Summary

• Use offsetof(config_t, checksum) instead of sizeof(config_t) - sizeof(uint32_t) to compute the CRC range in load_config() and save_config()

Problem

sizeof(config_t) - sizeof(uint32_t) assumes checksum occupies the last 4 bytes of the struct with no trailing padding. This holds today on ARM Cortex-M0+ because the current config_t layout happens to have 4-byte alignment.

However, screensaver_t contains uint64_t fields (idle_time_us, max_time_us), which give output_t an 8-byte alignment requirement on some ABIs. If config_t is extended with fields that break the current alignment (e.g., adding uint8_t fields before checksum), the compiler inserts tail padding after checksum, making sizeof(config_t) - 4 > offsetof(config_t, checksum).

When this happens, the CRC range includes the checksum field itself (and padding bytes), causing load_config() to always fail validation and silently reset config to defaults on every boot — even after a successful save_config().

Fix

Replace sizeof(config_t) - sizeof(uint32_t) with offsetof(config_t, checksum), which always computes the exact byte range before the checksum field regardless of struct padding.

Testing

Verified on RP2040 hardware (DeskHop PCB v1.1):

• save_config() → flash verify OK → simulated load_config() CRC matches
• Config persists across reboots

Test plan

• Firmware builds without warnings on arm-none-eabi-gcc
• Config save/load verified on hardware
• No functional change for current upstream config_t layout (offsetof == sizeof - 4)

🤖 Generated with Claude Code