linux-kernel - Re: [PATCH] TTY: export NR_LDISC and N_* line discipline numbers to user-space

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Date:	Tue, 24 May 2011 15:17:23 +0200
From:	Florian Fainelli <ffainelli@...ebox.fr>
To:	Greg KH <gregkh@...e.de>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Maxime Bizon <mbizon@...ebox.fr>,
	Tilman Schmidt <tilman@...p.cc>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] TTY: export NR_LDISC and N_* line discipline numbers to user-space

Hello,

On Tuesday 24 May 2011 14:40:44 Greg KH wrote:
> On Tue, May 24, 2011 at 10:43:03AM +0200, Florian Fainelli wrote:
> > From: Florian Fainelli <ffainelli@...ebox.fr>
> > 
> > Since commit (4564f9e5: consolidate line discipline number definitions)
> > the patch moved all line discipline number from a per-architecture
> > termios.h to a shared one: tty.h. However, prior to this consolidation
> > work, the line discipline numbers were outside of an ifdef
> > __KERNEL__/endif block so these numbers used to be exported to
> > user-space.
> > 
> > Since such numbers are kernel ABI anyway, and tty.h is already included
> > for user- space header processing, just move these relevant defines
> > outside of the ifdef __KERNEL__/endif block in include/linux/tty.h.
> 
> Any reason you didn't copy _any_ of the people who signed off on that
> patch, or the original author of that patch?

No particular reason but my mistake, fixed that.

> 
> Anyway, this is been this way for four years, since the 2.6.21 kernel
> release.  Since no one has noticed, or complained, since then, how much
> a problem is this really?

This is a problem because you may have out of tree drivers using a line 
discpline number, and you do not want your companion user-space application to 
have to use a different line discpline number but the one the kernel should 
provide.

> 
> Meaning, why has it taken this long for anyone to notice?

- long standing user-space applications such as pppd got their N_PPP long ago, 
and do not have to worry about N_PPP being renumbered so they can just define 
it in pppd's sources it won't break

- I started implementing a line discipline driver only a couple of months ago 
and then realized that such ABI should have been exported
--
Florian
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