docs: blank-check only works reliably for code flash

Author: vjardin

src/main.c

@@ -35,7 +35,7 @@ usage(int status) {
       "  write <file>[:<addr>] ...  Write file(s) to flash memory\n"
       "  verify <file>  Verify flash memory against file\n"
       "  erase          Erase flash sectors\n"
-      "  blank-check    Check if flash region is erased (all 0xFF)\n"
+      "  blank-check    Check if flash region is erased (code flash only)\n"
       "  crc            Calculate CRC-32 of flash region\n"
       "  dlm            Show Device Lifecycle Management state\n"
       "  dlm-transit <state>  Transition DLM state (ssd/nsecsd/dpl/lck_dbg/lck_boot)\n"

src/radfu.h

@@ -74,6 +74,11 @@ int ra_verify(
 /*
  * Check if flash memory region is blank (all 0xFF)
  * Reports first non-blank byte if found
+ *
+ * NOTE: This only works reliably for code flash. Data flash returns
+ * undefined values after erase (not 0xFF) per Renesas RA hardware spec.
+ * The bootloader protocol does not expose the hardware blank-check command.
+ *
  * Returns: 0 on success (blank), -1 on error or non-blank
  */
 int ra_blank_check(ra_device_t *dev, uint32_t start, uint32_t size);