RE: [gentoo-accessibility] brltty udev rules

[Keith Wessel]

Of course, this might also be irrelevant for dinosaurs like my serial
PowerBraille as the udev rules will only help with USB and Bluetooth
displays, correct? For serial displays, best option is to just use init and
let brltty remain running, automatically detecting the presence/absence of
the display if I understand correctly. Or is there more to udev than I know
about here?

Keith


-----Original Message-----
From: Chris Brannon [mailto:teiresias@g.o] 
Sent: Wednesday, February 12, 2014 2:07 AM
To: gentoo-accessibility@l.g.o
Subject: [gentoo-accessibility] brltty udev rules

Here's a fragment of the brltty udev rules:

LABEL="brltty_add"
SYMLINK+="brltty/$env{BRLTTY_BRAILLE_DRIVER}-$env{BRLTTY_BRAILLE_DEVICE}"
RUN+="/bin/brltty -E -P$env{BRLTTY_PID_FILE}"
GOTO="brltty_end"

LABEL="brltty_remove"
RUN+="/bin/sh -c 'kill -TERM `cat $env{BRLTTY_PID_FILE}`'"
GOTO="brltty_end"

So basically, this starts *an instance* of brltty whenever a braille
display is connected and kills the corresponding instance when the
display is unplugged.
Yes, this seems quite reasonable.  It's plug-and-play, and I
can see why folks like that, even though I'm an old curmudgeon.
But if you're using those rules, you'd better not start the daemon at
all from your init system.  If you do, you've got a problem, because
you'll have two running daemons trying to communicate with the same
display!
At least, that's what happens with the default configuration, because it
autodetects the braille driver.
Also, if you have a speech-driver setting in your /etc/brltty.conf,
you'll probably end up with two daemons trying to provide speech.  I
haven't verified this, however.

This is only a problem if you want to use brltty for both speech and
braille.  If you just want it for braille, then don't start it from the
init system.  Let udev start it automatically for you, and you're
golden.  If you want to use brltty for both speech and braille,
you're going to have to work around this somehow.

As far as I can tell, there's no way to select a "null" braille driver.
So if you want to use this udev autostart scheme, you don't have the
option of spawning a brltty that will only be used for speech.
In short, the udev scheme is incompatible with using brltty for both
speech and braille.

Probably your best bet is to override the udev rules.  First copy
/lib/udev/rules.d/70-brltty.rules to /etc/udev/rules.d/70-brltty.rules.
Edit /etc/udev/rules.d/70-brltty.rules, and delete the lines that
contain RUN+=
Right now, there are just two of them:
RUN+="/bin/brltty -E -P$env{BRLTTY_PID_FILE}"
and
RUN+="/bin/sh -c 'kill -TERM `cat $env{BRLTTY_PID_FILE}`'"
You'll need to repeat this procedure every time you upgrade brltty, just
to keep your modified rules from getting out of sync with the rules from
/lib/udev/rules.d.
Yeah, overriding files in /lib or /usr/lib with files in /etc is
fragile in the face of updates, but that's the way udev does things...
Anyway, this disables autostarting of brltty from udev.

Now, someone please tell me that there's a better way to do this, and
that I'm seeing problems that do not exist.  I'd love to believe that
I'm delusional!

-- Chris