xen/arm32: head: Document create_pages_tables()

Document the behavior and the main registers usage within the function.
Note that r6 is now only used within the function, so it does not need
to be part of the common register.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm32/head.S

@@ -53,7 +53,7 @@
  *   r3  -
  *   r4  -
  *   r5  -
- *   r6  - identity map in place
+ *   r6  -
  *   r7  -
  *   r8  - DTB address (boot CPU only)
  *   r9  - paddr(start)
@@ -302,17 +302,25 @@ cpu_init_done:
         mov   pc, r5                        /* Return address is in r5 */
 ENDPROC(cpu_init)
 
+/*
+ * Rebuild the boot pagetable's first-level entries. The structure
+ * is described in mm.c.
+ *
+ * After the CPU enables paging it will add the fixmap mapping
+ * to these page tables, however this may clash with the 1:1
+ * mapping. So each CPU must rebuild the page tables here with
+ * the 1:1 in place.
+ *
+ * Inputs:
+ *   r9 : paddr(start)
+ *   r10: phys offset
+ *
+ * Clobbers r0 - r6
+ *
+ * Register usage within this function:
+ *   r6 : Identity map in place
+ */
 create_page_tables:
-        /*
-         * Rebuild the boot pagetable's first-level entries. The structure
-         * is described in mm.c.
-         *
-         * After the CPU enables paging it will add the fixmap mapping
-         * to these page tables, however this may clash with the 1:1
-         * mapping. So each CPU must rebuild the page tables here with
-         * the 1:1 in place.
-         */
-
         /*
          * If Xen is loaded at exactly XEN_VIRT_START then we don't
          * need an additional 1:1 mapping, the virtual mapping will