arm: spl: Add an API to detect when U-Boot is started from SPL

For secure boot systems it is common to have a read-only U-Boot which starts
the machine and jumps to a read-write U-Boot for actual booting the OS. This
allows the read-write U-Boot to be upgraded without risk of permanently
bricking the machine. In the event that the read-write U-Boot is corrupted,
the read-only U-Boot can detect this with a checksum and boot into a
recovery flow.

To support this, add a way to detect when U-Boot is run from SPL as opposed
to some other method, such as booted directly (no SPL) or started from
another source (e.g. a primary U-Boot). This works by putting a special value
in r0.

For now we rely on board-specific code to actually check the register and
set a flag. At some point this could be generalised, perhaps by using a spare
register and passing a flag to _main and/or board_init_f().

This commit does not implement any feature, but merely provides the API for
boards to implement.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/spl.h

@@ -11,6 +11,8 @@
 #include <linux/compiler.h>
 #include <asm/spl.h>
 
+/* Value in r0 indicates we booted from U-Boot */
+#define UBOOT_NOT_LOADED_FROM_SPL	0x13578642
 
 /* Boot type */
 #define MMCSD_MODE_UNDEFINED	0
@@ -82,4 +84,15 @@ int spl_load_image_ext_os(block_dev_desc_t *block_dev, int partition);
 #ifdef CONFIG_SPL_BOARD_INIT
 void spl_board_init(void);
 #endif
+
+/**
+ * spl_was_boot_source() - check if U-Boot booted from SPL
+ *
+ * This will normally be true, but if U-Boot jumps to second U-Boot, it will
+ * be false. This should be implemented by board-specific code.
+ *
+ * @return true if U-Boot booted from SPL, else false
+ */
+bool spl_was_boot_source(void);
+
 #endif