linux-kernel - Re: [PATCH] $ARCH: Enable arbitary speed tty ioctls and split input/output speed

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Date:	Mon, 28 May 2007 14:47:40 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	David Woodhouse <dwmw2@...radead.org>, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] $ARCH: Enable arbitary speed tty ioctls and split
 input/output speed

Alan Cox writes:

> On Thu, 24 May 2007 09:45:37 -0400
> David Woodhouse <dwmw2@...radead.org> wrote:
> 
> > On Thu, 2007-05-24 at 14:41 +0100, Alan Cox wrote:
> > > Most people copied the x86 behaviour which makes it easy to transplant.
> > > Some are just smoking something (see ioctls.h for sh-64 and weep), others
> > > have slightly odd behaviour for historical compatibility reasons (Sparc)
> > 
> > Likewise, I assume the lack of IBSHIFT on PowerPC is because of AIX?
> 
> I assume nobody ever got around to it. CIBAUD is in the PowerPC System V
> API supplement for example and is supported by other Power OS's.

The guys that did the original ppc linux port (mainly Gary Thomas and
Cort Dougan, I believe) tended to follow the alpha and x86 linux ports
rather than the System V ABI.  There aren't any SysV-based OSes for
PowerPC still extant so it doesn't really matter.

And yes, nobody has got around to implementing CIBAUD/IBSHIFT.

> > Why bother introducing new IBSHIFT stuff when it can be declared
> > obsolete already -- if you want different input and output baud rates,
> > just set BOTHER and have different values c_ispeed and c_ospeed.
> 
> BOTHER is for the output speed. BOTHER << IBSHIFT is for the input speed
> if it different to the default (and you need B0 << IBSHIFT to know if an
> input speed is being specifically set in the first place).
> 
> CIBAUD and IBSHIFT come from standards documents so it appears to make
> sense at some level to support those standards. As everyone else in the
> Power world supports it I don't see why Linux should be different.

Sure, we can add them.  I suggest IBSHIFT = 16 and CIBAUD = 0xff0000
(or 077600000 if we are going to continue the peculiar convention of
using octal for the C* constants).

Paul.
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