linux-kernel - Re: Serial custom speed deprecated?

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Message-ID: <m34pw1cc9a.fsf@defiant.localdomain>
Date:	Fri, 25 Aug 2006 12:58:57 +0200
From:	Krzysztof Halasa <khc@...waw.pl>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-serial@...r.kernel.org,
	"'LKML'" <linux-kernel@...r.kernel.org>,
	libc-alpha@...rces.redhat.com
Subject: Re: Serial custom speed deprecated?

Alan Cox <alan@...rguk.ukuu.org.uk> writes:

> We could implement an entirely new TCSETS/TCGETS/TCSETSA/SAW which used
> different B* values so B9600 was 9600 etc and the data was stored in
> c_ospeed/c_ispeed type separate fields and we'd support arbitary speeds
> for input and output once and for all, shoot all the multiplier hacks
> etc. As it happens the kernel code for this is easy owing to some
> fortuitous good design long ago in the tty layer.

I think it makes most sense.

> We could also implement a Linux "improved" TCSET* new set of ioctls that
> had sensible speed fields, utf-8 characters for the _cc[] array and new
> flags for all the utf-8 handling and the like. That would be less
> compatible though.

I think compatibility at the source level is good here. UTF-8 looks
nice, though.

I think it could remain compatible - c_cc[] could grow into array of
multibyte characters with:
#define VINTR 0
#define VQUIT (1 * n)
#define VERASE (2 * n)
#define VKILL (3 * n)

where n is max number of UTF-8 bytes (5 for 32-bit UCS?)

I'm not sure if UTF-8 control codes are needed in practice, though
(I mean I just don't know).

> 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.

A bit messy I think. I think the first way is much better. Especially
when we have multiple changes (speed and UTF-8, for example).

>> Not sure if we want int, uint, or long long for speed values :-)
>
> You want speed_t according to POSIX.

Sure, I meant what does speed_t resolve to.

> I've no idea what the glibc impact of this kind of thing would be
> (consider new glibc, old kernel etc).  I've cc'd the libc folks but I am
> not sure it is practical to do.

While obviously I'm not glibc (nor termios) expert I don't think
we should expect problems. New glibc would just issue the old ioctl
if the new one isn't available. I think similar things are already
in place.
Glibc could be compiled with minimum kernel version = 2.6.20 or so
to assume the new ioctls are always present.
-- 
Krzysztof Halasa
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