Per-core static stat on RP2040

The RP2040 is a dual-core microcontroller. The rp2040-hal multicore module makes it easy to start code running on the second core, but doing so adds some complications. Any static variables are shared in memory, causing the same difficulties that static variables do in multithreaded systems.

This package enables the use of the unstable #[thread_local] attribute to mark a static variable as core-local. Each core gets its own instance of any such variables and can use and manipulate them completely independently.

Because these variables are not shared, their contents need not be Send or Sync, so you can, for example, use a RefCell's interior mutability to manage them.

Unlike thread-local variables, their states are initialized once on system reset, not when code is spawned onto a core. Therefore if you spawn multiple times onto core 1, then later spawns will see the state left behind by the previous runs.

Notes on Usage

The implementation needs the linker script to set up the layout of per-core state in memory. You therefore need to copy the relevant portions of memory.x into your own linker script.

It also needs this crate to define the __aeabi_read_tp function to read the per-"thread" state. You need to add

extern crate rp2040_multicore_per_cpu;

to make sure this crate is pulled in so that this symbols will be linked in.