feat(core): add THP transport-level PING/PONG messages

diff --git a/core/src/trezor/wire/thp/__init__.py b/core/src/trezor/wire/thp/__init__.py
index 7e47fab4..3a8b0baa 100644
--- a/core/src/trezor/wire/thp/__init__.py
+++ b/core/src/trezor/wire/thp/__init__.py
@@ -30,6 +30,9 @@ ACK_MESSAGE = const(0x20)
 CHANNEL_ALLOCATION_REQ = const(0x40)
 _CHANNEL_ALLOCATION_RES = const(0x41)
 _ERROR = const(0x42)
+PING = const(0x43)
+_PONG = const(0x44)
+
 CONTINUATION_PACKET = const(0x80)
 
 
@@ -168,6 +171,13 @@ class PacketHeader:
         """
         return cls(_CHANNEL_ALLOCATION_RES, BROADCAST_CHANNEL_ID, length)
 
+    @classmethod
+    def get_pong_header(cls, length: int) -> Self:
+        """
+        Returns header for pong message.
+        """
+        return cls(_PONG, BROADCAST_CHANNEL_ID, length)
+
 
 _DEFAULT_ENABLED_PAIRING_METHODS = [
     ThpPairingMethod.CodeEntry,
diff --git a/core/src/trezor/wire/thp/interface_context.py b/core/src/trezor/wire/thp/interface_context.py
index d623918c..96357470 100644
--- a/core/src/trezor/wire/thp/interface_context.py
+++ b/core/src/trezor/wire/thp/interface_context.py
@@ -15,6 +15,7 @@ from ..errors import WireBufferError
 from . import (
     CHANNEL_ALLOCATION_REQ,
     CODEC_V1,
+    PING,
     PacketHeader,
     ThpError,
     ThpErrorType,
@@ -33,7 +34,7 @@ if TYPE_CHECKING:
     from trezorio import WireInterface
     from typing import Awaitable, Iterable
 
-_CID_REQ_PAYLOAD_LENGTH = const(12)
+_BROADCAST_PAYLOAD_LENGTH = const(12)
 
 
 class ThpContext:
@@ -69,7 +70,8 @@ class ThpContext:
             assert packet_len == len(packet)
             self._iface.read(packet, 0)
 
-            if _get_ctrl_byte(packet) == CODEC_V1:
+            ctrl_byte = _get_ctrl_byte(packet)
+            if ctrl_byte == CODEC_V1:
                 await self._handle_codec_v1(packet)
                 continue
 
@@ -127,17 +129,25 @@ class ThpContext:
             await self._write_packets([response])
 
     async def _handle_broadcast(self, packet: bytes) -> None:
-        if _get_ctrl_byte(packet) != CHANNEL_ALLOCATION_REQ:
-            raise ThpError("Unexpected ctrl_byte in a broadcast channel packet")
+        ctrl_byte, _, payload_length = ustruct.unpack(">BHH", packet)
 
-        data = packet[: PacketHeader.INIT_LENGTH + _CID_REQ_PAYLOAD_LENGTH]
-        if not checksum.is_valid(data[-CHECKSUM_LENGTH:], data[:-CHECKSUM_LENGTH]):
-            raise ThpError("Checksum is not valid")
+        packet = packet[: PacketHeader.INIT_LENGTH + payload_length]
+        if not checksum.is_valid(packet[-CHECKSUM_LENGTH:], packet[:-CHECKSUM_LENGTH]):
+            raise ThpError("Invalid checksum")
 
-        length, nonce = ustruct.unpack(">H8s", packet[3:])
-        if length != _CID_REQ_PAYLOAD_LENGTH:
+        if payload_length != _BROADCAST_PAYLOAD_LENGTH:
             raise ThpError("Invalid length in broadcast channel packet")
 
+        nonce = packet[PacketHeader.INIT_LENGTH : -CHECKSUM_LENGTH]
+
+        if ctrl_byte == PING:
+            response_header = PacketHeader.get_pong_header(_BROADCAST_PAYLOAD_LENGTH)
+            return await self.write_payload(response_header, nonce)
+
+        if ctrl_byte != CHANNEL_ALLOCATION_REQ:
+            raise ThpError("Unexpected ctrl_byte in a broadcast channel packet")
+
+        log.info(__name__, "got alloc: %s", utils.hexlify_if_bytes(packet))
         channel_cache = channel_manager.create_new_channel(self._iface)
         channel = self._load_channel(channel_cache)
 
diff --git a/python/src/trezorlib/debuglink.py b/python/src/trezorlib/debuglink.py
index 2b4de411..1a47f7dd 100644
--- a/python/src/trezorlib/debuglink.py
+++ b/python/src/trezorlib/debuglink.py
@@ -1440,6 +1440,10 @@ class TrezorClientDebugLink(TrezorClient):
                 except Exception:
                     pass
 
+        if self.protocol_version is ProtocolVersion.V2:
+            assert isinstance(self.protocol, ProtocolV2Channel)
+            self.protocol.sync_responses()
+
     def mnemonic_callback(self, _) -> str:
         word, pos = self.debug.read_recovery_word()
         if word:
diff --git a/python/src/trezorlib/transport/thp/message_header.py b/python/src/trezorlib/transport/thp/message_header.py
index f58ddc77..2f33c7f0 100644
--- a/python/src/trezorlib/transport/thp/message_header.py
+++ b/python/src/trezorlib/transport/thp/message_header.py
@@ -33,6 +33,9 @@ _ERROR = 0x42
 CHANNEL_ALLOCATION_REQ = 0x40
 _CHANNEL_ALLOCATION_RES = 0x41
 
+PING = 0x43
+PONG = 0x44
+
 TREZOR_STATE_UNPAIRED = b"\x00"
 TREZOR_STATE_PAIRED = b"\x01"
 
@@ -80,6 +83,9 @@ class MessageHeader:
             and self.ctrl_byte == _CHANNEL_ALLOCATION_RES
         )
 
+    def is_pong(self):
+        return self.cid == BROADCAST_CHANNEL_ID and self.ctrl_byte == PONG
+
     def is_handshake_init_response(self) -> bool:
         return self.ctrl_byte & DATA_MASK == HANDSHAKE_INIT_RES
 
@@ -96,3 +102,7 @@ class MessageHeader:
     @classmethod
     def get_channel_allocation_request_header(cls, length: int):
         return cls(CHANNEL_ALLOCATION_REQ, BROADCAST_CHANNEL_ID, length)
+
+    @classmethod
+    def get_ping_header(cls, length: int):
+        return cls(PING, BROADCAST_CHANNEL_ID, length)
diff --git a/python/src/trezorlib/transport/thp/protocol_v2.py b/python/src/trezorlib/transport/thp/protocol_v2.py
index eb57ffc5..6573d697 100644
--- a/python/src/trezorlib/transport/thp/protocol_v2.py
+++ b/python/src/trezorlib/transport/thp/protocol_v2.py
@@ -137,6 +137,23 @@ class ProtocolV2Channel(Channel):
         self.sync_bit_send = 0
         self.sync_bit_receive = 0
 
+    def sync_responses(
+        self, retries: int = MAX_RETRANSMISSION_COUNT, timeout: float = 10.0
+    ) -> None:
+        """Make sure the event loop is running and ready."""
+        nonce = os.urandom(8)
+        thp_io.write_payload_to_wire_and_add_checksum(
+            self.transport,
+            MessageHeader.get_ping_header(len(nonce) + CHECKSUM_LENGTH),
+            nonce,
+        )
+        for _ in range(1 + retries):
+            header, payload = self._read_until_valid_crc_check(timeout=timeout)
+            if self._is_valid_pong(header, payload, nonce):
+                break
+        else:
+            raise RuntimeError("Invalid ping response")
+
     def _do_channel_allocation(self, retries: int = 0) -> None:
         channel_allocation_nonce = os.urandom(8)
         self._send_channel_allocation_request(channel_allocation_nonce)
@@ -149,7 +166,9 @@ class ProtocolV2Channel(Channel):
     def _send_channel_allocation_request(self, nonce: bytes):
         thp_io.write_payload_to_wire_and_add_checksum(
             self.transport,
-            MessageHeader.get_channel_allocation_request_header(12),
+            MessageHeader.get_channel_allocation_request_header(
+                len(nonce) + CHECKSUM_LENGTH
+            ),
             nonce,
         )
 
@@ -382,6 +401,17 @@ class ProtocolV2Channel(Channel):
             return False
         return True
 
+    def _is_valid_pong(
+        self, header: MessageHeader, payload: bytes, original_nonce: bytes
+    ) -> bool:
+        if not header.is_pong():
+            LOG.error("Received message is not a pong")
+            return False
+        if payload != original_nonce:
+            LOG.error("Invalid pong payload (nonce mismatch)")
+            return False
+        return True
+
 
 def _get_error_from_int(error_code: int) -> str:
     # TODO FIXME improve this (ThpErrorType)
diff --git a/tests/ui_tests/__init__.py b/tests/ui_tests/__init__.py
index bd5f6fa0..48350646 100644
--- a/tests/ui_tests/__init__.py
+++ b/tests/ui_tests/__init__.py
@@ -9,12 +9,9 @@ from contextlib import contextmanager
 import pytest
 from _pytest.nodes import Node
 from _pytest.outcomes import Failed
-from noise.exceptions import NoiseInvalidMessage
 
 from trezorlib.client import ProtocolVersion
 from trezorlib.debuglink import TrezorClientDebugLink as Client
-from trezorlib.exceptions import ThpError
-from trezorlib.transport import Timeout
 
 LOG = logging.getLogger(__name__)
 
@@ -76,19 +73,10 @@ def screen_recording(
     finally:
         if client_callback:
             client = client_callback()
-        client.sync_responses()
-        # Wait for response to Initialize, which gives the emulator time to catch up
-        # and redraw the homescreen. Otherwise there's a race condition between that
-        # and stopping recording.
 
-        # Instead of client.init_device() we create a new management session
-        # `Ping` is sent to make sure the device is available.
-        try:
-            client.get_seedless_session().ping(message="", timeout=1)
-        except (ThpError, NoiseInvalidMessage, Timeout):
-            # Do not raise for unsuccessful ping
-            LOG.exception("Ping failed")
-            pass
+        # Wait for response, which gives the emulator time to catch up and redraw the homescreen.
+        # Otherwise there's a race condition between that and stopping recording.
+        client.sync_responses()
         client.debug.stop_recording()
 
     result = testcase.build_result(request)