[BRLTTY] Problem connecting the braille display Brailliant BI 40

[Mario Lang]

Didier Spaier <didier at slint.fr> writes:

> On 12/11/2015 23:29, Mario Lang wrote:
>> Shérab <Sebastien.Hinderer at ens-lyon.org> writes:
>> 
>>> Hello Didier,
>>>
>>> Thanks a lot for your response.
>>>
>>> Didier Spaier (2015/11/12 21:13 +0100):
>>>> I am not sure that I understand your question.
>>>>
>>>> In brltty-5.2 this is done installing the file
>>>> 40-usb-brltty.rules in /lib/udev, or do I miss something?
>>>
>>> I am on Deban andhave brltty 5.2~20141018-5+b2 installed,it seems this
>>> is themost recent version of thepackage.
>>>
>>> I used this command:
>>>
>>> dpkg -L brltty | grep udev
>>>
>>> And the only result I got was:
>>>
>>> /usr/share/doc/brltty/examples/udev.rules.gz
>>>
>>> And I confirm that under/lib/udev/ nothing matches *brltty*
>> 
>> Yes, because udev-activation is not enabled on Debian by default.
>> You are free to use the example file provided to configure your system
>> to use it.
>> I'd be interested in hearing how it works for you, after you give it
>> some time for testing.
>> 
>>> So I think that indeed if the file you mention would be there everything
>>> would work fine but it's not! :)
>> 
>> I am not quite sure that *everything* would work just fine.
>> The problem here is that udev rules can basically just be used
>> to do auto-activation if you use USB.  However, not all
>> configurations do exclusively use USB.  My laptop, for instance,
>> has BRLTTY configured to find my display *either* via USB or Bluetooth.
>> This has prooven to be very helpful once my bluetooth stack broke.
>> 
>> Additionally, some people are still using serial braille displays.
>> These can also not be handled by udev (AFAIK).
>> 
>> If we enable udev activation by default, we need to teach the users
>> additionally about the fact that in some cases, they will have to enable
>> the daemon, and in some other cases they dont.
>> 
>> I am personally not ready to open that can of worms yet.
>> Especially, because I don't really see what we would be gaining.
>> Most of us have BRLTTY around on their systems most of the time anyway.
>> And if resource consumption is a problem, for instance, on an embedded
>> system, you can still manually drop these udev rules into your system
>> configuration and use them.  However, maybe I am missing an obvious
>> point.
>> Why is it particularily pressing to give up control about launching
>> brltty?  Just another link in a chian of things that should never break.
>> 
> Well, I am not acquainted with Debian and am a Slackware user so all the
> systemd thing escapes me, but what I do is as simple a shipping the
> udev rules and:
> _ If a Braille display was used at during installation (it was if brltty
>   was set in the command line), then make the startup script
>   /etc/rc.d/rc.brltty executable
> _ If not, just let it not executable (changing that is just a magtter of
>   "chmod 755/etc/rc.d.rc.brltty")
>
> At boot, /etc/rc.d/rc.S starts the daemon if and only if the startup script
> is executable.
>
> I imagine that something similar could be done with the systemd services?
>
> Furthermore I fail to see an inconvenience to start the daemon even if no
> Braille display is in use, or if a Bluetooth connected device is used:
> the rules wont be triggered then, that's all.

BRLTTY is already automatically searching the USB bus for new devices it
knows to handle.  Since you already
mentioned that there is actually no inconvenience of having it running,
I fail to see why adding udev rules by default is gaining us anything at
this point.

A motivation for having udev fire up BRLTTY would be to support
connecting several displays to a single host.  While interesting, I
suspect that use case is rather rare.  And again, if it turns up for
someone to be useful, they should already be able to use the provided
example rules as a starting point.

-- 
CYa,
  ⡍⠁⠗⠊⠕