Provide support for Picolibc

CMakeLists.txt

@@ -926,11 +926,14 @@ if(CONFIG_USERSPACE)
     ${APP_SMEM_UNALIGNED_LD}
     )
 
+  if(CONFIG_PICOLIBC)
+    set(LIBC_PARTITIONS -l libc.a z_libc_partition)
+  endif()
   if(CONFIG_NEWLIB_LIBC)
-    set(NEWLIB_PART -l libc.a z_libc_partition)
+    set(LIBC_PARTITIONS -l libc.a z_libc_partition)
   endif()
   if(CONFIG_NEWLIB_LIBC_NANO)
-    set(NEWLIB_PART -l libc_nano.a z_libc_partition)
+    set(LIBC_PARTITIONS -l libc_nano.a z_libc_partition)
   endif()
 
   add_custom_command(
@@ -939,7 +942,7 @@ if(CONFIG_USERSPACE)
     ${ZEPHYR_BASE}/scripts/gen_app_partitions.py
     -d ${OBJ_FILE_DIR}
     -o ${APP_SMEM_UNALIGNED_LD}
-    ${NEWLIB_PART}
+    ${LIBC_PARTITIONS}
     $<TARGET_PROPERTY:zephyr_property_target,COMPILE_OPTIONS>
     $<$<BOOL:${CMAKE_VERBOSE_MAKEFILE}>:--verbose>
     DEPENDS
@@ -990,7 +993,7 @@ if(CONFIG_USERSPACE)
     ${ZEPHYR_BASE}/scripts/gen_app_partitions.py
     -e $<TARGET_FILE:app_smem_unaligned_prebuilt>
     -o ${APP_SMEM_ALIGNED_LD}
-    ${NEWLIB_PART}
+    ${LIBC_PARTITIONS}
     $<TARGET_PROPERTY:zephyr_property_target,COMPILE_OPTIONS>
     $<$<BOOL:${CMAKE_VERBOSE_MAKEFILE}>:--verbose>
     DEPENDS

doc/getting_started/toolchain_3rd_party_x_compilers.rst

@@ -70,6 +70,14 @@ You can build toolchains from source code using crosstool-NG.
 
       ./go.sh <arch>
 
+   You can use this method to build a toolchain using picolibc as
+   well. ``sdk-ng`` has configurations for arm64-picolibc, arm-picolibc and
+   riscv64-picolibc (which supports both 32- and 64- bit RISC-V targets).
+
+   .. code-block:: console
+
+      ./go.sh <arch>-picolibc
+
    .. note::
 
       Currently, only i586 and Arm toolchain builds are verified.

doc/getting_started/toolchain_other_x_compilers.rst

@@ -59,3 +59,33 @@ the common prefix is ``/usr/bin/arm-none-eabi-`` (including the trailing dash,
 ``-``).  If your toolchain is installed in ``/opt/mytoolchain/bin`` with binary
 names based on target triplet ``myarch-none-elf``, ``CROSS_COMPILE`` would be
 set to ``/opt/mytoolchain/bin/myarch-none-elf-``.
+
+Other C Libraries
+#################
+
+Once you have a cross compiler installed and configured, you may need to install
+a C library. For example, you might install the picolibc-arm-none-eabi package
+on Debian-based Linux systems:
+
+   .. code-block:: console
+
+      # On Debian
+      sudo apt-get install picolibc-arm-none-eabi
+
+Or, you may install picolibc from source. To get the picolibc source, download a
+tarball from github:
+
+      $ wget https://github.com/picolibc/picolibc/archive/1.4.6.tar.gz
+      $ tar xf 1.4.6.tar.gz
+      $ cd picolibc-1.4.6
+
+There are sample configurations for a range of cross compilers provided with
+picolibc, including RISC-V and ARM. For example, to build picolibc using an
+arm-none-eabi compiler:
+
+   ..code-block:: console
+
+      $ mkdir build
+      $ cd build
+      $ sh ../do-arm-configure
+      $ ninja install

doc/guides/c_library.rst

@@ -18,6 +18,13 @@ support for ``newlib`` is a set of hooks available under
 :file:`lib/libc/newlib/libc-hooks.c` which integrates the c library with basic
 kernel services.
 
+Zephyr can be used with ``picolibc`` by selecting the
+:option:`CONFIG_PICOLIBC` in the application configuration file. Part of the
+support for ``picolibc`` is a set of hooks available under
+:file:`lib/libc/picolibc/libc-hooks.c` which integrates the c library with basic
+kernel services. Applications that do not require floating point scanf or printf
+functionality can select the
+:option:`CONFIG_PICOLIBC`
 
 Minimal C Library
 *****************

include/sys/libc-hooks.h

@@ -16,7 +16,7 @@
  * that need to call into the kernel as system calls
  */
 
-#ifdef CONFIG_NEWLIB_LIBC
+#if defined(CONFIG_PICOLIBC) || defined(CONFIG_NEWLIB_LIBC)
 
 /* syscall generation ignores preprocessor, ensure this is defined to ensure
  * we don't have compile errors
@@ -58,7 +58,21 @@ extern struct k_mem_partition z_malloc_partition;
  */
 #define Z_MALLOC_PARTITION_EXISTS 1
 #endif /* CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE > 0 */
-#endif /* CONFIG_MINIMAL_LIBC */
+#elif defined(CONFIG_PICOLIBC)
+/* If we are using picolibc, the heap arena is in one of two areas:
+ *  - If we have an MPU that requires power of two alignment, the heap bounds
+ *    must be specified in Kconfig via CONFIG_PICOLIBC_ALIGNED_HEAP_SIZE.
+ *  - Otherwise, the heap arena on most arches starts at a suitably
+ *    aligned base addreess after the `_end` linker symbol, through to the end
+ *    of system RAM.
+ */
+#if (!defined(CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT) || \
+     (defined(CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT) && \
+      CONFIG_PICOLIBC_ALIGNED_HEAP_SIZE))
+#define Z_MALLOC_PARTITION_EXISTS 1
+extern struct k_mem_partition z_malloc_partition;
+#endif
+#endif /* CONFIG_PICOLIBC */
 
 #ifdef Z_MALLOC_PARTITION_EXISTS
 /* Memory partition containing the libc malloc arena. Configuration controls
@@ -68,7 +82,7 @@ extern struct k_mem_partition z_malloc_partition;
 #endif
 
 #if defined(CONFIG_NEWLIB_LIBC) || defined(CONFIG_STACK_CANARIES) || \
-    defined(CONFIG_NEED_LIBC_MEM_PARTITION)
+    defined(CONFIG_PICOLIBC) || defined(CONFIG_NEED_LIBC_MEM_PARTITION)
 /* Minimal libc has no globals. We do put the stack canary global in the
  * libc partition since it is not worth placing in a partition of its own.
  *

kernel/Kconfig

@@ -197,9 +197,17 @@ config THREAD_USERSPACE_LOCAL_DATA
 	depends on USERSPACE
 	default y if ERRNO && !ERRNO_IN_TLS
 
+config LIBC_ERRNO
+	bool
+	help
+	  Use external libc errno, not the internal one. This makes sure that
+	  ERRNO is not set for libc configurations that provide their own
+	  implementation.
+
 config ERRNO
 	bool "Enable errno support"
 	default y
+	depends on !LIBC_ERRNO
 	help
 	  Enable per-thread errno in the kernel. Application and library code must
 	  include errno.h provided by the C library (libc) to use the errno

kernel/errno.c

@@ -44,10 +44,13 @@ static inline int *z_vrfy_z_errno(void)
 #include <syscalls/z_errno_mrsh.c>
 
 #else
+#ifndef CONFIG_PICOLIBC
 int *z_impl_z_errno(void)
 {
 	return &_current->errno_var;
 }
+#endif /* CONFIG_PICOLIBC */
+
 #endif /* CONFIG_USERSPACE */
 
 #endif /* CONFIG_ERRNO_IN_TLS */

lib/libc/CMakeLists.txt

@@ -1,7 +1,11 @@
 # SPDX-License-Identifier: Apache-2.0
 
+if(CONFIG_PICOLIBC)
+  add_subdirectory(picolibc)
+else()
 if(CONFIG_NEWLIB_LIBC)
   add_subdirectory(newlib)
 else()
   add_subdirectory(minimal)
 endif()
+endif()

lib/libc/Kconfig

@@ -24,9 +24,19 @@ config MINIMAL_LIBC
 	help
 	  Build with minimal C library.
 
+config PICOLIBC
+	bool "Picolibc library"
+	depends on !NATIVE_APPLICATION
+	select THREAD_LOCAL_STORAGE if ARCH_HAS_THREAD_LOCAL_STORAGE
+	select LIBC_ERRNO
+	help
+	  Build with picolibc library. The picolibc library is expected to be
+	  part of the SDK in this case.
+
 config NEWLIB_LIBC
 	bool "Newlib C library"
 	depends on !NATIVE_APPLICATION
+	select LIBC_ERRNO
 	help
 	  Build with newlib library. The newlib library is expected to be
 	  part of the SDK in this case.
@@ -41,11 +51,36 @@ endchoice # LIBC_IMPLEMENTATION
 config HAS_NEWLIB_LIBC_NANO
 	bool
 
+if PICOLIBC
+
+config PICOLIBC_INTEGER_PRINTF
+	bool "Build with picolibc integer-only printf"
+	help
+	  Build with floating point printf disabled. This will reduce the size
+	  of the image.
+
+config PICOLIBC_ALIGNED_HEAP_SIZE
+	int "Picolibc aligned heap size (bytes)"
+	depends on MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT
+	depends on USERSPACE
+	default 0
+	help
+	  If user mode is enabled, and MPU hardware has requirements that
+	  regions be sized to a power of two and aligned to their size,
+	  and user mode threads need to access this heap, then this is necessary
+	  to properly define an MPU region for the heap.
+
+	  If this is left at 0, then remaining system RAM will be used for this
+	  area and it may not be possible to program it as an MPU region.
+
+endif # PICOLIBC
+
 if NEWLIB_LIBC
 
 config NEWLIB_LIBC_NANO
 	bool "Build with newlib-nano C library"
 	depends on HAS_NEWLIB_LIBC_NANO
+	select LIBC_ERRNO
 	default y
 	help
 	  Build with newlib-nano library, for small embedded apps.

lib/libc/picolibc/CMakeLists.txt

@@ -0,0 +1,41 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_library()
+zephyr_library_sources(libc-hooks.c)
+
+# Zephyr normally uses -ffreestanding, which with current GNU toolchains
+# means that the flag macros used by picolibc <inttypes.h> to signal
+# support for PRI.64 macros are not present.  To make them available we
+# need to hook into the include path before the system files and
+# explicitly include the picolibc header that provides those macros.
+zephyr_include_directories(include)
+
+# define __LINUX_ERRNO_EXTENSIONS__ so we get errno defines like -ESHUTDOWN
+# used by the network stack
+zephyr_compile_definitions(__LINUX_ERRNO_EXTENSIONS__)
+
+zephyr_link_libraries(
+  m
+  c
+  gcc # Lib C depends on libgcc.
+  )
+
+# The -T/dev/null avoids pulling in picolibc.ld
+zephyr_link_libraries(
+  --specs=picolibc.specs
+  -T/dev/null
+  )
+
+zephyr_compile_options(
+  --specs=picolibc.specs
+  -D_GNU_SOURCE
+  )
+
+if(CONFIG_PICOLIBC_INTEGER_PRINTF)
+  zephyr_compile_options(
+    -DPICOLIBC_INTEGER_PRINTF_SCANF
+    )
+  zephyr_link_libraries(
+    -DPICOLIBC_INTEGER_PRINTF_SCANF
+    )
+endif()