linux-kernel - Re: Serial custom speed deprecated?

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Message-ID: <Pine.LNX.4.61.0608280817030.32531@chaos.analogic.com>
Date:	Mon, 28 Aug 2006 08:17:39 -0400
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
To:	<linux@...izon.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: Serial custom speed deprecated?

On Sat, 26 Aug 2006 linux@...izon.com wrote:

>> Or we could just add a standardised extra set of speed ioctls, but then
>> we need to decide what occurs if I set the speed and then issue a
>> termios call - does it override or not.
>
> Actually, we're not QUITE out of bits.  CBAUDEX | B0 is not taken.

B0 is not a bit (there are no bits in 0). It won't work.

> That would make a reasonable encoding for a custom speed.
> (But I haven't checked glibc... ah, yes, it should work!
> See glibc-2.4/sysdeps/unix/sysv/linux/speed.c; browse at
> http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/sysdeps/unix/sysv/linux/?cvsroot=glibc
> if you don't have a local copy source handy.)
>
> What I'd do is, when converting to the old-style for tcgetattr, if the
> current baud rate is not representable, cache it somewhere and return that
> (or some other magic value).  If a tcsetatt call comes in that specifies
> that magic value, use the cached baud rate.
>
> If you make the cache just the current baud rate setting (the magic
> value on set means "don't alter"), that will handle a lot of programs
> that just want to play with handshaking.
>
> If you make the cache separate, you can also survive an
> old-interface-using program switching to a different baud rate and then
> switching back.
>
>
> Also note that if you truly want to support all baud rates in historical
> use, you'll need to include at least one fractional bit for 134.5 baud.
> (Unless you're sure that IBM 2741 terminals are truly dead. :-))
>
> Alternatively, you could observe that asynchronous communications only
> requires agreement withing 5% between sender and receiver, so specifying
> a baud rate to much better than 1% is not too important.
>
> Half-precision floating point would be ideal for the job. :-)
> -
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.62 BogoMips).
New book: http://www.AbominableFirebug.com/
_
..

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