litex: ensure we don't re-enable interrups during ISR

During an interrupt handler, interrupts are implicitly disabled. They
will be re-enabled when the interrupt handler returns.

Due to some changes that were made, varous calls will re-enable
interrupts after they're finished. Examples of this include calling
`CALLBACK_CRITICAL_END` and getting the number of ticks with
`port_get_raw_ticks()`.

This patch prevents this from happening by doing two things:

1. Use standard calls in `port_get_raw_ticks()` to disable and re-enable
interrupts, preventing nesting issues, and
2. Increase the nesting count inside `isr()`, reflecting the implicit
call that is made by hardware when an interrupt is handled

This helps to address adafruit#3841.

Signed-off-by: Sean Cross <[email protected]>

Author: xobs

ports/litex/mphalport.c

@@ -44,16 +44,31 @@ void mp_hal_delay_us(mp_uint_t delay) {
 
 extern void SysTick_Handler(void);
 
+// This value contains the number of times "common_hal_mcu_disable_interrupts()"
+// has been called without calling "common_hal_mcu_enable_interrupts()". Since
+// this is the interrupt handler, that means we're handling an interrupt, so
+// this value should be `0`.
+//
+// Interrupts should already be disabled when this handler is running, which means
+// this value is logically already `1`. If we didn't do this, then interrupts would
+// be prematurely enabled by interrupt handlers that enable and disable interrupts.
+extern volatile uint32_t nesting_count;
+
 __attribute__((section(".ramtext")))
 void isr(void) {
     uint8_t irqs = irq_pending() & irq_getmask();
 
+    // Increase the "nesting count". Note: This should be going from 0 -> 1.
+    nesting_count += 1;
 #ifdef CFG_TUSB_MCU
     if (irqs & (1 << USB_INTERRUPT))
         usb_irq_handler();
 #endif
     if (irqs & (1 << TIMER0_INTERRUPT))
         SysTick_Handler();
+
+    // Decrease the "nesting count". Note: This should be going from 1 -> 0.
+    nesting_count -= 1;
 }
 
 mp_uint_t cpu_get_regs_and_sp(mp_uint_t *regs) {

ports/litex/supervisor/port.c

@@ -32,6 +32,8 @@
 #include "irq.h"
 #include "csr.h"
 
+#include "shared-bindings/microcontroller/__init__.h"
+
 // Global millisecond tick count. 1024 per second because most RTCs are clocked with 32.768khz
 // crystals.
 volatile uint64_t raw_ticks = 0;
@@ -129,9 +131,9 @@ uint32_t port_get_saved_word(void) {
 
 uint64_t port_get_raw_ticks(uint8_t* subticks) {
     // Reading 64 bits may take two loads, so turn of interrupts while we do it.
-    irq_setie(false);
+    common_hal_mcu_disable_interrupts();
     uint64_t raw_tick_snapshot = raw_ticks;
-    irq_setie(true);
+    common_hal_mcu_enable_interrupts();
     return raw_tick_snapshot;
 }