[zep noup] Use picolibc instead of newlib-nano

 1. Support picolibc stdio. There was already picolibc support code
    present in the library for the ARM LLVM toolchain. The
    conditionals which selected it have been changed to use
    __PICOLIBC__ instead of __clang_major__.

 2. Add _exit stub. Code using assert or abort end up calling _exit
    through the picolibc signal handling code.

 3. Switch to picolibc.specs. This is needed for toolchains which
    don't use picolibc by default, and can also be used without
    trouble in toolchains where picolibc is the default.

 4. Define CONFIG_PICOLIBC when using picolibc. This is enabled
    by default, but can be disabled on the cmake command line.

 5. Define picolibc_startup to initialize all .data and .bss
    segments using the standard tf-m arrays.

 6. Add picolibc.c to many source lists when CONFIG_PICOLIBC is
    defined. If this file is missign, picolibc_startup will
    not be defined which should cause a failure at link time.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

bl1/bl1_1/CMakeLists.txt

@@ -40,6 +40,7 @@ target_link_options(bl1_1
 target_sources(bl1_1
     PRIVATE
         main.c
+        $<$<BOOL:${CONFIG_PICOLIBC}>:${CMAKE_SOURCE_DIR}/platform/ext/common/picolibc.c>
         $<$<BOOL:${CONFIG_GNU_SYSCALL_STUB_ENABLED}>:${CMAKE_SOURCE_DIR}/platform/ext/common/syscalls_stub.c>
 )
 

bl1/bl1_2/CMakeLists.txt

@@ -42,6 +42,7 @@ target_link_options(bl1_2
 target_sources(bl1_2
     PRIVATE
         main.c
+        $<$<BOOL:${CONFIG_PICOLIBC}>:${CMAKE_SOURCE_DIR}/platform/ext/common/picolibc.c>
         $<$<BOOL:${CONFIG_GNU_SYSCALL_STUB_ENABLED}>:${CMAKE_SOURCE_DIR}/platform/ext/common/syscalls_stub.c>
 )
 

bl2/CMakeLists.txt

@@ -128,6 +128,7 @@ add_executable(bl2
     $<$<BOOL:${DEFAULT_MCUBOOT_FLASH_MAP}>:src/default_flash_map.c>
     $<$<BOOL:${MCUBOOT_DATA_SHARING}>:src/shared_data.c>
     $<$<BOOL:${PLATFORM_DEFAULT_PROVISIONING}>:src/provisioning.c>
+    $<$<BOOL:${CONFIG_PICOLIBC}>:${CMAKE_SOURCE_DIR}/platform/ext/common/picolibc.c>
     $<$<BOOL:${CONFIG_GNU_SYSCALL_STUB_ENABLED}>:${CMAKE_SOURCE_DIR}/platform/ext/common/syscalls_stub.c>
 )
 

cmake/install.cmake

@@ -254,6 +254,11 @@ install(DIRECTORY   $<BUILD_INTERFACE:${CMSIS_PATH}/CMSIS/Core/Include>
                     $<BUILD_INTERFACE:${CMSIS_PATH}/CMSIS/Driver/Include>
         DESTINATION ${INSTALL_PLATFORM_NS_DIR}/ext/cmsis)
 
+if(CONFIG_PICOLIBC)
+    install(FILES       ${PLATFORM_DIR}/ext/common/picolibc.c
+            DESTINATION ${INSTALL_PLATFORM_NS_DIR}/ext/common)
+endif()
+
 if(PLATFORM_DEFAULT_UART_STDOUT)
     install(FILES       ${PLATFORM_DIR}/ext/common/uart_stdout.c
                         ${PLATFORM_DIR}/ext/common/uart_stdout.h

cmake/spe-CMakeLists.cmake

@@ -104,6 +104,7 @@ add_subdirectory(platform)
 
 target_sources(platform_ns
     PRIVATE
+        $<$<BOOL:${CONFIG_PICOLIBC}>:${CMAKE_CURRENT_SOURCE_DIR}/platform/ext/common/picolibc.c>
         $<$<BOOL:${PLATFORM_DEFAULT_UART_STDOUT}>:${CMAKE_CURRENT_SOURCE_DIR}/platform/ext/common/uart_stdout.c>
 )
 

platform/CMakeLists.txt

@@ -81,6 +81,7 @@ target_sources(platform_s
         $<$<BOOL:${PLATFORM_DEFAULT_PROVISIONING}>:ext/common/provisioning.c>
         $<$<OR:$<BOOL:${TEST_S_FPU}>,$<BOOL:${TEST_NS_FPU}>>:${CMAKE_SOURCE_DIR}/platform/ext/common/test_interrupt.c>
         $<$<BOOL:${TFM_SANITIZE}>:ext/common/tfm_sanitize_handlers.c>
+        $<$<BOOL:${CONFIG_PICOLIBC}>:ext/common/picolibc.c>
         ./ext/common/tfm_fatal_error.c
 )
 
@@ -203,6 +204,7 @@ if(BL2)
             $<$<OR:$<BOOL:${PLATFORM_DEFAULT_NV_COUNTERS}>,$<BOOL:${PLATFORM_DEFAULT_OTP}>>:ext/common/template/flash_otp_nv_counters_backend.c>
             $<$<BOOL:${PLATFORM_DEFAULT_OTP}>:ext/common/template/otp_flash.c>
             $<$<BOOL:${BL2_SANITIZE}>:ext/common/tfm_sanitize_handlers.c>
+            $<$<BOOL:${CONFIG_PICOLIBC}>:ext/common/picolibc.c>
             ./ext/common/tfm_fatal_error.c
     )
 
@@ -304,6 +306,7 @@ if(BL1 AND PLATFORM_DEFAULT_BL1)
             $<$<BOOL:${PLATFORM_DEFAULT_OTP}>:ext/common/template/flash_otp_nv_counters_backend.c>
             $<$<BOOL:${PLATFORM_DEFAULT_OTP}>:ext/common/template/otp_flash.c>
             $<$<OR:$<BOOL:${BL1_1_SANITIZE}>,$<BOOL:${TFM_BL1_2_SANITIZE}>>:ext/common/tfm_sanitize_handlers.c>
+            $<$<BOOL:${CONFIG_PICOLIBC}>:ext/common/picolibc.c>
             ./ext/common/tfm_fatal_error.c
     )
 
@@ -359,6 +362,7 @@ if(BL1 AND PLATFORM_DEFAULT_BL1)
             $<$<BOOL:${PLATFORM_DEFAULT_NV_COUNTERS}>:ext/common/template/nv_counters.c>
             $<$<OR:$<BOOL:${PLATFORM_DEFAULT_NV_COUNTERS}>,$<BOOL:${PLATFORM_DEFAULT_OTP}>>:ext/common/template/flash_otp_nv_counters_backend.c>
             $<$<BOOL:${PLATFORM_DEFAULT_OTP}>:ext/common/template/otp_flash.c>
+            $<$<BOOL:${CONFIG_PICOLIBC}>:ext/common/picolibc.c>
     )
 
     target_link_libraries(platform_bl1_2

platform/ext/common/picolibc.c

@@ -0,0 +1,56 @@
+/*
+ * Copyright © 2025, Keith Packard <keithp@keithp.com>
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#include <stdint.h>
+#include <string.h>
+#include "config_impl.h"
+
+#ifdef __PICOLIBC__
+
+/*
+ * Picolibc's startup code only initializes a single data and bss
+ * segment. Replace that to initialize all of the segments using
+ * the lists provided by the linker script.
+ */
+
+void __libc_init_array(void);
+void _start(void);
+int main(int, char **);
+
+void
+_start(void)
+{
+  typedef struct __copy_table {
+    uint32_t const* src;
+    uint32_t* dest;
+    uint32_t  wlen;
+  } __copy_table_t;
+
+  typedef struct __zero_table {
+    uint32_t* dest;
+    uint32_t  wlen;
+  } __zero_table_t;
+
+  extern const __copy_table_t __copy_table_start__;
+  extern const __copy_table_t __copy_table_end__;
+  extern const __zero_table_t __zero_table_start__;
+  extern const __zero_table_t __zero_table_end__;
+
+  for (__copy_table_t const* pTable = &__copy_table_start__; pTable < &__copy_table_end__; ++pTable) {
+    memcpy(pTable->dest, pTable->src, pTable->wlen << 2);
+  }
+
+  for (__zero_table_t const* pTable = &__zero_table_start__; pTable < &__zero_table_end__; ++pTable) {
+    memset(pTable->dest, 0, pTable->wlen << 2);
+  }
+
+  __libc_init_array();
+  main(0, NULL);
+  return;
+}
+
+#endif /* __PICOLIBC__ */

platform/ext/common/provisioning_bundle/CMakeLists.txt

@@ -54,6 +54,7 @@ target_sources(provisioning_bundle
     PRIVATE
         ./provisioning_code.c
         ./provisioning_data.c
+        $<$<BOOL:${CONFIG_PICOLIBC}>:${CMAKE_SOURCE_DIR}/platform/ext/common/picolibc.c>
         $<$<BOOL:${CONFIG_GNU_SYSCALL_STUB_ENABLED}>:${CMAKE_SOURCE_DIR}/platform/ext/common/syscalls_stub.c>
 )
 

platform/ext/common/syscalls_stub.c

@@ -14,6 +14,14 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#ifdef __PICOLIBC__
+__attribute__((weak))
+void _exit(int status)
+{
+    (void) status;
+    for(;;);
+}
+#else
 __attribute__((weak))
 void _close(void)
 {
@@ -53,3 +61,5 @@ __attribute__((weak))
 void _write(void)
 {
 }
+
+#endif /* !__PICOLIBC__ */

platform/ext/common/uart_stdout.c

@@ -85,7 +85,7 @@ int fputc(int ch, FILE *f)
 /* Redirect sdtio for PicoLib in LLVM toolchain
    as per https://github.com/picolibc/picolibc/blob/main/doc/os.md
    'fputch()' named intentionally different from 'fputc()' from picolib */
-#elif defined(__clang_major__)
+#elif defined(__PICOLIBC__)
 
 int fputch(char ch, struct __file *f)
 {