tinyboot v0.2.1 Released - User-Flash Path Fixes

Turns out shipping a bootloader and actually booting real firmware through it are two different things. The user-flash path was quietly broken, and the week after the initial release was mostly spent hunting down the bugs that only show up once an application is sitting behind the bootloader.

If you’re new here, tinyboot is my minimal serial bootloader for the CH32V003 and friends, squeezed into 1920 bytes of system flash. The 0.2.0 release got the protocol and the system-flash variant solid. 0.2.1 is the follow-up that makes the user-flash variant actually usable.

The headline fixes: a hardcoded mtvec in qingke-rt that pointed the interrupt vector table at the wrong place, APB2 peripherals leaking state from the bootloader into the app, a broken app_version read, an alignment UB in the boot metadata path, and a defmt panic on reset-into-bootloader. Plus another ~180 bytes shaved off the system-flash build, because every byte matters when you have 1920 of them.

User-Flash Fixes

Broken Interrupt Vector Table

This was the main issue. qingke-rt hardcodes mtvec to 0x0, which is correct when your application starts at the beginning of flash. But in the user-flash layout, the app is loaded behind the bootloader at a non-zero address, so the interrupt vector table points to the wrong place and everything falls apart. I added a fix_mtvec! macro on the application side to patch this up at startup.

Peripheral Cleanup

The bootloader was not properly resetting APB2 peripherals before jumping to the application. If the bootloader had initialized UART or GPIO, those peripherals would still be in a configured state when the app started, which could cause conflicts. Now everything gets cleaned up before the jump.

App Version Display

The app version was not being read correctly in the user-flash case, which also broke the app boot path. Fixed the app_version read along with the rest of the user-flash flow.

Other Fixes

Memory Alignment UB

The boot metadata reads had an alignment issue. I was reading raw bytes and reinterpreting them directly, which is undefined behavior when the source buffer is not properly aligned. The fix was to read into a u32 buffer first and cast back to a u8 array.

Defmt Panics

When the application triggered a reset back into bootloader mode, defmt would panic during startup. This turned out to be a conflict in the startup assembly. I ended up splitting the bootloader runtime into a minimal and a full startup path to avoid the issue.

Optimizations

I managed to shave off another ~180 bytes from the system-flash bootloader through a mix of aggressive inlining, merging the CRC and payload handling, batching RCC enable calls, and writing custom read_exact/write_all implementations. Every byte matters when you have 1920 bytes to work with.

Other Improvements

All CH32V003 chip variants are now included in CI, and tinyboot-cli picked up env_logger support. Set RUST_LOG=debug for protocol-level diagnostics.