fix: correctly handle calls for GetRNG() when being made from nrf devices with SoftDevice enabled

src/machine/machine_nrf.go

@@ -318,9 +318,9 @@ func (i2c *I2C) signalStop() error {
 
 var rngStarted = false
 
-// GetRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
+// getRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
 // According to Nordic's documentation, the random output is suitable for cryptographic purposes.
-func GetRNG() (ret uint32, err error) {
+func getRNG() (ret uint32, err error) {
 	// There's no apparent way to check the status of the RNG peripheral's task, so simply start it
 	// to avoid deadlocking while waiting for output.
 	if !rngStarted {

src/machine/machine_nrf_bare.go

@@ -0,0 +1,9 @@
+//go:build nrf && !softdevice
+
+package machine
+
+// GetRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
+// According to Nordic's documentation, the random output is suitable for cryptographic purposes.
+func GetRNG() (ret uint32, err error) {
+	return getRNG()
+}

src/machine/machine_nrf_sd.go

@@ -0,0 +1,59 @@
+//go:build nrf && softdevice
+
+package machine
+
+import (
+	"device/arm"
+	"device/nrf"
+
+	"errors"
+)
+
+// avoid a heap allocation in GetRNG.
+var (
+	softdeviceEnabled uint8
+	bytesAvailable    uint8
+	buf               [4]uint8
+
+	errNoSoftDeviceSupport = errors.New("rng: softdevice not supported on this device")
+	errNotEnoughRandomData = errors.New("rng: not enough random data available")
+)
+
+// GetRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
+// According to Nordic's documentation, the random output is suitable for cryptographic purposes.
+func GetRNG() (ret uint32, err error) {
+	// First check whether the SoftDevice is enabled.
+	// sd_rand_application_bytes_available_get cannot be called when the SoftDevice is not enabled.
+	arm.SVCall1(0x12, &softdeviceEnabled) // sd_softdevice_is_enabled
+
+	if softdeviceEnabled == 0 {
+		return getRNG()
+	}
+
+	// call into the SoftDevice to get random data bytes available
+	switch nrf.Device {
+	case "nrf51":
+		// sd_rand_application_bytes_available_get: SOC_SVC_BASE_NOT_AVAILABLE + 4
+		arm.SVCall1(0x2B+4, &bytesAvailable)
+	case "nrf52", "nrf52840", "nrf52833":
+		// sd_rand_application_bytes_available_get: SOC_SVC_BASE_NOT_AVAILABLE + 4
+		arm.SVCall1(0x2C+4, &bytesAvailable)
+	default:
+		return 0, errNoSoftDeviceSupport
+	}
+
+	if bytesAvailable < 4 {
+		return 0, errNotEnoughRandomData
+	}
+
+	switch nrf.Device {
+	case "nrf51":
+		// sd_rand_application_vector_get: SOC_SVC_BASE_NOT_AVAILABLE + 5
+		arm.SVCall2(0x2B+5, &buf, 4)
+	case "nrf52", "nrf52840", "nrf52833":
+		// sd_rand_application_vector_get: SOC_SVC_BASE_NOT_AVAILABLE + 5
+		arm.SVCall2(0x2C+5, &buf, 4)
+	}
+
+	return uint32(buf[0]) | uint32(buf[1])<<8 | uint32(buf[2])<<16 | uint32(buf[3])<<24, nil
+}