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

Boot CPU and secondary CPUs will use different entry point to C code. At
the moment, the decision on which entry to use is taken within launch().

In order to avoid using conditional instruction and make the call
clearer, launch() is reworked to take in parameters the entry point and its
arguments.

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

@@ -163,6 +163,11 @@ primary_switched:
         /* Use a virtual address to access the UART. */
         mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
 #endif
+        PRINT("- Ready -\r\n")
+        /* Setup the arguments for start_xen and jump to C world */
+        mov   r0, r10                /* r0 := Physical offset */
+        mov   r1, r8                 /* r1 := paddr(FDT) */
+        ldr   r2, =start_xen
         b     launch
 ENDPROC(start)
 
@@ -227,6 +232,9 @@ secondary_switched:
         /* Use a virtual address to access the UART. */
         mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
 #endif
+        PRINT("- Ready -\r\n")
+        /* Jump to C world */
+        ldr   r2, =start_secondary
         b     launch
 ENDPROC(init_secondary)
 
@@ -517,19 +525,25 @@ setup_fixmap:
         mov   pc, lr
 ENDPROC(setup_fixmap)
 
+/*
+ * Setup the initial stack and jump to the C world
+ *
+ * Inputs:
+ *   r0 : Argument 0 of the C function to call
+ *   r1 : Argument 1 of the C function to call
+ *   r2 : C entry point
+ *
+ * Clobbers r3
+ */
 launch:
-        PRINT("- Ready -\r\n")
-
-        ldr   r0, =init_data
-        add   r0, #INITINFO_stack    /* Find the boot-time stack */
-        ldr   sp, [r0]
+        ldr   r3, =init_data
+        add   r3, #INITINFO_stack    /* Find the boot-time stack */
+        ldr   sp, [r3]
         add   sp, #STACK_SIZE        /* (which grows down from the top). */
         sub   sp, #CPUINFO_sizeof    /* Make room for CPU save record */
-        teq   r12, #0
-        moveq r0, r10                /* Marshal args: - phys_offset */
-        moveq r1, r8                 /*               - DTB address */
-        beq   start_xen              /* and disappear into the land of C */
-        b     start_secondary        /* (to the appropriate entry point) */
+
+        /* Jump to C world */
+       bx    r2
 ENDPROC(launch)
 
 /* Fail-stop */