rockchip: Print a message when returning to the bootrom

At present if the return to bootrom fails (e.g. because you are not using
the Rockchip's bootrom's pointer table in MMC) then the board prints
SPL message and hangs. Print a message first if we can, to help in
understanding what happened when it hangs.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>

arch/arm/include/asm/arch-rockchip/bootrom.h

@@ -13,10 +13,15 @@
  */
 extern u32 SAVE_SP_ADDR;
 
-/*
+/**
  * Hand control back to the bootrom to load another
  * boot stage.
  */
-extern void back_to_bootrom(void);
+void back_to_bootrom(void);
+
+/**
+ * Assembler component for the above (do not call this directly)
+ */
+void _back_to_bootrom_s(void);
 
 #endif

arch/arm/mach-rockchip/Makefile

@@ -4,6 +4,8 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+obj-$(CONFIG_ROCKCHIP_BROM_HELPER) += bootrom.o
+
 ifdef CONFIG_TPL_BUILD
 obj-$(CONFIG_ROCKCHIP_RK3188) += rk3188-board-tpl.o
 obj-$(CONFIG_ROCKCHIP_BROM_HELPER) += save_boot_param.o

arch/arm/mach-rockchip/bootrom.c

@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) 2017 Google, Inc
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <asm/arch/bootrom.h>
+
+void back_to_bootrom(void)
+{
+#if defined(CONFIG_SPL_LIBGENERIC_SUPPORT) && !defined(CONFIG_TPL_BUILD)
+	printf("Returning to boot ROM...");
+#endif
+	_back_to_bootrom_s();
+}

arch/arm/mach-rockchip/save_boot_param.S

@@ -23,10 +23,10 @@ ENTRY(save_boot_params)
 ENDPROC(save_boot_params)
 
 
-.globl back_to_bootrom
-ENTRY(back_to_bootrom)
+.globl _back_to_bootrom_s
+ENTRY(_back_to_bootrom_s)
 	ldr	r0, =SAVE_SP_ADDR
 	ldr	sp, [r0]
 	mov	r0, #0
 	pop	{r1-r12, pc}
-ENDPROC(back_to_bootrom)
+ENDPROC(_back_to_bootrom_s)