rp2: Fix build failure if threads are disabled.

Regression in 3af006e meant that pendsv.c no longer compiled if threads
were disabled in the build config. Add an implementation based on the
earlier one (simple counter) for the non-threads case.

It seems like with the current usage patterns there's no need for the
counter to be incremented/decremented atomically on a single core config.

This work was funded through GitHub Sponsors.

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

Author: projectgus

ports/rp2/pendsv.c

@@ -43,10 +43,14 @@
 
 static pendsv_dispatch_t pendsv_dispatch_table[PENDSV_DISPATCH_NUM_SLOTS];
 
+static inline void pendsv_resume_run_dispatch(void);
+
 void PendSV_Handler(void);
 
-// Using the nowait variant here as softtimer updates PendSV from the loop of mp_wfe_or_timeout(),
-// where we don't want the CPU event bit to be set.
+#if MICROPY_PY_THREAD
+
+// Important to use a 'nowait' mutex here as softtimer updates PendSV from the
+// loop of mp_wfe_or_timeout(), where we don't want the CPU event bit to be set.
 static mp_thread_recursive_mutex_t pendsv_mutex;
 
 void pendsv_init(void) {
@@ -62,7 +66,40 @@ void pendsv_suspend(void) {
 
 void pendsv_resume(void) {
     mp_thread_recursive_mutex_unlock(&pendsv_mutex);
+    pendsv_resume_run_dispatch();
+}
+
+static inline int pendsv_suspend_count(void) {
+    return pendsv_mutex.mutex.enter_count;
+}
+
+#else
+
+// Without threads we don't include any pico-sdk mutex in the build,
+// but also we don't need to worry about cross-thread contention (or
+// races with interrupts that update this counter).
+static int pendsv_lock;
+
+void pendsv_init(void) {
+}
+
+void pendsv_suspend(void) {
+    pendsv_lock++;
+}
+
+void pendsv_resume(void) {
+    assert(pendsv_lock > 0);
+    pendsv_lock--;
+    pendsv_resume_run_dispatch();
+}
 
+static inline int pendsv_suspend_count(void) {
+    return pendsv_lock;
+}
+
+#endif
+
+static inline void pendsv_resume_run_dispatch(void) {
     // Run pendsv if needed.  Find an entry with a dispatch and call pendsv dispatch
     // with it.  If pendsv runs it will service all slots.
     int count = PENDSV_DISPATCH_NUM_SLOTS;
@@ -76,7 +113,7 @@ void pendsv_resume(void) {
 
 void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
     pendsv_dispatch_table[slot] = f;
-    if (pendsv_mutex.mutex.enter_count == 0) {
+    if (pendsv_suspend_count() == 0) {
         #if PICO_ARM
         // There is a race here where other core calls pendsv_suspend() before
         // ISR can execute, but dispatch will happen later when other core
@@ -97,13 +134,17 @@ void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
 // PendSV interrupt handler to perform background processing.
 void PendSV_Handler(void) {
 
+    #if MICROPY_PY_THREAD
     if (!mp_thread_recursive_mutex_lock(&pendsv_mutex, 0)) {
         // Failure here means core 1 holds pendsv_mutex. ISR will
         // run again after core 1 calls pendsv_resume().
         return;
     }
     // Core 0 should not already have locked pendsv_mutex
     assert(pendsv_mutex.mutex.enter_count == 1);
+    #else
+    assert(pendsv_suspend_count() == 0);
+    #endif
 
     #if MICROPY_PY_NETWORK_CYW43
     CYW43_STAT_INC(PENDSV_RUN_COUNT);
@@ -117,5 +158,7 @@ void PendSV_Handler(void) {
         }
     }
 
+    #if MICROPY_PY_THREAD
     mp_thread_recursive_mutex_unlock(&pendsv_mutex);
+    #endif
 }