linux-kernel - Re: [RFC 1/8] serial:st-asc: Add ST ASC driver.

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Message-ID: <20130510232955.GY18614@n2100.arm.linux.org.uk>
Date:	Sat, 11 May 2013 00:29:55 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Srinivas KANDAGATLA <srinivas.kandagatla@...com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Will Deacon <will.deacon@....com>, jslaby@...e.cz,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Nicolas Pitre <nico@...aro.org>, linux-doc@...r.kernel.org,
	Stephen Gallimore <stephen.gallimore@...com>,
	linux-serial@...r.kernel.org, Jason Cooper <jason@...edaemon.net>,
	devicetree-discuss@...ts.ozlabs.org,
	Rob Herring <rob.herring@...xeda.com>,
	Stuart Menefy <stuart.menefy@...com>,
	Stephen Warren <swarren@...dia.com>,
	Dong Aisheng <dong.aisheng@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/8] serial:st-asc: Add ST ASC driver.

On Wed, May 08, 2013 at 09:36:13AM -0700, Greg KH wrote:
> On Wed, May 08, 2013 at 06:31:48PM +0200, Arnd Bergmann wrote:
> > On Wednesday 08 May 2013, Greg KH wrote:
> > > > just mention there is not hardware reason to not use the generic ttySx
> > > > in place of ttyAS as we have only one IP that handle serial on this
> > > > family of SoC
> > > > 
> > > > personally I'll switch to ttySx
> > > 
> > > Great, then you can use the same major/minor range as well, so there's
> > > no more objection from me about this :)
> > 
> > Does that work these days when you have kernel with multiple built-in
> > uart drivers?
> 
> It "should", as the major/minor registration should only happen when the
> hardware is found, but I haven't tested it out, so I can't say for sure.

serial stuff has never operated like that.  More specifically, it's a
limitation with the tty stuff that the way stuff works is that a
tty driver can only drive a single bunch of contiguous minor numbers.
No interleaving is allowed.

That limitation has existed for years, and I don't see it going away.
As long as that limitation exists, you can only ever have one serial
driver driving a set of contiguous minor numbers.

There has been an attempt to "work around" this by making the 8250
driver "special" which was a complete hack to get it to work.  That
was while I maintained this stuff and I outright refused to make one
serial driver magically special.
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