USB Serial on Linux
This documents my journey to getting a USB serial adapter working on Linux so that I can flash Sonoff Basic R2 with ESPHome. I had a cheap one based on a CH340G hanging around that I could not get working so I forked out for one based on a a FTDI FT232RL which seemed to work out of the box.
See what's going on
Let's look at lsusb and dmesg and see what we get when the CH340G adapter is plugged in.
dmesg
[ 2898.744517] usb 3-8.3: new full-speed USB device number 12 using xhci_hcd
[ 2898.894344] usb 3-8.3: New USB device found, idVendor=1a86, idProduct=7523, bcdDevice= 2.63
[ 2898.894353] usb 3-8.3: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[ 2898.894356] usb 3-8.3: Product: USB2.0-Serial
[ 2898.903026] ch341 3-8.3:1.0: ch341-uart converter detected
[ 2898.904184] usb 3-8.3: ch341-uart converter now attached to ttyUSB0
lsusb
*snip*
Bus 003 Device 015: ID 1a86:7523 QinHeng Electronics CH340 serial converter
*snip*
That looks promising. It's detected and recognised as a USB Serial Device. But hang on a minute, almost immediately after we get.
[ 2899.501293] input: BRLTTY 6.4 Linux Screen Driver Keyboard as /devices/virtual/input/input35
[ 2899.502800] usb 3-8.3: usbfs: interface 0 claimed by ch341 while 'brltty' sets config #1
[ 2899.503336] ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0
[ 2899.503352] ch341 3-8.3:1.0: device disconnected
It looks like the device get's disconnected by brltty, which is apparently a daemon for interfacing with a refreshable braille display. Thankfully I'm not blind and don't need a braille display, so what's going on here. A little searching and I found this Stack Exchange answer that confirms it is indeed a problem.
But hang on a minute. If I plug the FTDI device in it does not seem to be disconnected.
dmesg output from connecting the FTDI adapter
[ 3205.419824] usb 3-8.3: USB disconnect, device number 12
[ 3208.506882] usb 3-8.3: new full-speed USB device number 13 using xhci_hcd
[ 3208.663598] usb 3-8.3: New USB device found, idVendor=0403, idProduct=6001, bcdDevice= 6.00
[ 3208.663608] usb 3-8.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 3208.663611] usb 3-8.3: Product: FT232R USB UART
[ 3208.663613] usb 3-8.3: Manufacturer: FTDI
[ 3208.663616] usb 3-8.3: SerialNumber: AQ027NY5
[ 3208.672641] ftdi_sio 3-8.3:1.0: FTDI USB Serial Device converter detected
[ 3208.672689] usb 3-8.3: Detected FT232RL
[ 3208.673439] usb 3-8.3: FTDI USB Serial Device converter now attached to ttyUSB0
Until I plug the CH340G in, and then both get disconnected (and incidentally my X keyboard config is lost as well).
both adapters being disconnected by BRLTTY
[ 3282.474013] input: BRLTTY 6.4 Linux Screen Driver Keyboard as /devices/virtual/input/input36
[ 3282.476515] usb 3-8.1: usbfs: interface 0 claimed by ch341 while 'brltty' sets config #1
[ 3282.477350] ch341-uart ttyUSB1: ch341-uart converter now disconnected from ttyUSB1
[ 3282.477381] ch341 3-8.1:1.0: device disconnected
[ 3283.642093] usb 3-8.3: usbfs: interface 0 claimed by ftdi_sio while 'brltty' sets config #1
[ 3283.643357] ftdi_sio ttyUSB0: FTDI USB Serial Device converter now disconnected from ttyUSB0
[ 3283.643383] ftdi_sio 3-8.3:1.0: device disconnected
Fix the BRLTTY issue
It looks like there are a couple of solutions to the problem. I went with
this less aggressive approach, just commenting
out the offending udev rule in /usr/lib/udev/rules.d/*-brltty-device.rules.
I did also take this users advice
and removed modemmanager as I don't need it on my laptop and don't fancy any more surprises.