port/rp2: Make tickless, remove 1ms timeout when idle.

projectgus · projectgus · commit f5e32506f237 · 2023-10-31T16:42:10.000+11:00
The main motivation for doing this was to reduce the latency when the
system is woken by a USB interrupt. The best_effort_wfe_or_timeout()
function calls into the pico-sdk dynamic timer framework which sets up a
new dynamic timer instance each time, and then has to tear it down before
continuing after a WFE.

Testing Python interrupt latency it seems to improved by about
12us (from average of 46us to 34us running a Pin IRQ). C-based
"scheduled nodes" should see even lower latency.

The risk with this change is that it exposes cases where a driver expects
execution to continue without an interrupt. Alternative is to enable the
standard ARM SysTick interrupt to wake the CPU at tick intervals.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton &lt;angus@redyak.com.au&gt;
diff --git a/ports/rp2/modmachine.c b/ports/rp2/modmachine.c
@@ -111,7 +111,7 @@ STATIC mp_obj_t machine_freq(size_t n_args, const mp_obj_t *args) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_freq_obj, 0, 1, machine_freq);
 
 STATIC mp_obj_t machine_idle(void) {
-    best_effort_wfe_or_timeout(make_timeout_time_ms(1));
+    __wfe();
     return mp_const_none;
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(machine_idle_obj, machine_idle);
diff --git a/ports/rp2/mpconfigport.h b/ports/rp2/mpconfigport.h
@@ -274,7 +274,7 @@ extern void mp_thread_end_atomic_section(uint32_t);
 #define MICROPY_EVENT_POLL_HOOK \
     do { \
         MICROPY_EVENT_POLL_HOOK_FAST; \
-        best_effort_wfe_or_timeout(make_timeout_time_ms(1)); \
+        __wfe(); \
     } while (0);
 
 #define MICROPY_MAKE_POINTER_CALLABLE(p) ((void *)((mp_uint_t)(p) | 1))

Original file line number	Diff line number	Diff line change
`@@ -111,7 +111,7 @@ STATIC mp_obj_t machine_freq(size_t n_args, const mp_obj_t *args) {`
`111`	`111`	`STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_freq_obj, 0, 1, machine_freq);`
`112`	`112`
`113`	`113`	`STATIC mp_obj_t machine_idle(void) {`
`114`		`- best_effort_wfe_or_timeout(make_timeout_time_ms(1));`
	`114`	`+ __wfe();`
`115`	`115`	`return mp_const_none;`
`116`	`116`	`}`
`117`	`117`	`STATIC MP_DEFINE_CONST_FUN_OBJ_0(machine_idle_obj, machine_idle);`