linux-kernel - Re: How do I choose an arbitrary minor number for my tty device?

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Message-ID: <4CEBF668.2080708@freescale.com>
Date:	Tue, 23 Nov 2010 11:14:16 -0600
From:	Timur Tabi <timur@...escale.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Greg KH <gregkh@...e.de>, Arnd Bergmann <arnd@...db.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Scott Wood <scottwood@...escale.com>,
	Stuart Yoder <stuart.yoder@...escale.com>
Subject: Re: How do I choose an arbitrary minor number for my tty device?

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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