xen/arm32: head: Introduce print_reg

At the moment, the user should save r14/lr if it cares about it.

Follow-up patches will introduce more use of putn in place where lr
should be preserved.

Furthermore, any user of putn should also move the value to register r0
if it was stored in a different register.

For convenience, a new macro is introduced to print a given register.
The macro will take care for us to move the value to r0 and also
preserve lr.

Lastly the new macro is used to replace all the callsite of putn. This
will simplify rework/review later on.

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

xen/arch/arm/arm32/head.S

@@ -79,8 +79,25 @@
 98:     .asciz _s          ;\
         .align 2           ;\
 99:
+
+/*
+ * Macro to print the value of register \rb
+ *
+ * Clobbers r0 - r4
+ */
+.macro print_reg rb
+        mov   r0, \rb
+        mov   r4, lr
+        bl    putn
+        mov   lr, r4
+.endm
+
 #else /* CONFIG_EARLY_PRINTK */
 #define PRINT(s)
+
+.macro print_reg rb
+.endm
+
 #endif /* !CONFIG_EARLY_PRINTK */
 
         .arm
@@ -159,8 +176,7 @@ GLOBAL(init_secondary)
 #ifdef CONFIG_EARLY_PRINTK
         mov_w r11, EARLY_UART_BASE_ADDRESS   /* r11 := UART base address */
         PRINT("- CPU ")
-        mov   r0, r7
-        bl    putn
+        print_reg r7
         PRINT(" booting -\r\n")
 #endif
 
@@ -211,8 +227,7 @@ skip_bss:
         bne   1f
         mov   r4, r0
         PRINT("- Missing processor info: ")
-        mov   r0, r4
-        bl    putn
+        print_reg r4
         PRINT(" -\r\n")
         b     fail
 1: