- * Have we been asked to return to the FEL portion of the boot ROM?
- * TODO: We need a more robust test here, or bracket this with
- * #ifdef CONFIG_SPL_FEL.
+ * When booting from the SD card, the "eGON.BT0" signature is expected
+ * to be found in memory at the address 0x0004 (see the "mksunxiboot"
+ * tool, which generates this header).
+ *
+ * When booting in the FEL mode over USB, this signature is patched in
+ * memory and replaced with something else by the 'fel' tool. This other
+ * signature is selected in such a way, that it can't be present in a
+ * valid bootable SD card image (because the BROM would refuse to
+ * execute the SPL in this case).
+ *
+ * This branch is just making a decision at runtime whether to load
+ * the main u-boot binary from the SD card (if the "eGON.BT0" signature
+ * is found) or return to the FEL code in the BROM to wait and receive
+ * the main u-boot binary over USB.