Add option to disable bit rate switching for socket can (#373)

When running in CAN-FD mode, but with same nominal and data baudrate (just 64B data payload, but without transmitting data faster), BRS bit has to remain at zero. That indicates that there is no bitrate switch happening for data and data is transmitted at same speed. If I send message with BRS=1, even with data & nominal speed configured the same, I get framing errors on the bus. Even single message like this kills my whole bus.

This introduces option to do that via uavcan.udp.disable_brs set to true, or determines this automatically when uavcan.udp.bitrate indicates same bitrate for header & data.

---------

Co-authored-by: Pavel Kirienko <pavel.kirienko@gmail.com>

Author: LukasLendvorsky

CHANGELOG.rst

@@ -3,6 +3,11 @@
 Changelog
 =========
 
+v1.25
+-----
+
+- SocketCAN: Add option to disable bit-rate switching (BRS bit) for FD-CAN with uavcan.udp.disable_brs.  (`#373 <https://github.com/OpenCyphal/pycyphal/pull/373>`_)
+
 v1.24
 -----
 

pycyphal/_version.py

@@ -1 +1 @@
-__version__ = "1.24.5"
+__version__ = "1.25.0"

pycyphal/application/_transport_factory.py

@@ -283,6 +283,7 @@ def init(name: str, default: RelaxedValue) -> ValueProxy:
     iface_list = str(init("iface", "")).split()
     mtu = int(init("mtu", Natural16([64])))
     br_arb, br_data = init("bitrate", Natural32([1_000_000, 4_000_000])).ints
+    disable_brs = bool(init("disable_brs", br_arb == br_data))
 
     if iface_list:
         from pycyphal.transport.can import CANTransport
@@ -292,7 +293,7 @@ def init(name: str, default: RelaxedValue) -> ValueProxy:
             if iface.lower().startswith("socketcan:"):
                 from pycyphal.transport.can.media.socketcan import SocketCANMedia
 
-                media = SocketCANMedia(iface.split(":", 1)[-1], mtu=mtu)
+                media = SocketCANMedia(iface.split(":", 1)[-1], mtu=mtu, disable_brs=disable_brs)
             elif iface.lower().startswith("candump:"):
                 from pycyphal.transport.can.media.candump import CandumpMedia
 

pycyphal/transport/can/media/socketcan/_socketcan.py

@@ -41,7 +41,13 @@ class SocketCANMedia(Media):
     SocketCAN documentation: https://www.kernel.org/doc/Documentation/networking/can.txt
     """
 
-    def __init__(self, iface_name: str, mtu: int, loop: typing.Optional[asyncio.AbstractEventLoop] = None) -> None:
+    def __init__(
+        self,
+        iface_name: str,
+        mtu: int,
+        disable_brs: bool = False,
+        loop: typing.Optional[asyncio.AbstractEventLoop] = None,
+    ) -> None:
         """
         CAN Classic/FD is selected automatically based on the MTU. It is not possible to use CAN FD with MTU of 8 bytes.
 
@@ -50,6 +56,9 @@ def __init__(self, iface_name: str, mtu: int, loop: typing.Optional[asyncio.Abst
         :param mtu: The maximum data field size in bytes. CAN FD is used if this value > 8, Classic CAN otherwise.
             This value must belong to Media.VALID_MTU_SET.
 
+        :param disable_brs: When true, will disable bitrate switching for CAN FD frames. Meaning that the data bitrate
+            will be the same as the nominal bitrate.
+
         :param loop: Deprecated.
         """
         # This can't be made a class attribute because these errnos are only available on GNU/Linux.
@@ -68,6 +77,8 @@ def __init__(self, iface_name: str, mtu: int, loop: typing.Optional[asyncio.Abst
         self._mtu = int(mtu)
         if self._mtu not in self.VALID_MTU_SET:
             raise ValueError(f"Invalid MTU: {self._mtu} not in {self.VALID_MTU_SET}")
+        self._disable_brs: bool = disable_brs
+
         if loop:
             warnings.warn("The loop argument is deprecated", DeprecationWarning)
 
@@ -260,7 +271,7 @@ def _read_frame(self, ts_mono_ns: int) -> typing.Tuple[Timestamp, Envelope]:
                 return timestamp, Envelope(out, loopback=loopback)
 
     def _compile_native_frame(self, source: DataFrame) -> bytes:
-        flags = _CANFD_BRS if self._is_fd else 0
+        flags = _CANFD_BRS if (self._is_fd and not self._disable_brs) else 0
         ident = source.identifier | (_CAN_EFF_FLAG if source.format == FrameFormat.EXTENDED else 0)
         header = _FRAME_HEADER_STRUCT.pack(ident, len(source.data), flags)
         out = header + source.data.ljust(self._native_frame_data_capacity, b"\x00")