linux-kernel - Re: post 3.14 serial regression

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Date:	Wed, 09 Apr 2014 13:04:03 -0700
From:	Dave Hansen <dave@...1.net>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Dave Hansen <dave.hansen@...el.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Geert Uytterhoeven <geert+renesas@...ux-m68k.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>
Subject: Re: post 3.14 serial regression

On 04/09/2014 12:41 PM, Geert Uytterhoeven wrote:
> Dave: If I understand it correctly, you use console=ttyS2, while the kernel
> suddenly changed the order of the serial devices, so your port is no
> longer ttyS2, but ttyS4. Hence the serial port is not found, and
> uart_remove_one_port() is called on it, taking away your /dev/console for
> userspace?

Right.

> Why does the serial port driver call uart_add_one_port() for ports that
> don't exist?
> Or if it does exist ("inaccessible and buried inside the system somewhere"),
> how is it removed again?

The ones buried inside the system are immaterial here.  I just wanted to
explain why I was using a separate PCI card instead of the other serial
ports.

>>>From the backtrace, it's the call below to uart_remove_one_port()
> that removes the port?
> 
> /**
>  *      serial8250_register_8250_port - register a serial port
>  *      @up: serial port template
>  *
>  *      Configure the serial port specified by the request. If the
>  *      port exists and is in use, it is hung up and unregistered
>  *      first.
>  *
>  *      The port is then probed and if necessary the IRQ is autodetected
>  *      If this fails an error is returned.
>  *
>  *      On success the port is ready to use and the line number is returned.
>  */
> int serial8250_register_8250_port(struct uart_8250_port *up)
> {
>         ...
> 
>         if (uart && uart->port.type != PORT_8250_CIR) {
>                 if (uart->port.dev)
>                         uart_remove_one_port(&serial8250_reg, &uart->port);
> 
> where was it added before?

Remember, serial8250_find_match_or_unused() will also reuse *EXISTING*
uart_ports if the port is of 'unknown' type.

I believe that port got added during the addition of the
serial8250_isa_devs, and now we're trying to reuse it since it is an
unknown port type.
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