rp2: Fix lightsleep to work with interrupts and cyw43 driver.

This commit prevents the device from "hanging" when using lightsleep while
the WiFi chip is active.

Whenever the WiFi chip wants to interrupt the microcontroller to notify it
for a new package, it sets the CYW43_PIN_WL_HOST_WAKE pin to high,
triggering an IRQ.  However, as polling the chip cannot happen in an
interrupt handler, it subsequently notifies the pendsv-service to do a poll
as soon as the interrupt handler ended.  In order to prevent a new
interrupt from happening immediately afterwards, even before the poll has
run, the IRQ handler disables interrupts from the pin.

The first problem occurs, when a WiFi package arrives while the main loop
is in cyw43-code.  In order to prevent concurrent access of the hardware,
the network code blocks pendsv from running again while entering lwIP code.

The same holds for direct cyw43 code (like changing the cyw43-gpios, i.e.
the LED on the Pico W).  While the pendsv is disabled, interrupts can still
occur to schedule a poll (and disable further interrupts), but it will not
run.  This can happen while the microcontroller is anywhere in rp2040 code.

In order to preserve power while waiting for cyw43 responses,
cyw43_configport.h defines CYW43_DO_IOCTL_WAIT and
CYW43_SDPCM_SEND_COMMON_WAIT to __WFI().  While this might work in most
cases, there are 2 edge cases where it fails:
- When an interrupt has already been received by the cyw43 stack, for
  example due to an incoming ethernet packet.
- When the interrupt from the cyw43 response comes before the
  microcontroller entered the __WFI() instruction.

When that happens, wfi will just block forever as no further interrupts are
received.  The only way to safely use wfi to wake up from an interrupt is
inside a critical section, as this delays interrupts until the wfi is
entered, possibly resuming immediately until interrupts are reenabled and
the interrupt handler is run.  Additionally this critical section needs to
check whether the interrupt has already been disabled and pendsv was
triggered, as in such a case, wfi can never be woken up, and needs to be
skipped, because there is already a package from the network chip waiting.
Note that this turns cyw43_yield into a nop (and thereby the cyw43-loops
into busy waits) from the second time onwards, as after the first call, a
pendsv request will definitely be pending.  More logic could be added, to
explicitly enable the interrupt in this case.

Regarding lightsleep, this code has a similar problem.  When an interrupt
occurs during lightsleep, the IRQ and pendsv handler and thereby poll are
run immediately, with the clocks still disabled, causing the SPI transfers
to fail.  If we don't want to add complex logic inside the IRQ handler we
need to protect the whole lightsleep procedure form interrupts with a
critical section, exiting out early if an interrupt is pending for whatever
reason.  Only then we can start to shut down clocks and only enable
interrupts when the system is ready again.  Other interrupt handlers might
also be happy, that they are only run when the system is fully operational.

Tested on a Pico W, calling machine.lightsleep() within an endless loop and
pinging from the outside.

ports/rp2/cyw43_configport.h

@@ -49,12 +49,12 @@
 
 #define CYW43_SDPCM_SEND_COMMON_WAIT \
     if (get_core_num() == 0) { \
-        __WFI(); \
+        cyw43_yield(); \
     } \
 
 #define CYW43_DO_IOCTL_WAIT \
     if (get_core_num() == 0) { \
-        __WFI(); \
+        cyw43_yield(); \
     } \
 
 #define CYW43_ARRAY_SIZE(a)             MP_ARRAY_SIZE(a)
@@ -88,6 +88,15 @@
 #define cyw43_schedule_internal_poll_dispatch(func) pendsv_schedule_dispatch(PENDSV_DISPATCH_CYW43, func)
 
 void cyw43_post_poll_hook(void);
+extern volatile int cyw43_has_pending;
+
+static inline void cyw43_yield(void) {
+    uint32_t my_interrupts = save_and_disable_interrupts();
+    if (!cyw43_has_pending) {
+        __WFI();
+    }
+    restore_interrupts(my_interrupts);
+}
 
 static inline void cyw43_delay_us(uint32_t us) {
     uint32_t start = mp_hal_ticks_us();
@@ -99,7 +108,7 @@ static inline void cyw43_delay_ms(uint32_t ms) {
     uint32_t us = ms * 1000;
     int32_t start = mp_hal_ticks_us();
     while (mp_hal_ticks_us() - start < us) {
-        __WFI();
+        cyw43_yield();
         MICROPY_EVENT_POLL_HOOK_FAST;
     }
 }

ports/rp2/modmachine.c

@@ -48,6 +48,9 @@
 #include "pico/bootrom.h"
 #include "pico/stdlib.h"
 #include "pico/unique_id.h"
+#if MICROPY_PY_NETWORK_CYW43
+#include "lib/cyw43-driver/src/cyw43.h"
+#endif
 
 #if MICROPY_PY_MACHINE
 
@@ -141,6 +144,13 @@ STATIC mp_obj_t machine_lightsleep(size_t n_args, const mp_obj_t *args) {
 
     const uint32_t xosc_hz = XOSC_MHZ * 1000000;
 
+    uint32_t my_interrupts = save_and_disable_interrupts();
+    #if MICROPY_PY_NETWORK_CYW43
+    if (cyw43_has_pending) {
+        restore_interrupts(my_interrupts);
+        return mp_const_none;
+    }
+    #endif
     // Disable USB and ADC clocks.
     clock_stop(clk_usb);
     clock_stop(clk_adc);
@@ -165,6 +175,9 @@ STATIC mp_obj_t machine_lightsleep(size_t n_args, const mp_obj_t *args) {
     rosc_hw->ctrl = ROSC_CTRL_ENABLE_VALUE_DISABLE << ROSC_CTRL_ENABLE_LSB;
 
     if (n_args == 0) {
+        #if MICROPY_PY_NETWORK_CYW43
+        gpio_set_dormant_irq_enabled(CYW43_PIN_WL_HOST_WAKE, GPIO_IRQ_LEVEL_HIGH, true);
+        #endif
         xosc_dormant();
     } else {
         uint32_t sleep_en0 = clocks_hw->sleep_en0;
@@ -190,6 +203,7 @@ STATIC mp_obj_t machine_lightsleep(size_t n_args, const mp_obj_t *args) {
 
     // Bring back all clocks.
     clocks_init();
+    restore_interrupts(my_interrupts);
 
     return mp_const_none;
 }

ports/rp2/mpnetworkport.c

@@ -48,6 +48,7 @@ static alarm_id_t lwip_alarm_id = -1;
 #define CYW43_SHARED_IRQ_HANDLER_PRIORITY PICO_SHARED_IRQ_HANDLER_HIGHEST_ORDER_PRIORITY
 
 uint32_t cyw43_country_code = CYW43_COUNTRY_WORLDWIDE;
+volatile int cyw43_has_pending = 0;
 
 static void gpio_irq_handler(void) {
     uint32_t events = gpio_get_irq_event_mask(CYW43_PIN_WL_HOST_WAKE);
@@ -56,6 +57,7 @@ static void gpio_irq_handler(void) {
         // So disable the interrupt until this is done.  It's re-enabled again by
         // CYW43_POST_POLL_HOOK which is called at the end of cyw43_poll_func.
         gpio_set_irq_enabled(CYW43_PIN_WL_HOST_WAKE, CYW43_IRQ_LEVEL, false);
+        cyw43_has_pending = 1;
         pendsv_schedule_dispatch(PENDSV_DISPATCH_CYW43, cyw43_poll);
         CYW43_STAT_INC(IRQ_COUNT);
     }
@@ -68,6 +70,7 @@ void cyw43_irq_init(void) {
 }
 
 void cyw43_post_poll_hook(void) {
+    cyw43_has_pending = 0;
     gpio_set_irq_enabled(CYW43_PIN_WL_HOST_WAKE, CYW43_IRQ_LEVEL, true);
 }