stm32:desig: Add DFU compatible serial generation

Reviewed-by: Karl Palsson <karlp@tweak.net.au>
Modified to provide better doxygen and consistent api names.

include/libopencm3/stm32/desig.h

@@ -47,11 +47,19 @@ void desig_get_unique_id(uint32_t *result);
  * Read the full 96 bit unique identifier and return it as a
  * zero-terminated string
  * @param string memory region to write the result to
- 8 @param string_len the size of string in bytes
+ * @param string_len the size of string in bytes
  */
 void desig_get_unique_id_as_string(char *string,
 				   unsigned int string_len);
 
+/** 
+ * Returns the same serial number that the factory DFU 
+ * bootloader reports (via USB descriptors). 
+ * @param string memory region to write the result to. This is 
+ *        expected to be a 13-byte buffer.
+ */
+void desig_get_unique_id_as_dfu(char *string);
+
 END_DECLS
 
 #endif

lib/stm32/desig.c

@@ -53,3 +53,36 @@ void desig_get_unique_id_as_string(char *string,
 	string[len] = '\0';
 }
 
+/**
+ * Generate a serial number from the unique id registers as the DFU bootloader.
+ * This document: http://www.usb.org/developers/docs/devclass_docs/usbmassbulk_10.pdf
+ * says that the serial number has to be at least 12 digits long and that
+ * the last 12 digits need to be unique. It also stipulates that the valid
+ * character set is that of upper-case hexadecimal digits.
+ * The onboard DFU bootloader produces a 12-digit serial based on the
+ * 96-bit unique ID. Show the serial with ```dfu-util -l``` while the
+ * MCU is in DFU mode.
+ * @see https://my.st.com/52d187b7 for the algorithim used.
+ * @param string pointer to store serial in, must be at least 13 bytes
+ */
+void desig_get_unique_id_as_dfu(char *string) {
+	uint8_t *id = (uint8_t *)DESIG_UNIQUE_ID_BASE;
+
+	uint8_t serial[6];
+	serial[0] = id[11];
+	serial[1] = id[10] + id[2];
+	serial[2] = id[9];
+	serial[3] = id[8] + id[0];
+	serial[4] = id[7];
+	serial[5] = id[6];
+
+	uint8_t *ser = &serial[0];
+	uint8_t *end = &serial[6];
+	const char hex_digit[] = "0123456789ABCDEF";
+
+	for (; ser < end; ser++) {
+		*string++ = hex_digit[(*ser >> 4) & 0x0f];
+		*string++ = hex_digit[(*ser >> 0) & 0x0f];
+	}
+	*string = '\0';
+}