arm/dt: Make __vet_atags also accept a dtb image

The dtb is passed to the kernel via register r2, which is the same
method that is used to pass an atags pointer.  This patch modifies
__vet_atags to not clear r2 when it encounters a dtb image.

v2: fixed bugs pointed out by Nicolas Pitre
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/arm/kernel/head-common.S

@@ -15,6 +15,12 @@
 #define ATAG_CORE_SIZE ((2*4 + 3*4) >> 2)
 #define ATAG_CORE_SIZE_EMPTY ((2*4) >> 2)
 
+#ifdef CONFIG_CPU_BIG_ENDIAN
+#define OF_DT_MAGIC 0xd00dfeed
+#else
+#define OF_DT_MAGIC 0xedfe0dd0 /* 0xd00dfeed in big-endian */
+#endif
+
 /*
  * Exception handling.  Something went wrong and we can't proceed.  We
  * ought to tell the user, but since we don't have any guarantee that
@@ -28,20 +34,26 @@
 
 /* Determine validity of the r2 atags pointer.  The heuristic requires
  * that the pointer be aligned, in the first 16k of physical RAM and
- * that the ATAG_CORE marker is first and present.  Future revisions
+ * that the ATAG_CORE marker is first and present.  If CONFIG_OF_FLATTREE
+ * is selected, then it will also accept a dtb pointer.  Future revisions
  * of this function may be more lenient with the physical address and
  * may also be able to move the ATAGS block if necessary.
  *
  * Returns:
- *  r2 either valid atags pointer, or zero
+ *  r2 either valid atags pointer, valid dtb pointer, or zero
  *  r5, r6 corrupted
  */
 __vet_atags:
 	tst	r2, #0x3			@ aligned?
 	bne	1f
 
-	ldr	r5, [r2, #0]			@ is first tag ATAG_CORE?
-	cmp	r5, #ATAG_CORE_SIZE
+	ldr	r5, [r2, #0]
+#ifdef CONFIG_OF_FLATTREE
+	ldr	r6, =OF_DT_MAGIC		@ is it a DTB?
+	cmp	r5, r6
+	beq	2f
+#endif
+	cmp	r5, #ATAG_CORE_SIZE		@ is first tag ATAG_CORE?
 	cmpne	r5, #ATAG_CORE_SIZE_EMPTY
 	bne	1f
 	ldr	r5, [r2, #4]
@@ -49,7 +61,7 @@ __vet_atags:
 	cmp	r5, r6
 	bne	1f
 
-	mov	pc, lr				@ atag pointer is ok
+2:	mov	pc, lr				@ atag/dtb pointer is ok
 
 1:	mov	r2, #0
 	mov	pc, lr
@@ -61,7 +73,7 @@ ENDPROC(__vet_atags)
  *
  *  r0  = cp#15 control register
  *  r1  = machine ID
- *  r2  = atags pointer
+ *  r2  = atags/dtb pointer
  *  r9  = processor ID
  */
 	__INIT

arch/arm/kernel/head.S

@@ -59,7 +59,7 @@
  *
  * This is normally called from the decompressor code.  The requirements
  * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
- * r1 = machine nr, r2 = atags pointer.
+ * r1 = machine nr, r2 = atags or dtb pointer.
  *
  * This code is mostly position independent, so if you link the kernel at
  * 0xc0008000, you call this at __pa(0xc0008000).
@@ -91,7 +91,7 @@ ENTRY(stext)
 #endif
 
 	/*
-	 * r1 = machine no, r2 = atags,
+	 * r1 = machine no, r2 = atags or dtb,
 	 * r8 = phys_offset, r9 = cpuid, r10 = procinfo
 	 */
 	bl	__vet_atags
@@ -339,7 +339,7 @@ __secondary_data:
  *
  *  r0  = cp#15 control register
  *  r1  = machine ID
- *  r2  = atags pointer
+ *  r2  = atags or dtb pointer
  *  r4  = page table pointer
  *  r9  = processor ID
  *  r13 = *virtual* address to jump to upon completion
@@ -376,7 +376,7 @@ ENDPROC(__enable_mmu)
  *
  *  r0  = cp#15 control register
  *  r1  = machine ID
- *  r2  = atags pointer
+ *  r2  = atags or dtb pointer
  *  r9  = processor ID
  *  r13 = *virtual* address to jump to upon completion
  *