xen/arm32: head: Rework and document setup_fixmap()

At the moment, the fixmap table is only hooked when earlyprintk is used.
This is fine today because in C land, the fixmap is not used by anyone
until the the boot CPU is switching to the runtime page-tables.

In the future, the boot CPU will not switch between page-tables to
avoid TLB incoherency. Thus, the fixmap table will need to be always
hooked beofre any use. Let's start doing it now in setup_fixmap().

Lastly, document the behavior and the main registers usage within the
function.

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

xen/arch/arm/arm32/head.S

@@ -546,8 +546,21 @@ identity_mapping_removed:
         mov   pc, lr
 ENDPROC(remove_identity_mapping)
 
+/*
+ * Map the UART in the fixmap (when earlyprintk is used) and hook the
+ * fixmap table in the page tables.
+ *
+ * The fixmap cannot be mapped in create_page_tables because it may
+ * clash with the 1:1 mapping.
+ *
+ * Inputs:
+ *   r10: Physical offset
+ *   r11: Early UART base physical address
+ *
+ * Clobbers r1 - r4
+ */
 setup_fixmap:
-#if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
+#if defined(CONFIG_EARLY_PRINTK)
         /* Add UART to the fixmap table */
         ldr   r1, =xen_fixmap        /* r1 := vaddr (xen_fixmap) */
         lsr   r2, r11, #THIRD_SHIFT
@@ -556,6 +569,7 @@ setup_fixmap:
         orr   r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */
         mov   r3, #0x0
         strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */
+#endif
 
         /* Map fixmap into boot_second */
         ldr   r1, =boot_second       /* r1 := vaddr (boot_second) */
@@ -570,7 +584,7 @@ setup_fixmap:
 
         /* Ensure any page table updates made above have occurred. */
         dsb   nshst
-#endif
+
         mov   pc, lr
 ENDPROC(setup_fixmap)