fix: correctly handle interrupt disable on esp32-c3

Priority 0 disables an interrupt on ESP32-C3. This corrects
the code to actually use that.

Signed-off-by: deadprogram <ron@hybridgroup.com>

builder/sizes_test.go

@@ -42,7 +42,7 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 3668, 280, 0, 2244},
+		{"hifive1b", "examples/echo", 3680, 280, 0, 2252},
 		{"microbit", "examples/serial", 2694, 342, 8, 2248},
 		{"wioterminal", "examples/pininterrupt", 7074, 1510, 120, 7248},
 

src/device/riscv/handleinterrupt.S

@@ -75,6 +75,10 @@ handleInterruptASM:
     SFREG   f30,(30 + 16)*REGSIZE(sp)
     SFREG   f31,(31 + 16)*REGSIZE(sp)
 #endif
+    // Save ra to a global so handleException can print the caller of the NULL
+    // function pointer.
+    la      t0, tinygo_saved_ra
+    SREG    ra, 0(t0)
     call    handleInterrupt
 #ifdef __riscv_flen
     LFREG   f0, (31 + 16)*REGSIZE(sp)
@@ -128,3 +132,10 @@ handleInterruptASM:
     LREG    ra, 0*REGSIZE(sp)
     addi    sp, sp, NREG*REGSIZE
     mret
+
+.section .bss.tinygo_saved_ra
+.global tinygo_saved_ra
+.type tinygo_saved_ra,@object
+.align 2
+tinygo_saved_ra:
+    .space REGSIZE

src/runtime/interrupt/interrupt_esp32c3.go

@@ -10,6 +10,9 @@ import (
 	"unsafe"
 )
 
+//go:extern tinygo_saved_ra
+var tinygo_saved_ra uintptr
+
 // Enable register CPU interrupt with interrupt.Interrupt.
 // The ESP32-C3 has 31 CPU independent interrupts.
 // The Interrupt.New(x, f) (x = [1..31]) attaches CPU interrupt to function f.
@@ -88,7 +91,13 @@ const (
 
 const (
 	defaultThreshold = 5
-	disableThreshold = 10
+	// Priority 0 disables an interrupt on ESP32-C3 (per the TRM,
+	// priority 0 = masked).  During handling we set the current
+	// interrupt's priority to 0 so it cannot re-fire while MIE is
+	// re-enabled for nesting.  This is critical for level-triggered
+	// interrupts where the hardware line stays asserted until the
+	// peripheral source is serviced by the handler.
+	disableThreshold = 0
 )
 
 //go:inline
@@ -225,6 +234,7 @@ func handleException(mcause uintptr) {
 	println("*** Exception:     pc:", riscv.MEPC.Get())
 	println("*** Exception:   code:", uint32(mcause&0x1f))
 	println("*** Exception: mcause:", mcause)
+	println("*** Exception:     ra:", tinygo_saved_ra)
 	switch uint32(mcause & 0x1f) {
 	case riscv.InstructionAccessFault:
 		println("***    virtual address:", riscv.MTVAL.Get())