tud_task() inhibits wfe on rp2350, turns low power loops into busy loops

[rlcamp]

The common pattern of

while (!condition) {
    tud_task();
    __wfe();
}

which should put the processor to sleep between interrupts, becomes a busy loop by default on RP2350 due to the use of pico-sdk spinlocks within tud_task(), which behave like sev() due to reasons discussed in #1812. Mitigations include disabling EXTEXCALL (which modifies the behaviour of ALL atomics and other exclusive load/stores between cores), or putting

#undef CFG_TUSB_OS
#define CFG_TUSB_OS OPT_OS_NONE

in tusb_config.h, disabling the pico-specific code paths in the TinyUSB OS abstraction layer. The latter is less of a big hammer but still has consequences wr interacting with TinyUSB from both cores.

[peterharperuk]

Maybe you should do something like this instead?

    absolute_time_t timeout_time = make_timeout_time_ms(10);
    do {
      if (tud_task_event_ready()) break;
    } while (!best_effort_wfe_or_timeout(timeout_time));
    tud_task(); // tinyusb device task