Bug#721763: brltty grabs tty from CP2102/CP2109 device disconnecting default kernel driver
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Package: brltty
Severity: critical
Tags: upstream
Justification: breaks unrelated software
Dear Maintainer,
On connecting a CERN SPEC board (see www.ohwr.org White Rabbit
project) through it's USB to serial convertor (CP2102/CP2109,
vid=10C4h, pid=EA60h) to the system, expecting a ttyUSB device to be
installed using the default kernel driver, brltty unexpectedly
replaced the default kernel driver with it's own USB Braille terminal
driver. Since the Seika Braille reader uses the CP2102/CP2109 default
vid/pid, brltty addressed the SPEC board as such.
The output of dmesg is given here:
[ 3548.128782] usb 3-1.5: new full-speed USB device number 7 using
ehci_hcd
[ 3548.222649] usb 3-1.5: New USB device found, idVendor=10c4,
idProduct=ea60
[ 3548.222654] usb 3-1.5: New USB device strings: Mfr=1, Product=2,
SerialNumber=3
[ 3548.222657] usb 3-1.5: Product: CP2102 USB to UART Bridge Controller
[ 3548.222660] usb 3-1.5: Manufacturer: Silicon Labs
[ 3548.222662] usb 3-1.5: SerialNumber: 0001
[ 3548.223562] cp210x 3-1.5:1.0: cp210x converter detected
[ 3548.296321] usb 3-1.5: reset full-speed USB device number 7 using
ehci_hcd
[ 3548.389167] usb 3-1.5: cp210x converter now attached to ttyUSB0
[ 3548.389243] usb 3-1.5: usbfs: interface 0 claimed by cp210x while
'brltty' sets config #1
[ 3548.389670] cp210x ttyUSB0: cp210x converter now disconnected from
ttyUSB0
[ 3548.389691] cp210x 3-1.5:1.0: device disconnected
To resolve the issue I uninstalled the brltty package. Withholding
brltty from connecting by default to the conflicting USB to serial
convertors involved, solves this problem, but seems not easily
possible at the moment.
As far as the sources concern, the vid/pid combination is found in
Drivers/Braille/Seika/braille.c connectResource() function, but also
in the Hotplug/udev.rules file. I cannot find the latter installed,
and thus assume that the c-code is used. Furtheron I read that alls
supported USB devices are enabled by default in the
/usr/share/doc/brltty/NEWS.Debian.gz file.
If I look at the udev.rules file in the source the news means that
this bug presumably also holds for the following devices:
# Device: 10C4:EA80
# Generic Identifier
# Vendor: Cygnal Integrated Products, Inc.
# Product: CP210x UART Bridge
# Seika [Note Taker]
# Device: 0403:6001
# Generic Identifier
# Vendor: Future Technology Devices International, Ltd
# Product: FT232 USB-Serial (UART) IC
# Albatross [all models]
# Cebra [all models]
# HIMS [Sync Braille]
# HandyTech [FTDI chip]
it holds for my device:
# Device: 10C4:EA60
# Generic Identifier
# Vendor: Cygnal Integrated Products, Inc.
# Product: CP210x UART Bridge / myAVR mySmartUSB light
# Seika [Braille Display]
I had hoped to be able to disable the conflicting device by disabling
the brltty udev rules for these devices, which is alas not possible at
the moment, since it seems not to be used.
I have had contact with:
The devellopers, among others Eric van der Bij and Tomasz Wlostowski,
of the SPEC board, whose strong conviction is that the default kernel
driver should be used to yield a ttyUSB device, and that changing to a
custom vid/pid for this harware will cause confusion. Not to mention
the problems this would possibly cause in Windows land, since this
means programming (and certification) of a custom serial port driver.
A develloper, Dave Mielke, of brltty, whose conviction is that the
SPEC usage of the default vid/pid is correct, but that brltty should
enable as many devices as possible for ease of use by blind people,
the Braille devices are their screen. A standpoint that can be
understood from his perspective. Dave is investigating if the device
can be distinghuished from the default chip via other/additional USB
descriptors.
The manufacturer, Seika, of the Braille reader, who says the reader
can identify itself over the serial bus. No outcome on what is
possible with the vid/pid is given up to this moment. Programming a
custom serial port driver (and having it certified) on the Windows
platform might be a problem in this case too (this is speculative from
my side).
Although the CP210x chips in principle allow reprogramming, it seems
not so easy to solve this problem on the hardware side of the Braille
readers, since existing Braille displays are not easily replaced
because of their high cost and users might not understand why they
should update vid/pid if it could be done in a proper way.
My expectation of the outcome is: the default kernel driver being
respected for the default vid/pid of USB to serial convertors, but a
possibility to enable the use of brltty if needed will be provided. If
possible, clear communication to the blind users of brltty is given on
forehand, that specific devices will become disabled because of this
issue.
Yours,
Tjeerd Pinkert
- -- System Information:
Debian Release: jessie/sid
APT prefers oldstable-updates
APT policy: (500, 'oldstable-updates'), (500, 'testing'), (500,
'stable')
Architecture: i386 (i686)
Kernel: Linux 3.10-2-686-pae (SMP w/2 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.14 (GNU/Linux)
Comment: Using GnuPG with Icedove - http://www.enigmail.net/
iEYEARECAAYFAlImRtkACgkQ9xQaBfeouaoxVQCfUbhK6KrbZgkNBvu6ZJMUGuw+
sHsAn2ZQNrF7e6nYIag1ra0KJ4T27GCa
=im+i
-----END PGP SIGNATURE-----
Reply to: