Trying to make some use of this new MCU, while avoiding bloated and proprietary tools

I have a CH32X035F8U6-R0-1v0 evaluation board with bootloader v2.6.0, YMMV.

Preparations:

# Go to a directory with your projects and clone this repo:
git clone https://github.com/Nable80/ch32x035_experiments

# Clone a repo with simple firmware flashing tools too:
git clone https://github.com/wagiminator/MCU-Flash-Tools

# Install libc and compiler packages for bare-metal RISC-V targets:
sudo apt update -y
sudo apt install picolibc-riscv64-unknown-elf gcc-riscv64-unknown-elf

# Install udev rules to avoid unnecessary privileges:
cd ch32x035_experiments
sudo install -v -m 0644 99-wch-isp.rules /etc/udev/rules.d/
sudo udevadm control --reload-rules

Building and uploading firmware:

chprog.py is the only tool that works for me right now:

make build && ../MCU-Flash-Tools/chprog.py main.bin

Pin reminders:

USB and bootloader:

• Connect PC17 to VCC with a resistor (4.7k or so) to enter USB bootloader on power-on
• PC17 - D+
• PC16 - D-

Serial Debug Interface (SDI, seems to be incompatible with SWD):

• PC18 - DIO aka SWDIO
• PC19 - DCK aka SWCLK

USART without remapping:

• PA2 - TX2 (USART2_TX)
• PA3 - RX2 (USART2_RX)
• PB3 - TX3 (note: RX3 and RX4 require remaps)
• PB0 - TX4
• PB11 - RX1 (note: F8U6 package doesn't provide TX1 at all)

SPI:

• PA4 - CS
• PA5 - SCK
• PA6 - MISO
• PA7 - MOSI

NOTE:

NOTE: original WCH's toolchain uses -march=rv32imacxw to enable useful proprietary extentions (non-standard compressed instructions, fast interrupt handling, etc) and all of this isn't available in a regular toolchain (did they publish any patches?).

I changed this to -march=rv32imac_zicsr to be able to compile the code at least somehow but I don't know much about RISC-V, feel free to correct me.