __pte_to_phys(pte)	(pte_val(pte) & PTE_ADDR_MASK)