arm: spear: Support returning to BootROM

Implement the weak board_return_to_bootrom() function so that when
enabling the spl_bootrom.c driver, one can make use of usbboot on
spear platforms. All necessary information to return to the BootROM
are stored in the BootROM's stack. The SPL stack pointer is reset so
we save the BootROM's stack pointer into the SPL .data section.
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NStefan Roese <sr@denx.de>

arch/arm/cpu/arm926ejs/spear/spl.c

@@ -16,6 +16,12 @@
 #include <asm/arch/spr_syscntl.h>
 #include <linux/mtd/st_smi.h>
 
+/* Reserve some space to store the BootROM's stack pointer during SPL operation.
+ * The BSS cannot be used for this purpose because it will be zeroed after
+ * having stored the pointer, so force the location to the data section.
+ */
+u32 bootrom_stash_sp __attribute__((section(".data")));
+
 static void ddr_clock_init(void)
 {
 	struct misc_regs *misc_p = (struct misc_regs *)CONFIG_SPEAR_MISCBASE;
@@ -223,8 +229,9 @@ u32 spl_boot_device(void)
 {
 	u32 mode = 0;
 
-	/* Currently only SNOR is supported as the only */
-	if (snor_boot_selected()) {
+	if (usb_boot_selected()) {
+		mode = BOOT_DEVICE_BOOTROM;
+	} else if (snor_boot_selected()) {
 		/* SNOR-SMI initialization */
 		snor_init();
 
@@ -252,3 +259,27 @@ void board_init_f(ulong dummy)
 	mpmc_init();
 	spear_late_init();
 }
+
+/*
+ * In a few cases (Ethernet, UART or USB boot, we might want to go back into the
+ * BootROM code right after having initialized a few components like the DRAM).
+ * The following function is called from SPL common code (board_init_r).
+ */
+void board_return_to_bootrom(void)
+{
+	/*
+	 * Retrieve the BootROM's stack pointer and jump back to the start of
+	 * the SPL, where we can easily branch back into the BootROM. Don't do
+	 * it right here because SPL might be compiled in Thumb mode while the
+	 * BootROM expects ARM mode.
+	 */
+	asm volatile ("ldr r0, =bootrom_stash_sp;"
+		      "ldr r0, [r0];"
+		      "mov sp, r0;"
+#if defined(CONFIG_SPL_SYS_THUMB_BUILD)
+		      "blx back_to_bootrom;"
+#else
+		      "bl back_to_bootrom;"
+#endif
+		      );
+}

arch/arm/cpu/arm926ejs/spear/start.S

@@ -29,13 +29,17 @@
  */
 
 	.globl	reset
+	.globl	back_to_bootrom
 
 reset:
 	/*
 	* SPL has to return back to BootROM in a few cases (eg. Ethernet boot,
-	* UART boot, USB boot): save registers in BootROM's stack.
+	* UART boot, USB boot): save registers in BootROM's stack and then the
+	* BootROM's stack pointer in the SPL's data section.
 	*/
 	push	{r0-r12,lr}
+	ldr	r0, =bootrom_stash_sp
+	str	sp, [r0]
 
 	/*
 	 * Flush v4 I/D caches
@@ -56,4 +60,5 @@ reset:
 	 */
 	bl	_main	/* _main will call board_init_f */
 
+back_to_bootrom:
 	pop	{r0-r12,pc}