rp2: Stop machine.idle() blocking indefinitely.

Updates rp2 port to always resume from idle within 1ms max.

When rp2 port went tickless the behaviour of machine.idle() changed as
there is no longer a tick interrupt to wake it up every millisecond. On a
quiet system it would now block indefinitely. No other port does this.

See parent commit for justification of why this change is useful.

Also adds a test case that fails without this change.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/rp2/modmachine.c

@@ -103,7 +103,7 @@ static void mp_machine_set_freq(size_t n_args, const mp_obj_t *args) {
 }
 
 static void mp_machine_idle(void) {
-    __wfe();
+    MICROPY_INTERNAL_WFE(1);
 }
 
 static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {

tests/ports/rp2/rp2_machine_idle.py

@@ -0,0 +1,36 @@
+import machine
+import time
+
+# Verify that machine.idle() resumes execution within 0.1 and 1.1ms (should be
+# 1ms max but allowing for some overhead).
+#
+# (A minimum sleep time for machine.idle() isn't specified but in a system like
+# this with no active interrupts then we should expect some idle time before
+# resuming. If it's consistently resuming immediately then that indicates a bug
+# is preventing proper idle.)
+#
+# This test doesn't contain any rp2-specific code, but rp2 is currently the only
+# tickless port - which is what led to the bug this is a regression test for.
+# Some other ports (unix, esp32) have idle behaviour that resumes immediately on
+# a quiet system, so this test is also less useful for those.
+#
+# Verification uses the average idle time, as individual iterations will always
+# have outliers due to interrupts, scheduler, etc.
+
+ITERATIONS = 500
+total = 0
+
+for _ in range(ITERATIONS):
+    before = time.ticks_us()
+    machine.idle()
+    total += time.ticks_diff(time.ticks_us(), before)
+
+total /= 1000  # us to ms
+average = total / ITERATIONS
+
+# print(f"Total {total}ms average {average}ms")  # uncomment for debug
+
+if 0.1 < average < 1.1:
+    print("PASS")
+else:
+    print(f"Total {total}ms average {average}ms, out of spec")

tests/ports/rp2/rp2_machine_idle.py.exp

@@ -0,0 +1 @@
+PASS