Message-ID: <4CEBF668.2080708@freescale.com>
Date: Tue, 23 Nov 2010 11:14:16 -0600
From: Timur Tabi <timur@freescale.com>
Organization: Freescale
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: Greg KH <gregkh@suse.de>, Arnd Bergmann <arnd@arndb.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Scott Wood <scottwood@freescale.com>,
        Stuart Yoder <stuart.yoder@freescale.com>
Subject: Re: How do I choose an arbitrary minor number for my tty device?
References: <20101118163321.GA2723@suse.de>	<4CE5562B.8080604@freescale.com>	<20101118165136.GA3103@suse.de>	<4CE55ACB.80207@freescale.com>	<20101118171856.GA4283@suse.de>	<4CE5647F.5000203@freescale.com>	<20101118175832.GA4931@suse.de>	<4CE58005.7040308@freescale.com>	<20101118200230.GA25715@suse.de>	<4CE58736.4090703@freescale.com>	<20101118201035.GA26287@suse.de>	<4CE58FE5.3010502@freescale.com>	<20101118205647.677404ec@lxorguk.ukuu.org.uk>	<4CEA9B00.1000804@freescale.com> <20101123135621.5d100e81@lxorguk.ukuu.org.uk>
In-Reply-To: <20101123135621.5d100e81@lxorguk.ukuu.org.uk>
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Alan Cox wrote:
> Your console driver provides a device method (see kernel/printk.c).
> When /dev/console is opened the kernel iterates the console list looking
> for one with ->device and then calls that method. On success it expects
> the passed int * to contain the minor number to use.

Are you talking about this:

static struct tty_driver *ehv_bc_console_device(struct console *co, int *index)
{
	*index = co->index;

	return ehv_bc_driver;
}

I never really understood this function, but almost everyone does the same
thing, and it seems to work for me.  Looking at console_device(), it appears
that all of the xxx_console_device functions are called in order until one of
them returns non-NULL.

How is this related to add_preferred_console()?  When I call this function, I
also specify the same index and the name from the struct console device:

static struct console ehv_bc_console = {
	.name		= "ttyEHV",
	.write		= ehv_bc_console_write,
	.device		= ehv_bc_console_device,
	.flags		= CON_PRINTBUFFER | CON_ENABLED,
};

add_preferred_console(ehv_bc_console.name, ehv_bc_console.index, NULL);
register_console(&ehv_bc_console);

> I suspect in your case you probably want to attach the primary byte
> channel to minor 0 in the driver (and reserve it for that), or some
> similar rule.

Yes, that's a good idea.  It does simplify things a lot.

-- 
Timur Tabi
Linux kernel developer at Freescale

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