linux-kernel - Re: [patch 2/2] 8250_pnp: register x86 COM ports at the conventional ttyS names

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Date:	Wed, 16 Jan 2008 13:44:37 -0500
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Bjorn Helgaas <bjorn.helgaas@...com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	Russell King <rmk+serial@....linux.org.uk>,
	Jeff Garzik <jeff@...zik.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 2/2] 8250_pnp: register x86 COM ports at the conventional
 ttyS names

Bjorn Helgaas wrote:
> x86 users expect COM1-COM4 ports at the conventional ioport addresses
> to be named ttyS0-ttyS3.  For PNP devices, the BIOS determines the
> order we discover them, so we might discover COM2 before COM1.
> 
> We currently always get the correct names, even without this patch.
> But that's only because serial8250_isa_init_ports() first registers
> the hard-coded list of COM port addresses from SERIAL_PORT_DFNS.  When
> PNP rediscovers one of those ports, it gets the already-established
> line.
> 
> This patch removes the implicit dependency on SERIAL_PORT_DFNS by
> requesting the names we desire.
>  
>  static int __devinit
> +serial_pnp_line(struct uart_port *port)
> +{
> +#ifdef CONFIG_X86
> +	switch (port->iobase) {
> +	case 0x3f8:	return 0;	/* COM1 -> ttyS0 */
> +	case 0x2f8:	return 1;	/* COM2 -> ttyS1 */
> +	case 0x3e8:	return 2;	/* COM3 -> ttyS2 */
> +	case 0x2e8:	return 3;	/* COM4 -> ttyS3 */
> +	}
> +#endif
> +

Arguably, the right thing is to use the addresses present in the array 
at address 0x400.  In particular, COM3 and COM4 aren't always at those 
addresses.

	-hpa
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