platform(esp32c3): embiggen heap

[hawkw]

we currently have the heap hardcoded to 32KB. this is quite small, especially for async, because each async task is essentially moving a bit of stack into the heap.

@jamesmunns looked at the esp32c3-hal crate's linker script and determined that the memory region used for the stack and heap is 300KB.

we could probably get away with at least a 64KB heap before we have a significant danger of getting clobbered by a rogue stack frame. even 100KB is probably fine...

[jamesmunns]

the memory region used for the stack and heap is 300KB.

That's for now.

The region is also used for statics, and I have no idea if code size factors into it. Since heap + stack is "whatever you didn't use for other stuff", that size is likely to decrease as time goes on.

Something we COULD do is figure out how to modify/extend the esp32c3 linker scripts, in particular to:

• Set the size of heap, so it becomes a "stronger" reservation
• Set a MINIMUM stack size reservation
• Assert at link time that the heap size + min stack size don't overflow the memory region, giving us a link-time error if these assertions fail.

Until we have something like cargo-call-stack, the minimum stack size is going to be a best estimate, but that is better than the current "lol cross your fingers" approach.

[hawkw]

Where are statics placed in this region? Does more statics bring the heap start towards the stack start, or the stack start towards the heap start?

[jamesmunns]

All links from chat yesterday:

• https://github.com/esp-rs/esp-hal/blob/main/esp32c3-hal/ld/mb-riscv-link.x#L6
• https://github.com/esp-rs/esp-hal/blob/main/esp32c3-hal/ld/mb-riscv-link.x#L117-L124
• https://github.com/esp-rs/esp-hal/blob/main/esp32c3-hal/ld/mb-esp32c3-memory.x#L33
• https://github.com/esp-rs/esp-hal/blob/main/esp32c3-hal/ld/mb-esp32c3-memory.x#L60-L63

The full layout is here (if we are using mb mode?):

https://github.com/esp-rs/esp-hal/blob/main/esp32c3-hal/ld/mb-riscv-link.x#L41-L204

But tl;dr:

• "rwtext" (I dunno what this is)
• statics (.data and .bss)
• then heap (grows upward)
• then stack (starts at the end of ram, grows downward)