Support builds with picolibc

platforms/chibios/boards/SIPEED_LONGAN_NANO/configs/chconf.h

@@ -0,0 +1,23 @@
+/* Copyright 2021 QMK
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/* To compile the ChibiOS syscall stubs with picolibc
+ * the _reent struct has to be defined. */
+#if !defined(_FROM_ASM_) && defined(USE_PICOLIBC)
+struct _reent;
+#endif
+
+#include_next <chconf.h>

tmk_core/chibios.mk

@@ -301,7 +301,7 @@ endif
 #
 
 # Use defined stack sizes of the main thread in linker scripts
-LDSYMBOLS =,--defsym=__process_stack_size__=$(USE_PROCESS_STACKSIZE),--defsym=__main_stack_size__=$(USE_EXCEPTIONS_STACKSIZE)
+LDSYMBOLS =--defsym=__process_stack_size__=$(USE_PROCESS_STACKSIZE),--defsym=__main_stack_size__=$(USE_EXCEPTIONS_STACKSIZE)
 
 # Shared Compiler flags for all toolchains
 SHARED_CFLAGS = -fomit-frame-pointer \
@@ -311,7 +311,8 @@ SHARED_CFLAGS = -fomit-frame-pointer \
                 -fshort-wchar
 
 # Shared Linker flags for all toolchains
-SHARED_LDFLAGS = -Wl,--script=$(LDSCRIPT)$(LDSYMBOLS) \
+SHARED_LDFLAGS = -T $(LDSCRIPT) \
+                 -Wl,$(LDSYMBOLS) \
                  -Wl,--gc-sections \
                  -nostartfiles
 
@@ -329,7 +330,19 @@ ifeq ($(strip $(MCU)), risc-v)
             endif
         endif
     endif
-
+
+    # Default to compiling with picolibc for RISC-V targets if available,
+    # which is available by default on current (bullseye) debian based systems.
+    ifeq ($(shell $(TOOLCHAIN)gcc --specs=picolibc.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
+        # Toolchain specific Compiler flags
+        # Note that we still link with our own linker script
+        # by providing it via the -T flag above.
+        TOOLCHAIN_CFLAGS = --specs=picolibc.specs
+
+        # Tell QMK that we are compiling with picolibc.
+        OPT_DEFS += -DUSE_PICOLIBC
+    endif
+
     # MCU architecture flags
     MCUFLAGS = -march=$(MCU_ARCH) \
                -mabi=$(MCU_ABI) \

tmk_core/common/chibios/syscall-fallbacks.c

@@ -18,6 +18,12 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
+/* To compile the ChibiOS syscall stubs with picolibc
+ * the _reent struct has to be defined. */
+#if defined(USE_PICOLIBC)
+struct _reent;
+#endif
+
 #pragma GCC diagnostic ignored "-Wmissing-prototypes"
 
 __attribute__((weak, used)) int _open_r(struct _reent *r, const char *path, int flag, int m) {