esp32s2: Use the device's EUI-48 address as unique ID

On my hardware, esptool reports
    MAC: 7c:df:a1:02:6c:b8
after this change, the USB descriptor says SerialNumber: 7CDFA1026CB8
and microcontroller.cpu.id has
    >>> "".join("%02x" % byte for byte in microcontroller.cpu.uid)
    'c7fd1a20c68b'

Note that the nibble-swapping between USB and cpu.uid is typical.
For instance, an stm32 board has USB SerialNumber
24002500F005D42445632302 but hex-converted microcontroller.cpu.id
420052000f504d4254363220.

Author: jepler

ports/esp32s2/common-hal/microcontroller/Processor.c

@@ -26,10 +26,14 @@
  */
 
 #include <math.h>
+#include <string.h>
+
 #include "common-hal/microcontroller/Processor.h"
 #include "py/runtime.h"
 #include "supervisor/shared/translate.h"
 
+#include "soc/efuse_reg.h"
+
 float common_hal_mcu_processor_get_temperature(void) {
     return NAN;
 }
@@ -42,5 +46,23 @@ uint32_t common_hal_mcu_processor_get_frequency(void) {
     return 0;
 }
 
+STATIC uint8_t swap_nibbles(uint8_t v) {
+    return ((v << 4) | (v >> 4)) & 0xff;
+}
+
 void common_hal_mcu_processor_get_uid(uint8_t raw_id[]) {
+    memset(raw_id, 0, COMMON_HAL_MCU_PROCESSOR_UID_LENGTH);
+
+    uint8_t *ptr = &raw_id[COMMON_HAL_MCU_PROCESSOR_UID_LENGTH-1];
+    // MAC address contains 48 bits (6 bytes), 32 in the low order word
+    uint32_t mac_address_part = REG_READ(EFUSE_RD_MAC_SPI_SYS_0_REG);
+    *ptr-- = swap_nibbles(mac_address_part & 0xff); mac_address_part >>= 8;
+    *ptr-- = swap_nibbles(mac_address_part & 0xff); mac_address_part >>= 8;
+    *ptr-- = swap_nibbles(mac_address_part & 0xff); mac_address_part >>= 8;
+    *ptr-- = swap_nibbles(mac_address_part & 0xff);
+
+    // and 16 in the high order word
+    mac_address_part = REG_READ(EFUSE_RD_MAC_SPI_SYS_1_REG);
+    *ptr-- = swap_nibbles(mac_address_part & 0xff); mac_address_part >>= 8;
+    *ptr-- = swap_nibbles(mac_address_part & 0xff);
 }

ports/esp32s2/common-hal/microcontroller/Processor.h

@@ -27,7 +27,7 @@
 #ifndef MICROPY_INCLUDED_LITEX_COMMON_HAL_MICROCONTROLLER_PROCESSOR_H
 #define MICROPY_INCLUDED_LITEX_COMMON_HAL_MICROCONTROLLER_PROCESSOR_H
 
-#define COMMON_HAL_MCU_PROCESSOR_UID_LENGTH 15
+#define COMMON_HAL_MCU_PROCESSOR_UID_LENGTH 6
 
 #include "py/obj.h"
 

ports/esp32s2/mpconfigport.mk

@@ -7,7 +7,7 @@ MPY_TOOL_LONGINT_IMPL = -mlongint-impl=mpz
 INTERNAL_LIBM = 1
 
 # Chip supplied serial number, in bytes
-USB_SERIAL_NUMBER_LENGTH = 30
+USB_SERIAL_NUMBER_LENGTH = 12
 
 # Longints can be implemented as mpz, as longlong, or not
 LONGINT_IMPL = MPZ