linux-kernel - Re: [PATCH RFC] char/tty_io: fix legacy pty name when more than 256 pty devices are requested

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Message-ID: <4AA6DF50.3030603@zytor.com>
Date:	Tue, 08 Sep 2009 15:48:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mauro Carvalho Chehab <mchehab@...radead.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH RFC] char/tty_io: fix legacy pty name when more than 256
 pty devices are requested

On 09/08/2009 10:49 AM, Mauro Carvalho Chehab wrote:
> 
> It is important to preserve the old nomenclature for tty/pty devices for the
> first 256 devices, to avoid breakage on existing applications and with udev.
> 
> So, in order to allow more pty devices, the nomenclature were extended for
> the devices with minor 256 or above. For those, the nomenclature will be:
> 	ttyf0000-ttfpffff	(pty slave)
> 	ptyf0000-ttyfffff	(pty master)
> 

This would seem to be a somewhat silly way to extend it, especially
given the arbitrary artificial limit of just over 2^16 entries.

If you're going to do this, I would suggest using the following
namespace, as such:

	[pt]ty[p-za-e][0-9a-f]+

	sprintf(name, "%cty%c%x", slave ? 't' : 'p',
		"pqrstuvwxyzabcde"[(index >> 4) & 15],
		((index >> 4) & ~15) | (index & 15));

No arbitrary limits, and it still extends the existing namespace with
some reasonable continuity.  It means bits [7:4] are weirdly encoded,
but we get straightforward backwards compatibility as a result.

	-hpa
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