arm: Allow lr to be saved by board code

The link register value can be required on some boards (e.g. FEL mode on
sunxi) so use a branch instruction to jump to save_boot_params() instead
of a branch link.

This requires a branch back to save_boot_params_ret so adjust the users
to deal with this. For exynos just drop the function since it doesn't
do anything.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

arch/arm/cpu/armv7/exynos/spl_boot.c

@@ -309,4 +309,3 @@ void board_init_r(gd_t *id, ulong dest_addr)
 	while (1)
 		;
 }
-void save_boot_params(u32 r0, u32 r1, u32 r2, u32 r3) {}

arch/arm/cpu/armv7/omap-common/lowlevel_init.S

@@ -19,7 +19,7 @@
 ENTRY(save_boot_params)
 	ldr	r1, =OMAP_SRAM_SCRATCH_BOOT_PARAMS
 	str	r0, [r1]
-	bx	lr
+	b	save_boot_params_ret
 ENDPROC(save_boot_params)
 
 ENTRY(set_pl310_ctrl_reg)

arch/arm/cpu/armv7/omap3/lowlevel_init.S

@@ -23,7 +23,7 @@ ENTRY(save_boot_params)
 	ldr	r5, [r0, #0x4]
 	and	r5, r5, #0xff
 	str	r5, [r4]
-	bx	lr
+	b	save_boot_params_ret
 ENDPROC(save_boot_params)
 #endif
 

arch/arm/cpu/armv7/start.S

@@ -31,9 +31,12 @@
  *************************************************************************/
 
 	.globl	reset
+	.globl	save_boot_params_ret
 
 reset:
-	bl	save_boot_params
+	/* Allow the board to save important registers */
+	b	save_boot_params
+save_boot_params_ret:
 	/*
 	 * disable interrupts (FIQ and IRQ), also set the cpu to SVC32 mode,
 	 * except if in HYP mode already
@@ -96,7 +99,7 @@ ENDPROC(c_runtime_cpu_setup)
  *
  *************************************************************************/
 ENTRY(save_boot_params)
-	bx	lr			@ back to my caller
+	b	save_boot_params_ret		@ back to my caller
 ENDPROC(save_boot_params)
 	.weak	save_boot_params
 

arch/arm/include/asm/system.h

@@ -142,6 +142,21 @@ void flush_l3_cache(void);
 
 #ifndef __ASSEMBLY__
 
+/**
+ * save_boot_params() - Save boot parameters before starting reset sequence
+ *
+ * If you provide this function it will be called immediately U-Boot starts,
+ * both for SPL and U-Boot proper.
+ *
+ * All registers are unchanged from U-Boot entry. No registers need be
+ * preserved.
+ *
+ * This is not a normal C function. There is no stack. Return by branching to
+ * save_boot_params_ret.
+ *
+ * void save_boot_params(u32 r0, u32 r1, u32 r2, u32 r3);
+ */
+
 #define isb() __asm__ __volatile__ ("" : : : "memory")
 
 #define nop() __asm__ __volatile__("mov\tr0,r0\t@ nop\n\t");

board/nokia/rx51/lowlevel_init.S

@@ -37,7 +37,8 @@ ih_magic:		/* IH_MAGIC in big endian from include/image.h */
 
 .global save_boot_params
 save_boot_params:
-
+	/* Get return address */
+	ldr	lr, =save_boot_params_ret
 
 /* Copy valid attached kernel to address KERNEL_ADDRESS */