[Core] Add support for RISC-V builds and GD32VF103 MCU (#12508)

* Add support for RISC-V builds and GD32VF103 MCU

* Add toolchain selection in chibios.mk based on the mcu selected in
mcu_selection.mk
* Reorder and added comments to chibios.mk to have a streamlined makefile
* Add GD32VF103 mcu to possible targets for QMK.
* Add STM32 compatibility for GD32VF103 MCU, this is hacky but more efficent
  then rewriting every driver.
* Add GigaDevice DFU bootloader as flash target, please note that
  dfu-util of at least version 0.10 is needed.
* Add analog driver compatibility
* Add apa102 bitbang driver compatibility
* Add ws2812 bitbang driver compatibility
* Add eeprom in flash emulation compatibility
* Allow faster re-builds with ccache

* Add SiPeed Longan Nano to platform files

* Add SiPeed Longan Nano Onekeys

* Make quine compatible with other bootloaders

* Support builds with picolibc

* Add risc-v toolchain to arch and debian/ubuntu scripts

7 files changed, 377 insertions, 111 deletions

diff --git a/tmk_core/chibios.mk b/tmk_core/chibios.mk
index 18839710be..cae840fa4f 100644
--- a/tmk_core/chibios.mk
+++ b/tmk_core/chibios.mk
@@ -27,29 +27,62 @@ endif
 OPT_OS = chibios
 CHIBIOS = $(TOP_DIR)/lib/chibios
 CHIBIOS_CONTRIB = $(TOP_DIR)/lib/chibios-contrib
-# Startup files. Try a few different locations, for compability with old versions and
-# for things hardware in the contrib repository
-STARTUP_MK = $(CHIBIOS)/os/common/ports/ARMCMx/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
-ifeq ("$(wildcard $(STARTUP_MK))","")
-	STARTUP_MK = $(CHIBIOS)/os/common/startup/ARMCMx/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
-	ifeq ("$(wildcard $(STARTUP_MK))","")
-		STARTUP_MK = $(CHIBIOS_CONTRIB)/os/common/startup/ARMCMx/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
-	endif
+
+#
+# Startup, Port and Platform support selection
+##############################################################################
+
+ifeq ($(strip $(MCU)), risc-v)
+    # RISC-V Support
+    # As of 7.4.2021 there is only one supported RISC-V platform in Chibios-Contrib,
+    # therefore all required settings are hard-coded
+    STARTUP_MK = $(CHIBIOS_CONTRIB)/os/common/startup/RISCV-ECLIC/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
+    PORT_V = $(CHIBIOS_CONTRIB)/os/common/ports/RISCV-ECLIC/compilers/GCC/mk/port.mk
+    RULESPATH = $(CHIBIOS_CONTRIB)/os/common/startup/RISCV-ECLIC/compilers/GCC
+    PLATFORM_MK = $(CHIBIOS_CONTRIB)/os/hal/ports/GD/GD32VF103/platform.mk
+else
+    # ARM Support
+    # Startup files. Try a few different locations, for compability with old versions and
+    # for things hardware in the contrib repository
+    STARTUP_MK = $(CHIBIOS)/os/common/ports/ARMCMx/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
+    ifeq ("$(wildcard $(STARTUP_MK))","")
+        STARTUP_MK = $(CHIBIOS)/os/common/startup/ARMCMx/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
+        ifeq ("$(wildcard $(STARTUP_MK))","")
+            STARTUP_MK = $(CHIBIOS_CONTRIB)/os/common/startup/ARMCMx/compilers/GCC/mk/startup_$(MCU_STARTUP).mk
+        endif
+    endif
+
+    # Compability with old version
+    PORT_V = $(CHIBIOS)/os/rt/ports/ARMCMx/compilers/GCC/mk/port_v$(ARMV)m.mk
+    ifeq ("$(wildcard $(PORT_V))","")
+        PORT_V = $(CHIBIOS)/os/common/ports/ARMCMx/compilers/GCC/mk/port_v$(ARMV)m.mk
+    endif
+
+    RULESPATH = $(CHIBIOS)/os/common/ports/ARMCMx/compilers/GCC
+    ifeq ("$(wildcard $(RULESPATH)/rules.mk)","")
+        RULESPATH = $(CHIBIOS)/os/common/startup/ARMCMx/compilers/GCC
+    endif
 endif
-include $(STARTUP_MK)
-# HAL-OSAL files (optional).
-include $(CHIBIOS)/os/hal/hal.mk
 
 ifeq ("$(PLATFORM_NAME)","")
-	PLATFORM_NAME = platform
+    PLATFORM_NAME = platform
 endif
 
-PLATFORM_MK = $(CHIBIOS)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
 ifeq ("$(wildcard $(PLATFORM_MK))","")
-PLATFORM_MK = $(CHIBIOS_CONTRIB)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
+    PLATFORM_MK = $(CHIBIOS)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
+    ifeq ("$(wildcard $(PLATFORM_MK))","")
+        PLATFORM_MK = $(CHIBIOS_CONTRIB)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
+    endif
 endif
+
+include $(STARTUP_MK)
+include $(PORT_V)
 include $(PLATFORM_MK)
 
+#
+# Board support selection.
+##############################################################################
+
 BOARD_MK :=
 
 ifneq ("$(wildcard $(KEYBOARD_PATH_5)/boards/$(BOARD)/board.mk)","")
@@ -77,13 +110,19 @@ else ifneq ("$(wildcard $(TOP_DIR)/platforms/chibios/boards/$(BOARD)/board/board
 endif
 
 ifeq ("$(wildcard $(BOARD_MK))","")
-	BOARD_MK = $(CHIBIOS)/os/hal/boards/$(BOARD)/board.mk
-	ifeq ("$(wildcard $(BOARD_MK))","")
-		BOARD_MK = $(CHIBIOS_CONTRIB)/os/hal/boards/$(BOARD)/board.mk
-	endif
+    BOARD_MK = $(CHIBIOS)/os/hal/boards/$(BOARD)/board.mk
+    ifeq ("$(wildcard $(BOARD_MK))","")
+        BOARD_MK = $(CHIBIOS_CONTRIB)/os/hal/boards/$(BOARD)/board.mk
+    endif
 endif
 
-# Bootloader address
+include $(BOARD_MK)
+
+#
+# Bootloader selection.
+##############################################################################
+
+# Set bootloader address if supplied.
 ifdef STM32_BOOTLOADER_ADDRESS
     OPT_DEFS += -DSTM32_BOOTLOADER_ADDRESS=$(STM32_BOOTLOADER_ADDRESS)
 endif
@@ -113,6 +152,10 @@ else ifneq ("$(wildcard $(BOARD_PATH)/configs/bootloader_defs.h)","")
     OPT_DEFS += -include $(BOARD_PATH)/configs/bootloader_defs.h
 endif
 
+#
+# ChibiOS config selection.
+##############################################################################
+
 # Work out the config file directories
 ifneq ("$(wildcard $(KEYBOARD_PATH_5)/chconf.h)","")
     CHCONFDIR = $(KEYBOARD_PATH_5)
@@ -130,6 +173,10 @@ else ifneq ("$(wildcard $(TOP_DIR)/platforms/boards/chibios/common/configs/chcon
     CHCONFDIR = $(TOP_DIR)/platforms/chibios/boards/common/configs
 endif
 
+#
+# HAL config selection.
+##############################################################################
+
 ifneq ("$(wildcard $(KEYBOARD_PATH_5)/halconf.h)","")
     HALCONFDIR = $(KEYBOARD_PATH_5)
 else ifneq ("$(wildcard $(KEYBOARD_PATH_4)/halconf.h)","")
@@ -146,40 +193,10 @@ else ifneq ("$(wildcard $(TOP_DIR)/platforms/chibios/boards/common/configs/halco
     HALCONFDIR = $(TOP_DIR)/platforms/chibios/boards/common/configs
 endif
 
-# HAL-OSAL files (optional).
-include $(CHIBIOS)/os/hal/hal.mk
-
-ifeq ("$(PLATFORM_NAME)","")
-	PLATFORM_NAME = platform
-endif
-
-PLATFORM_MK = $(CHIBIOS)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
-ifeq ("$(wildcard $(PLATFORM_MK))","")
-PLATFORM_MK = $(CHIBIOS_CONTRIB)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
-endif
-include $(PLATFORM_MK)
-
-
-include $(BOARD_MK)
--include $(CHIBIOS)/os/hal/osal/rt/osal.mk         # ChibiOS <= 19.x
--include $(CHIBIOS)/os/hal/osal/rt-nil/osal.mk     # ChibiOS >= 20.x
-# RTOS files (optional).
-include $(CHIBIOS)/os/rt/rt.mk
-# Compability with old version
-PORT_V = $(CHIBIOS)/os/rt/ports/ARMCMx/compilers/GCC/mk/port_v$(ARMV)m.mk
-ifeq ("$(wildcard $(PORT_V))","")
-PORT_V = $(CHIBIOS)/os/common/ports/ARMCMx/compilers/GCC/mk/port_v$(ARMV)m.mk
-endif
-include $(PORT_V)
-# Other files (optional).
-include $(CHIBIOS)/os/hal/lib/streams/streams.mk
-
-RULESPATH = $(CHIBIOS)/os/common/ports/ARMCMx/compilers/GCC
-ifeq ("$(wildcard $(RULESPATH)/rules.mk)","")
-RULESPATH = $(CHIBIOS)/os/common/startup/ARMCMx/compilers/GCC
-endif
+#
+# Linker script selection.
+##############################################################################
 
-# Define linker script file here
 ifneq ("$(wildcard $(KEYBOARD_PATH_5)/ld/$(MCU_LDSCRIPT).ld)","")
     LDSCRIPT = $(KEYBOARD_PATH_5)/ld/$(MCU_LDSCRIPT).ld
 else ifneq ("$(wildcard $(KEYBOARD_PATH_4)/ld/$(MCU_LDSCRIPT).ld)","")
@@ -202,17 +219,30 @@ else
     LDSCRIPT = $(STARTUPLD)/$(MCU_LDSCRIPT).ld
 endif
 
+#
+# Include ChibiOS makefiles.
+##############################################################################
+
+# HAL-OSAL files (optional).
+include $(CHIBIOS)/os/hal/hal.mk
+-include $(CHIBIOS)/os/hal/osal/rt/osal.mk         # ChibiOS <= 19.x
+-include $(CHIBIOS)/os/hal/osal/rt-nil/osal.mk     # ChibiOS >= 20.x
+# RTOS files (optional).
+include $(CHIBIOS)/os/rt/rt.mk
+# Other files (optional).
+include $(CHIBIOS)/os/hal/lib/streams/streams.mk
+
 CHIBISRC = $(STARTUPSRC) \
-       $(KERNSRC) \
-       $(PORTSRC) \
-       $(OSALSRC) \
-       $(HALSRC) \
-       $(PLATFORMSRC) \
-       $(BOARDSRC) \
-       $(STREAMSSRC) \
-       $(CHIBIOS)/os/various/syscalls.c \
-       $(PLATFORM_COMMON_DIR)/syscall-fallbacks.c \
-       $(PLATFORM_COMMON_DIR)/wait.c
+        $(KERNSRC) \
+        $(PORTSRC) \
+        $(OSALSRC) \
+        $(HALSRC) \
+        $(PLATFORMSRC) \
+        $(BOARDSRC) \
+        $(STREAMSSRC) \
+        $(CHIBIOS)/os/various/syscalls.c \
+        $(PLATFORM_COMMON_DIR)/syscall-fallbacks.c \
+        $(PLATFORM_COMMON_DIR)/wait.c
 
 # Ensure the ASM files are not subjected to LTO -- it'll strip out interrupt handlers otherwise.
 QUANTUM_LIB_SRC += $(STARTUPASM) $(PORTASM) $(OSALASM) $(PLATFORMASM)
@@ -247,9 +277,9 @@ else ifneq ("$(wildcard $(TOP_DIR)/platforms/chibios/boards/$(BOARD)/configs/hal
 endif
 
 ifeq ($(strip $(USE_CHIBIOS_CONTRIB)),yes)
-	include $(CHIBIOS_CONTRIB)/os/hal/hal.mk
-	CHIBISRC += $(PLATFORMSRC_CONTRIB) $(HALSRC_CONTRIB)
-	EXTRAINCDIRS += $(PLATFORMINC_CONTRIB) $(HALINC_CONTRIB) $(CHIBIOS_CONTRIB)/os/various
+    include $(CHIBIOS_CONTRIB)/os/hal/hal.mk
+    CHIBISRC += $(PLATFORMSRC_CONTRIB) $(HALSRC_CONTRIB)
+    EXTRAINCDIRS += $(PLATFORMINC_CONTRIB) $(HALINC_CONTRIB) $(CHIBIOS_CONTRIB)/os/various
 endif
 
 #
@@ -267,61 +297,123 @@ ifneq ("$(wildcard $(BOARD_PATH)/configs/post_config.h)","")
 endif
 
 ##############################################################################
-# Compiler settings
+# Compiler and Linker configuration
 #
-CC = arm-none-eabi-gcc
-OBJCOPY = arm-none-eabi-objcopy
-OBJDUMP = arm-none-eabi-objdump
-SIZE = arm-none-eabi-size
-AR = arm-none-eabi-ar
-NM = arm-none-eabi-nm
-HEX = $(OBJCOPY) -O $(FORMAT)
-EEP =
-BIN = $(OBJCOPY) -O binary
-
-THUMBFLAGS = -DTHUMB_PRESENT -mno-thumb-interwork -DTHUMB_NO_INTERWORKING -mthumb -DTHUMB
-
-COMPILEFLAGS += -fomit-frame-pointer
-COMPILEFLAGS += -falign-functions=16
-COMPILEFLAGS += -ffunction-sections
-COMPILEFLAGS += -fdata-sections
-COMPILEFLAGS += -fno-common
-COMPILEFLAGS += -fshort-wchar
-COMPILEFLAGS += $(THUMBFLAGS)
-
-# FPU options default (Cortex-M4 and Cortex-M7 single precision).
-USE_FPU_OPT ?= -mfloat-abi=hard -mfpu=fpv4-sp-d16 -fsingle-precision-constant
-
-# FPU-related options
-USE_FPU ?= no
-ifneq ($(USE_FPU),no)
-  COMPILEFLAGS += $(USE_FPU_OPT)
-  OPT_DEFS += -DCORTEX_USE_FPU=TRUE
+
+# Use defined stack sizes of the main thread in linker scripts
+LDSYMBOLS =--defsym=__process_stack_size__=$(USE_PROCESS_STACKSIZE),--defsym=__main_stack_size__=$(USE_EXCEPTIONS_STACKSIZE)
+
+# Shared Compiler flags for all toolchains
+SHARED_CFLAGS = -fomit-frame-pointer \
+                -ffunction-sections \
+                -fdata-sections \
+                -fno-common \
+                -fshort-wchar
+
+# Shared Linker flags for all toolchains
+SHARED_LDFLAGS = -T $(LDSCRIPT) \
+                 -Wl,$(LDSYMBOLS) \
+                 -Wl,--gc-sections \
+                 -nostartfiles
+
+ifeq ($(strip $(MCU)), risc-v)
+    # RISC-V toolchain specific configuration
+    # Find suitable GCC compiler
+    ifeq ($(strip $(TOOLCHAIN)),)
+        ifneq ($(shell which riscv32-unknown-elf-gcc 2>/dev/null),)
+            TOOLCHAIN = riscv32-unknown-elf-
+        else
+            ifneq ($(shell which riscv64-unknown-elf-gcc 2>/dev/null),)
+                TOOLCHAIN = riscv64-unknown-elf-
+            else
+                $(error "No RISC-V toolchain found. Can't find riscv32-unknown-elf-gcc or riscv64-unknown-elf-gcc found in your systems PATH variable. Please install a valid toolchain and make it accessible!")
+            endif
+        endif
+    endif
+    
+    # Default to compiling with picolibc for RISC-V targets if available,
+    # which is available by default on current (bullseye) debian based systems.
+    ifeq ($(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) \
+               -mcmodel=$(MCU_CMODEL) \
+               -mstrict-align
 else
-  OPT_DEFS += -DCORTEX_USE_FPU=FALSE
+    # ARM toolchain specific configuration
+    TOOLCHAIN ?= arm-none-eabi-
+
+    # Toolchain specific Linker flags
+    TOOLCHAIN_LDFLAGS = -Wl,--no-wchar-size-warning \
+                        --specs=nano.specs
+
+    # MCU architecture flags
+    MCUFLAGS = -mcpu=$(MCU) \
+               -mthumb -DTHUMB_PRESENT \
+               -mno-thumb-interwork -DTHUMB_NO_INTERWORKING \
+               -mno-unaligned-access
+
+    # Some ARM cores like the M4 and M7 have floating point units which can be enabled
+    USE_FPU ?= no
+
+    ifneq ($(USE_FPU),no)
+        OPT_DEFS += -DCORTEX_USE_FPU=TRUE
+
+        # Default is single precision floats
+        USE_FPU_OPT ?= -mfloat-abi=hard \
+                       -mfpu=fpv4-sp-d16 \
+                       -fsingle-precision-constant
+
+        MCUFLAGS += $(USE_FPU_OPT)
+    else
+        OPT_DEFS += -DCORTEX_USE_FPU=FALSE
+    endif
 endif
 
-CFLAGS += $(COMPILEFLAGS)
+# Assembler flags
+ASFLAGS  += $(SHARED_ASFLAGS) $(TOOLCHAIN_ASFLAGS)
 
-ASFLAGS += $(THUMBFLAGS)
+# C Compiler flags
+CFLAGS   += $(SHARED_CFLAGS) $(TOOLCHAIN_CFLAGS)
 
-CXXFLAGS += $(COMPILEFLAGS)
-CXXFLAGS += -fno-rtti
+# C++ Compiler flags
+CXXFLAGS += $(CFLAGS) $(SHARED_CXXFLAGS) $(TOOLCHAIN_CXXFLAGS) -fno-rtti
 
-LDFLAGS +=-Wl,--gc-sections
-LDFLAGS +=-Wl,--no-wchar-size-warning
-LDFLAGS += -mno-thumb-interwork -mthumb
-LDSYMBOLS =,--defsym=__process_stack_size__=$(USE_PROCESS_STACKSIZE)
-LDSYMBOLS :=$(LDSYMBOLS),--defsym=__main_stack_size__=$(USE_EXCEPTIONS_STACKSIZE)
-LDFLAGS += -Wl,--script=$(LDSCRIPT)$(LDSYMBOLS)
-LDFLAGS += --specs=nano.specs
+# Linker flags
+LDFLAGS  += $(SHARED_LDFLAGS) $(TOOLCHAIN_LDFLAGS) $(MCUFLAGS)
 
+# Tell QMK that we are hosting it on ChibiOS.
 OPT_DEFS += -DPROTOCOL_CHIBIOS
 
-# Workaround to stop ChibiOS from complaining about new GCC -- it's been fixed for 7/8/9 already
-OPT_DEFS += -DPORT_IGNORE_GCC_VERSION_CHECK=1
+# Speed up recompilations by opt-in usage of ccache
+USE_CCACHE ?= no
+ifneq ($(USE_CCACHE),no)
+    CC_PREFIX ?= ccache
+endif
+
+# Construct GCC toolchain
+CC      = $(CC_PREFIX) $(TOOLCHAIN)gcc
+OBJCOPY = $(TOOLCHAIN)objcopy
+OBJDUMP = $(TOOLCHAIN)objdump
+SIZE    = $(TOOLCHAIN)size
+AR      = $(TOOLCHAIN)ar
+NM      = $(TOOLCHAIN)nm
+HEX     = $(OBJCOPY) -O $(FORMAT)
+EEP     =
+BIN     = $(OBJCOPY) -O binary
 
-MCUFLAGS = -mcpu=$(MCU)
+##############################################################################
+# Make targets
+#
 
 DEBUG = gdb
 
diff --git a/tmk_core/common/chibios/bootloader.c b/tmk_core/common/chibios/bootloader.c
index f9514ee5f3..5cadadeeeb 100644
--- a/tmk_core/common/chibios/bootloader.c
+++ b/tmk_core/common/chibios/bootloader.c
@@ -95,6 +95,28 @@ void enter_bootloader_mode_if_requested(void) {
     }
 }
 
+#elif defined(GD32VF103)
+
+#    define DBGMCU_KEY_UNLOCK 0x4B5A6978
+#    define DBGMCU_CMD_RESET 0x1
+
+__IO uint32_t *DBGMCU_KEY = (uint32_t *)DBGMCU_BASE + 0x0CU;
+__IO uint32_t *DBGMCU_CMD = (uint32_t *)DBGMCU_BASE + 0x08U;
+
+__attribute__((weak)) void bootloader_jump(void) {
+    /* The MTIMER unit of the GD32VF103 doesn't have the MSFRST
+     * register to generate a software reset request.
+     * BUT instead two undocumented registers in the debug peripheral
+     * that allow issueing a software reset. WHO would need the MSFRST
+     * register anyway? Source:
+     * https://github.com/esmil/gd32vf103inator/blob/master/include/gd32vf103/dbg.h */
+    *DBGMCU_KEY = DBGMCU_KEY_UNLOCK;
+    *DBGMCU_CMD = DBGMCU_CMD_RESET;
+}
+
+void enter_bootloader_mode_if_requested(void) { /* Jumping to bootloader is not possible from user code. */
+}
+
 #elif defined(KL2x) || defined(K20x) || defined(MK66F18) || defined(MIMXRT1062)  // STM32_BOOTLOADER_DUAL_BANK // STM32_BOOTLOADER_ADDRESS
 /* Kinetis */
 
diff --git a/tmk_core/common/chibios/chibios_config.h b/tmk_core/common/chibios/chibios_config.h
index f065948028..ad2f808a95 100644
--- a/tmk_core/common/chibios/chibios_config.h
+++ b/tmk_core/common/chibios/chibios_config.h
@@ -39,6 +39,25 @@
 #    endif
 #endif
 
+// GD32 compatibility
+#if defined(MCU_GD32V)
+#    define CPU_CLOCK GD32_SYSCLK
+
+#    if defined(GD32VF103)
+#        define USE_GPIOV1
+#        define USE_I2CV1
+#        define PAL_MODE_ALTERNATE_OPENDRAIN PAL_MODE_GD32_ALTERNATE_OPENDRAIN
+#        define PAL_MODE_ALTERNATE_PUSHPULL PAL_MODE_GD32_ALTERNATE_PUSHPULL
+#    endif
+#endif
+
+#if defined(GD32VF103)
+/* This chip has the same API as STM32F103, but uses different names for literally the same thing.
+ * As of 4.7.2021 QMK is tailored to use STM32 defines/names, for compatibility sake
+ * we just redefine the GD32 names. */
+#    include "gd32v_compatibility.h"
+#endif
+
 // teensy compatibility
 #if defined(MCU_KINETIS)
 #    define CPU_CLOCK KINETIS_SYSCLK_FREQUENCY
diff --git a/tmk_core/common/chibios/eeprom_stm32_defs.h b/tmk_core/common/chibios/eeprom_stm32_defs.h
index 3499796264..66904f247f 100644
--- a/tmk_core/common/chibios/eeprom_stm32_defs.h
+++ b/tmk_core/common/chibios/eeprom_stm32_defs.h
@@ -18,7 +18,7 @@
 #include <hal.h>
 
 #if !defined(FEE_PAGE_SIZE) || !defined(FEE_PAGE_COUNT)
-#    if defined(STM32F103xB) || defined(STM32F042x6)
+#    if defined(STM32F103xB) || defined(STM32F042x6) || defined(GD32VF103C8) || defined(GD32VF103CB)
 #        ifndef FEE_PAGE_SIZE
 #            define FEE_PAGE_SIZE 0x400  // Page size = 1KByte
 #        endif
@@ -45,7 +45,9 @@
 #if !defined(FEE_MCU_FLASH_SIZE)
 #    if defined(STM32F042x6)
 #        define FEE_MCU_FLASH_SIZE 32  // Size in Kb
-#    elif defined(STM32F103xB) || defined(STM32F072xB) || defined(STM32F070xB)
+#    elif defined(GD32VF103C8)
+#        define FEE_MCU_FLASH_SIZE 64  // Size in Kb
+#    elif defined(STM32F103xB) || defined(STM32F072xB) || defined(STM32F070xB) || defined(GD32VF103CB)
 #        define FEE_MCU_FLASH_SIZE 128  // Size in Kb
 #    elif defined(STM32F303xC) || defined(STM32F401xC)
 #        define FEE_MCU_FLASH_SIZE 256  // Size in Kb
diff --git a/tmk_core/common/chibios/flash_stm32.c b/tmk_core/common/chibios/flash_stm32.c
index 35fb0eebdb..72c41b8b78 100644
--- a/tmk_core/common/chibios/flash_stm32.c
+++ b/tmk_core/common/chibios/flash_stm32.c
@@ -23,6 +23,11 @@
 #    define FLASH_SR_WRPERR FLASH_SR_WRPRTERR
 #endif
 
+#if defined(MCU_GD32V)
+/* GigaDevice GD32VF103 is a STM32F103 clone at heart. */
+#    include "gd32v_compatibility.h"
+#endif
+
 #if defined(STM32F4XX)
 #    define FLASH_SR_PGERR (FLASH_SR_PGSERR | FLASH_SR_PGPERR | FLASH_SR_PGAERR)
 
diff --git a/tmk_core/common/chibios/gd32v_compatibility.h b/tmk_core/common/chibios/gd32v_compatibility.h
new file mode 100644
index 0000000000..f4dcfd8c55
--- /dev/null
+++ b/tmk_core/common/chibios/gd32v_compatibility.h
@@ -0,0 +1,120 @@
+/* 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/>.
+ */
+
+#pragma once
+
+/* GD32VF103 has the same API as STM32F103, but uses different names for literally the same thing.
+ * As of 23.7.2021 QMK is tailored to use STM32 defines/names, for compatibility sake
+ * we just redefine the GD32 names. */
+
+/* Close your eyes kids. */
+#define MCU_STM32
+
+/* AFIO redefines */
+#define MAPR PCF0
+#define AFIO_MAPR_USART1_REMAP AFIO_PCF0_USART0_REMAP
+#define AFIO_MAPR_USART2_REMAP AFIO_PCF0_USART1_REMAP
+#define AFIO_MAPR_USART3_REMAP_PARTIALREMAP AFIO_PCF0_USART2_REMAP_PARTIALREMAP
+#define AFIO_MAPR_USART3_REMAP_FULLREMAP AFIO_PCF0_USART2_REMAP_FULLREMAP
+
+/* DMA redefines. */
+#define STM32_DMA_STREAM(stream) GD32_DMA_STREAM(stream)
+#define STM32_DMA_STREAM_ID(peripheral, channel) GD32_DMA_STREAM_ID(peripheral - 1, channel - 1)
+#define STM32_DMA_CR_DIR_M2P GD32_DMA_CTL_DIR_M2P
+#define STM32_DMA_CR_PSIZE_WORD GD32_DMA_CTL_PWIDTH_WORD
+#define STM32_DMA_CR_MSIZE_WORD GD32_DMA_CTL_MWIDTH_WORD
+#define STM32_DMA_CR_MINC GD32_DMA_CTL_MNAGA
+#define STM32_DMA_CR_CIRC GD32_DMA_CTL_CMEN
+#define STM32_DMA_CR_PL GD32_DMA_CTL_PRIO
+#define STM32_DMA_CR_CHSEL GD32_DMA_CTL_CHSEL
+#define cr1 ctl0
+#define cr2 ctl1
+#define cr3 ctl2
+#define dier dmainten
+
+/* ADC redefines */
+#if HAL_USE_ADC
+#    define STM32_ADC_USE_ADC1 GD32_ADC_USE_ADC0
+
+#    define smpr1 sampt0
+#    define smpr2 sampt1
+#    define sqr1 rsq0
+#    define sqr2 rsq1
+#    define sqr3 rsq2
+
+#    define ADC_SMPR2_SMP_AN0 ADC_SAMPT1_SMP_SPT0
+#    define ADC_SMPR2_SMP_AN1 ADC_SAMPT1_SMP_SPT1
+#    define ADC_SMPR2_SMP_AN2 ADC_SAMPT1_SMP_SPT2
+#    define ADC_SMPR2_SMP_AN3 ADC_SAMPT1_SMP_SPT3
+#    define ADC_SMPR2_SMP_AN4 ADC_SAMPT1_SMP_SPT4
+#    define ADC_SMPR2_SMP_AN5 ADC_SAMPT1_SMP_SPT5
+#    define ADC_SMPR2_SMP_AN6 ADC_SAMPT1_SMP_SPT6
+#    define ADC_SMPR2_SMP_AN7 ADC_SAMPT1_SMP_SPT7
+#    define ADC_SMPR2_SMP_AN8 ADC_SAMPT1_SMP_SPT8
+#    define ADC_SMPR2_SMP_AN9 ADC_SAMPT1_SMP_SPT9
+
+#    define ADC_SMPR1_SMP_AN10 ADC_SAMPT0_SMP_SPT10
+#    define ADC_SMPR1_SMP_AN11 ADC_SAMPT0_SMP_SPT11
+#    define ADC_SMPR1_SMP_AN12 ADC_SAMPT0_SMP_SPT12
+#    define ADC_SMPR1_SMP_AN13 ADC_SAMPT0_SMP_SPT13
+#    define ADC_SMPR1_SMP_AN14 ADC_SAMPT0_SMP_SPT14
+#    define ADC_SMPR1_SMP_AN15 ADC_SAMPT0_SMP_SPT15
+
+#    define ADC_SQR3_SQ1_N ADC_RSQ2_RSQ1_N
+#endif
+
+/* FLASH redefines */
+#if defined(EEPROM_ENABLE)
+#    define SR STAT
+#    define FLASH_SR_BSY FLASH_STAT_BUSY
+#    define FLASH_SR_PGERR FLASH_STAT_PGERR
+#    define FLASH_SR_EOP FLASH_STAT_ENDF
+#    define FLASH_SR_WRPRTERR FLASH_STAT_WPERR
+#    define FLASH_SR_WRPERR FLASH_SR_WRPRTERR
+#    define FLASH_OBR_OPTERR FLASH_OBSTAT_OBERR
+#    define AR ADDR
+#    define CR CTL
+#    define FLASH_CR_PER FLASH_CTL_PER
+#    define FLASH_CR_STRT FLASH_CTL_START
+#    define FLASH_CR_LOCK FLASH_CTL_LK
+#    define FLASH_CR_PG FLASH_CTL_PG
+#    define KEYR KEY
+#endif
+
+/* Serial USART redefines. */
+#if HAL_USE_SERIAL
+#    if !defined(SERIAL_USART_CR1)
+#        define SERIAL_USART_CR1 (USART_CTL0_PCEN | USART_CTL0_PM | USART_CTL0_WL)  // parity enable, odd parity, 9 bit length
+#    endif
+#    if !defined(SERIAL_USART_CR2)
+#        define SERIAL_USART_CR2 (USART_CTL1_STB_1)  // 2 stop bits
+#    endif
+#    if !defined(SERIAL_USART_CR3)
+#        define SERIAL_USART_CR3 0x0
+#    endif
+#    define USART_CR3_HDSEL USART_CTL2_HDEN
+#    define CCR CHCV
+#endif
+
+/* SPI redefines. */
+#if HAL_USE_SPI
+#    define SPI_CR1_LSBFIRST SPI_CTL0_LF
+#    define SPI_CR1_CPHA SPI_CTL0_CKPH
+#    define SPI_CR1_CPOL SPI_CTL0_CKPL
+#    define SPI_CR1_BR_0 SPI_CTL0_PSC_0
+#    define SPI_CR1_BR_1 SPI_CTL0_PSC_1
+#    define SPI_CR1_BR_2 SPI_CTL0_PSC_2
+#endif
diff --git a/tmk_core/common/chibios/syscall-fallbacks.c b/tmk_core/common/chibios/syscall-fallbacks.c
index 739017ae1d..4569879c7c 100644
--- a/tmk_core/common/chibios/syscall-fallbacks.c
+++ b/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) {