rp2: Remove 1ms timeout to make idle waiting tickless.

projectgus · dpgeorge · commit 8c432ea2d437 · 2023-11-03T18:26:51.000+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 be improved by about 12us
(from average of 46us to 34us running a Pin IRQ).  C-based "scheduled
nodes" should see even lower latency.

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);`