Add pico_unique_id to inject ID access before main and provide accessor

src/rp2_common/CMakeLists.txt

@@ -37,6 +37,7 @@ if (NOT PICO_BARE_METAL)
     pico_add_subdirectory(boot_stage2)
 
     pico_add_subdirectory(pico_multicore)
+    pico_add_subdirectory(pico_unique_id)
 
     pico_add_subdirectory(pico_bit_ops)
     pico_add_subdirectory(pico_divider)

src/rp2_common/pico_unique_id/CMakeLists.txt

@@ -0,0 +1,9 @@
+add_library(pico_unique_id INTERFACE)
+
+target_sources(pico_unique_id INTERFACE
+        ${CMAKE_CURRENT_LIST_DIR}/unique_id.c
+)
+
+target_include_directories(pico_unique_id INTERFACE ${CMAKE_CURRENT_LIST_DIR}/include)
+
+target_link_libraries(pico_unique_id INTERFACE hardware_flash)

src/rp2_common/pico_unique_id/include/pico/unique_id.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef _PICO_UNIQUE_ID_H_
+#define _PICO_UNIQUE_ID_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** \file pico/unique_id.h
+ *  \defgroup unique_id unique_id
+ *
+ * Unique device ID access API
+ *
+ * RP2040 does not have an on-board unique identifier (all instances of RP2040
+ * silicon are identical and have no persistent state). However, RP2040 boots
+ * from serial NOR flash devices which have a 64-bit unique ID as a standard
+ * feature, and there is a 1:1 association between RP2040 and flash, so this
+ * is suitable for use as a unique identifier for an RP2040-based board.
+ *
+ * This library injects a call to the flash_get_unique_id function from the
+ * hardware_flash library, to run before main, and stores the result in a
+ * static location which can safely be accessed at any time via
+ * pico_get_unique_id().
+ *
+ * This avoids some pitfalls of the hardware_flash API, which requires any
+ * flash-resident interrupt routines to be disabled when called into.
+ */
+
+#define PICO_UNIQUE_ID_SIZE_BYTES 8
+
+/*! \brief Get unique ID
+ *  \ingroup unique_id
+ *
+
+ * Get the unique 64-bit device identifier which was retrieved from the
+ * external NOR flash device at boot.
+ *
+ * On PICO_NO_FLASH builds the unique identifier is set to all 0xEE.
+ *
+ * \param id_out an 8-byte buffer to which the identifer will be written.
+ */
+void pico_get_unique_id(uint8_t *id_out);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

src/rp2_common/pico_unique_id/unique_id.c

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include "hardware/flash.h"
+#include "pico/unique_id.h"
+
+static_assert(PICO_UNIQUE_ID_SIZE_BYTES == FLASH_UNIQUE_ID_SIZE_BYTES);
+
+static uint8_t retrieved_id[PICO_UNIQUE_ID_SIZE_BYTES];
+
+static void __attribute__((constructor)) _retrieve_unique_id_on_boot() {
+#if PICO_NO_FLASH
+    // The hardware_flash call will panic() if called directly on a NO_FLASH
+    // build. Since this constructor is pre-main it would be annoying to
+    // debug, so just produce something well-defined and obviously wrong.
+    for (int i = 0; i < PICO_UNIQUE_ID_SIZE_BYTES; i++)
+        retrieved_id[i] = 0xee;
+#else
+    flash_get_unique_id(retrieved_id);
+#endif
+}
+
+void pico_get_unique_id(uint8_t *id_out) {
+    for (int i = 0; i < PICO_UNIQUE_ID_SIZE_BYTES; i++)
+        id_out[i] = retrieved_id[i];
+}