shared/tinyusb: Add a helper for hex string conversion.

Change the rp2 and renesas-ra ports to use the helper function.

Saves copy-pasta, at the small cost of one more function call in the
firmware (if not using LTO).

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/renesas-ra/usbd.c

@@ -34,15 +34,8 @@
 
 void mp_usbd_port_get_serial_number(char *serial_buf) {
     const bsp_unique_id_t *id = R_BSP_UniqueIdGet();
-    // convert to hex
-    int hexlen = sizeof(id->unique_id_bytes) * 2;
-    MP_STATIC_ASSERT(hexlen <= MICROPY_HW_USB_DESC_STR_MAX);
-    for (int i = 0; i < hexlen; i += 2) {
-        static const char *hexdig = "0123456789abcdef";
-        serial_buf[i] = hexdig[id->unique_id_bytes[i / 2] >> 4];
-        serial_buf[i + 1] = hexdig[id->unique_id_bytes[i / 2] & 0x0f];
-    }
-    serial_buf[hexlen] = 0;
+    MP_STATIC_ASSERT(sizeof(id->unique_id_bytes) * 2 <= MICROPY_HW_USB_DESC_STR_MAX);
+    mp_usbd_hex_str(serial_buf, id->unique_id_bytes, sizeof(id->unique_id_bytes));
 }
 
 #endif

ports/rp2/usbd.c

@@ -36,15 +36,8 @@
 void mp_usbd_port_get_serial_number(char *serial_buf) {
     pico_unique_board_id_t id;
     pico_get_unique_board_id(&id);
-    // convert to hex
-    int hexlen = sizeof(id.id) * 2;
-    MP_STATIC_ASSERT(hexlen <= MICROPY_HW_USB_DESC_STR_MAX);
-    for (int i = 0; i < hexlen; i += 2) {
-        static const char *hexdig = "0123456789abcdef";
-        serial_buf[i] = hexdig[id.id[i / 2] >> 4];
-        serial_buf[i + 1] = hexdig[id.id[i / 2] & 0x0f];
-    }
-    serial_buf[hexlen] = 0;
+    MP_STATIC_ASSERT(sizeof(id.id) * 2 <= MICROPY_HW_USB_DESC_STR_MAX);
+    mp_usbd_hex_str(serial_buf, id.id, sizeof(id.id));
 }
 
 #endif

shared/tinyusb/mp_usbd.c

@@ -62,4 +62,14 @@ static void mp_usbd_task_callback(mp_sched_node_t *node) {
     mp_usbd_task();
 }
 
+void mp_usbd_hex_str(char *out_str, const uint8_t *bytes, size_t bytes_len) {
+    size_t hex_len = bytes_len * 2;
+    for (int i = 0; i < hex_len; i += 2) {
+        static const char *hexdig = "0123456789abcdef";
+        out_str[i] = hexdig[bytes[i / 2] >> 4];
+        out_str[i + 1] = hexdig[bytes[i / 2] & 0x0f];
+    }
+    out_str[hex_len] = 0;
+}
+
 #endif

shared/tinyusb/mp_usbd.h

@@ -36,4 +36,9 @@ void mp_usbd_task(void);
 // Can write a string up to MICROPY_HW_USB_DESC_STR_MAX characters long, plus terminating byte.
 extern void mp_usbd_port_get_serial_number(char *buf);
 
+// Most ports need to write a hexadecimal serial number from a byte array, this
+// is a helper function for this. out_str must be long enough to hold a string of total
+// length (2 * bytes_len + 1) (including NUL terminator).
+void mp_usbd_hex_str(char *out_str, const uint8_t *bytes, size_t bytes_len);
+
 #endif // MICROPY_INCLUDED_SHARED_TINYUSB_USBD_H