arm64: head: remove __PHYS_OFFSET

It's very easy to confuse __PHYS_OFFSET and PHYS_OFFSET. To clarify
things, let's remove __PHYS_OFFSET and use KERNEL_START directly, with
comments to show that we're using physical address, as we do for other
objects.

At the same time, update the comment regarding the kernel entry address
to mention __pa(KERNEL_START) rather than __pa(PAGE_OFFSET).

There should be no functional change as a result of this patch.

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20220629041207.1670133-1-anshuman.khandual@arm.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/head.S

@@ -37,8 +37,6 @@
 
 #include "efi-header.S"
 
-#define __PHYS_OFFSET	KERNEL_START
-
 #if (PAGE_OFFSET & 0x1fffff) != 0
 #error PAGE_OFFSET must be at least 2MB aligned
 #endif
@@ -51,9 +49,6 @@
  *   MMU = off, D-cache = off, I-cache = on or off,
  *   x0 = physical address to the FDT blob.
  *
- * This code is mostly position independent so you call this at
- * __pa(PAGE_OFFSET).
- *
  * Note that the callee-saved registers are used for storing variables
  * that are useful before the MMU is enabled. The allocations are described
  * in the entry routines.
@@ -409,7 +404,7 @@ SYM_FUNC_END(create_kernel_mapping)
 /*
  * The following fragment of code is executed with the MMU enabled.
  *
- *   x0 = __PHYS_OFFSET
+ *   x0 = __pa(KERNEL_START)
  */
 SYM_FUNC_START_LOCAL(__primary_switched)
 	adr_l	x4, init_task
@@ -793,7 +788,7 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 	adrp	x2, init_idmap_pg_dir
 	bl	__enable_mmu
 #ifdef CONFIG_RELOCATABLE
-	adrp	x23, __PHYS_OFFSET
+	adrp	x23, KERNEL_START
 	and	x23, x23, MIN_KIMG_ALIGN - 1
 #ifdef CONFIG_RANDOMIZE_BASE
 	mov	x0, x22
@@ -815,6 +810,6 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 	bl	__relocate_kernel
 #endif
 	ldr	x8, =__primary_switched
-	adrp	x0, __PHYS_OFFSET
+	adrp	x0, KERNEL_START		// __pa(KERNEL_START)
 	br	x8
 SYM_FUNC_END(__primary_switch)