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: <20090908203323.486c129c@caramujo.chehab.org>
Date:	Tue, 8 Sep 2009 20:33:23 -0300
From:	Mauro Carvalho Chehab <mchehab@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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

Em Tue, 08 Sep 2009 15:48:48 -0700
"H. Peter Anvin" <hpa@...or.com> escreveu:

> 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]+

To be backward compatible, the format should be, at least:

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

otherwise, it can potentially break backward compatibility, as the first 10 devices will be like:
	/dev/ttyp0
	/dev/ttyp1
	...

instead of:

	/dev/ttyp00
	/dev/ttyp01

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

We may use this, instead:

 	sprintf(name, "%cty%c%02x", 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.

But it will also be a little more weird.
> 




Cheers,
Mauro
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