"Payload length is too large" issue when attempting to connect to Frame from linux desktop

[rencire]

Hi, I'm running into an issue with connecting to the frame via this library.
I am using linux, and frame_sdk-1.2.2. Here's the example script:

import asyncio
from frame_sdk import Frame


async def main():
    async with Frame() as f:
        print(f"Connected: {f.bluetooth.is_connected()}")
        print(f"Frame battery: {await f.get_battery_level()}%")

    print("disconnected")

asyncio.run(main())

Here's the error I get:

/home/ren/projects/personal/frame_test/.venv/lib/python3.12/site-packages/bleak/backends/bluezdbus/client.py:632: UserWarning: Using default MTU value. Call _acquire_mtu() or set _mtu_size first to avoid this warning.
  warnings.warn(
Traceback (most recent call last):
  File "/home/ren/projects/personal/frame_test/main2.py", line 12, in <module>
    asyncio.run(main())
  File "/nix/store/l014xp1qxdl6gim3zc0jv3mpxhbp346s-python3-3.12.4/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/nix/store/l014xp1qxdl6gim3zc0jv3mpxhbp346s-python3-3.12.4/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/l014xp1qxdl6gim3zc0jv3mpxhbp346s-python3-3.12.4/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/ren/projects/personal/frame_test/main2.py", line 6, in main
    async with Frame() as f:
  File "/home/ren/projects/personal/frame_test/.venv/lib/python3.12/site-packages/frame_sdk/frame.py", line 32, in __aenter__
    await self.ensure_connected()
  File "/home/ren/projects/personal/frame_test/.venv/lib/python3.12/site-packages/frame_sdk/frame.py", line 46, in ensure_connected
    await self.inject_all_library_functions()
  File "/home/ren/projects/personal/frame_test/.venv/lib/python3.12/site-packages/frame_sdk/frame.py", line 224, in inject_all_library_functions
    response = await self.bluetooth.send_lua(f"frame.file.mkdir(\"lib-{library_version}\");print(\"c\")", await_print=True)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ren/projects/personal/frame_test/.venv/lib/python3.12/site-packages/frame_sdk/bluetooth.py", line 345, in send_lua
    await self._transmit(string.encode())
  File "/home/ren/projects/personal/frame_test/.venv/lib/python3.12/site-packages/frame_sdk/bluetooth.py", line 320, in _transmit
    raise Exception(f"Payload length is too large: {len(data)} > {self._btle_client.mtu_size - 3}")
Exception: Payload length is too large: 41 > 20

Interestingly, frameutils seems to work. I get the expected output from lua on frame when I run the following script:

import asyncio
from frameutils import Bluetooth

async def main():
    bluetooth = Bluetooth()
    await bluetooth.connect()

    print(await bluetooth.send_lua("print('hello world')", await_print=True))
    print(await bluetooth.send_lua('print(1 + 2)', await_print=True))

    await bluetooth.disconnect()

asyncio.run(main())

I remember hearing awhile back that there's a bug with bluetooth driver on linux, I wonder if that is related to this? Any pointers would be appreciated, thanks.

[ciborg971]

in bluetooth.py, add this line 218 :

self._btle_client._backend._mtu_size = 256

[glfharris]

Just going to elaborate on this issue, as this fix didn't quite work for me initially. Disclaimer that I have very minimal experience with bluetooth communications.

I don't know whether this is typical, but Frame doesn't appear to negotiate with the host regarding MTU, so just setting the MTU in bluetooth.py doesn't appear to be enough.

Setting the MTU to 256 was enough to allow communication via frame.bluetooth.send_lua, but would fail for anything requiring multiple packets of data. This meant that the library functions weren't transferred correctly, so anything requiring them would fail. Instead the line in bluetooth.py needs to be changed to reflect the MTU that Frame appears to be operating at. For me the magic number appears to be 247, however I'm not sure this is the case for everyone.

The way I found this was by getting Frame to display the MTU it was expecting and to change bluetooth.py accordingly.

async with Frame() as f:
    f.bluetooth._print_debugging = True
    # This gets Frame to display it's max payload length
    await f.bluetooth.send_lua("""
        mtu = frame.bluetooth.max_length()
        frame.display.text(tostring(mtu), 10, 200)
        frame.display.show()
        print('Success!')
        """)
    # This prints the current max payload as understood by Frame SDK
    print(f.bluetooth.max_data_payload())

Essentially you need to set self._btle_client._backend._mtu_size until these numbers are the same.

You may need to reset Frame after this to get it working. frame-sdk-python will not transfer the library functions if it recognises the file name already on the device, even if its 0 bytes because previous transfers have failed.