linux-kernel - Re: [RFC] tty (or char) bus?

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Date:	Sun, 19 Jul 2009 15:19:22 +0200
From:	Tilman Schmidt <tilman@...p.cc>
To:	Samuel Thibault <samuel.thibault@...-lyon.org>,
	Janusz Krzysztofik <jkrzyszt@....icnet.pl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>
Subject: Re: [RFC] tty (or char) bus?

Scripsit Samuel Thibault die 17.07.2009 19:54:
> Janusz Krzysztofik, le Tue 14 Jul 2009 17:31:23 +0200, a écrit :

>> AFAICS, even if tty lowlevel write() could be used unmodified, a
>> convenient way of reading characters from a tty is missing and should
>> be implemented in a line discipline. Please correct me if I am wrong.
> 
> Have you seen the receive_buf line discipline hook?  Indeed it's not a
> read() operation as from userland, but at least you can get the data
> from the tty that way.

That's as it should be. A read() operation that sleeps until some data
is available isn't very useful in kernel mode, as it can only be used
if you have the ability to sleep. A callback function which runs your
code as soon as the data arrives is a much better fit, although of
course it requires a bit of rethinking.

>> OTOH, I found that some kind of abstraction layer for acccessing devices 
>> over a tty could be convenient. Instead of allocating a new line 
>> discipline for each specific device, sometimes found on a specific board 
>> only, why not just create a new bus type?
> 
> I'd tend to agree with you, as I also have a use case for that: braille
> & speech synthesis devices.  However for now I haven't found a really
> convincing argument why line disciplines aren't enough.

I was in the same situation three years ago when I implemented the
ser_gigaset driver for an RS232 connected ISDN adapter, and found the
line discipline (LD) interface quite adequate once I had figured out
how to use it. The only inconvenience is how LDs are loaded and
attached to a serial interface, via the TIOCSETD ioctl, because you
need a userspace daemon which keeps the tty device open so that the
LD stays attached to it.

-- 
Tilman Schmidt                    E-Mail: tilman@...p.cc
Bonn, Germany
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