[sx126x][sx127x] Fixes from code review

- Without LoRa, require the SX127x radio to have a sync word or preamble
  detection, as listen before talk cannot see a packet arriving otherwise
- Warn once when a send finds the radio failed

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

esphome/components/packet_interface/radio_link.cpp

@@ -32,6 +32,13 @@ bool RadioLinkBase::skip_empty_() const {
   return true;
 }
 
+void RadioLinkBase::radio_not_ready_(bool failed) {
+  if (!failed || this->failed_warned_)
+    return;
+  this->failed_warned_ = true;
+  ESP_LOGW(TAG, "%s: The radio failed to set up, so nothing can be sent", this->name_);
+}
+
 void RadioLinkBase::radio_free_() {
   this->listen_before_talk_.end_busy();
   this->disable_loop();

esphome/components/packet_interface/radio_link.h

@@ -20,11 +20,14 @@ class RadioLinkBase : public PacketInterface {
   bool may_send_(bool busy);
   /// Returns true for an empty packet, which the radios cannot send, so there is nothing to retry.
   bool skip_empty_() const;
+  /// Warns once when a send finds the radio failed; until its setup has run, sends are refused quietly.
+  void radio_not_ready_(bool failed);
   /// Ends the busy period when loop() finds the radio free.
   void radio_free_();
   void receive_(const std::vector<uint8_t> &packet, float rssi, float snr);
 
   ListenBeforeTalk listen_before_talk_;
+  bool failed_warned_{false};
 };
 
 /**
@@ -54,8 +57,10 @@ class RadioLink : public RadioLinkBase, public Listener {
 
   bool send_to_interface(const PacketBuffer &data, const PacketMetaData &meta_data) override {
     // The radio is set up after the layers above, which may send from their own setup
-    if (!this->radio_->is_ready())
+    if (!this->radio_->is_ready()) {
+      this->radio_not_ready_(this->radio_->is_failed());
       return false;
+    }
     size_t size = data.size();
     if (size == 0)
       return this->skip_empty_();

esphome/components/sx127x/packet_interface/__init__.py

@@ -12,8 +12,10 @@ from .. import (
     CONF_MODULATION,
     CONF_PACKET_MODE,
     CONF_PAYLOAD_LENGTH,
+    CONF_PREAMBLE_DETECT,
     CONF_RX_START,
     CONF_SX127X_ID,
+    CONF_SYNC_VALUE,
     DOMAIN,
     SX127x,
     SX127xListener,
@@ -34,7 +36,10 @@ CONFIG_SCHEMA = radio_link_schema(SX127xPacketInterface, CONF_SX127X_ID, SX127x)
 
 
 def _check_fsk(config: ConfigType, radio: ConfigType) -> None:
-    """Without LoRa, the radio must send packets, not a continuous bit stream, and holds fewer bytes."""
+    """Without LoRa, the radio must send packets, not a continuous bit stream, and holds fewer bytes.
+
+    It can tell that a packet is arriving only from a detected preamble or sync word, which listen before talk needs.
+    """
     if radio[CONF_MODULATION] == "LORA":
         return
     if not radio.get(CONF_PACKET_MODE, False):
@@ -42,6 +47,12 @@ def _check_fsk(config: ConfigType, radio: ConfigType) -> None:
             f"Without LoRa modulation the radio's '{CONF_PACKET_MODE}' must be true, as a link sends packets",
             path=[CONF_SX127X_ID],
         )
+    if not radio.get(CONF_SYNC_VALUE) and not radio.get(CONF_PREAMBLE_DETECT):
+        raise cv.Invalid(
+            f"Without LoRa modulation the radio needs a '{CONF_SYNC_VALUE}' or '{CONF_PREAMBLE_DETECT}', so that "
+            "the link can tell when a packet is arriving and not send over it",
+            path=[CONF_SX127X_ID],
+        )
     if config[CONF_MAX_PACKET_SIZE] > FSK_MAX_PACKET_SIZE:
         raise cv.Invalid(
             f"Without LoRa modulation the radio sends at most {FSK_MAX_PACKET_SIZE} bytes",

tests/components/packet_interface/radio_link.cpp

@@ -22,6 +22,7 @@ class FakeRadio : public Component {
  public:
   bool is_receiving() { return this->receiving; }
   void register_listener(FakeListener *listener) { this->listener = listener; }
+  void fail() { this->mark_failed(); }
   FakeError transmit_packet(const uint8_t *data, size_t length) {
     this->sent.emplace_back(data, data + length);
     return FakeError::NONE;
@@ -97,6 +98,13 @@ TEST_F(RadioLinkTest, EmptyPacketCountsAsSentWithoutTransmitting) {
   EXPECT_TRUE(this->radio_.sent.empty());
 }
 
+TEST_F(RadioLinkTest, FailedRadioRefusesSends) {
+  this->radio_.fail();
+  EXPECT_FALSE(this->send_());
+  EXPECT_FALSE(this->send_());
+  EXPECT_TRUE(this->radio_.sent.empty());
+}
+
 TEST_F(RadioLinkTest, ReceivedPacketCarriesSignal) {
   Packet received;
   PacketMetaData meta;

tests/unit_tests/test_lora_packet_interface.py

@@ -106,7 +106,12 @@ class _FullConfig(dict):
         (
             sx127x_packet_interface,
             "sx127x",
-            {"modulation": "FSK", "packet_mode": False, "payload_length": 0},
+            {
+                "modulation": "FSK",
+                "packet_mode": False,
+                "payload_length": 0,
+                "sync_value": [0x12],
+            },
             {},
             "packet_mode",
             {"packet_mode": True},
@@ -114,11 +119,30 @@ class _FullConfig(dict):
         (
             sx127x_packet_interface,
             "sx127x",
-            {"modulation": "OOK", "packet_mode": True, "payload_length": 0},
+            {
+                "modulation": "OOK",
+                "packet_mode": True,
+                "payload_length": 0,
+                "sync_value": [0x12],
+            },
             {CONF_MAX_PACKET_SIZE: 65},
             "at most 64",
             None,
         ),
+        (
+            sx127x_packet_interface,
+            "sx127x",
+            {
+                "modulation": "FSK",
+                "packet_mode": True,
+                "payload_length": 0,
+                "sync_value": [],
+                "preamble_detect": 0,
+            },
+            {},
+            "sync_value",
+            {"preamble_detect": 2},
+        ),
     ],
 )
 def test_radio_settings_checked(
@@ -129,7 +153,9 @@ def test_radio_settings_checked(
     match: str,
     fix: dict | None,
 ) -> None:
-    """Packets vary in length, and FSK or OOK on an SX127x needs packet mode and holds at most 64 bytes."""
+    """Packets vary in length; FSK or OOK on an SX127x needs packet mode, a sync word or preamble detection, and
+    holds at most 64 bytes.
+    """
     config = {f"{domain}_id": "radio", CONF_MAX_PACKET_SIZE: 64, **link}
     token = fv.full_config.set(_FullConfig(domain, radio))
     try: