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

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Message-Id: <E1JFEwb-0000zs-2q@be1.7eggert.dyndns.org>
Date:	Wed, 16 Jan 2008 21:33:04 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Bjorn Helgaas <bjorn.helgaas@...com>,
	"H. Peter Anvin" <hpa@...or.com>, 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>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [patch 2/2] 8250_pnp: register x86 COM ports at the conventional ttyS names

Bjorn Helgaas <bjorn.helgaas@...com> wrote:
> On Wednesday 16 January 2008 11:44:37 am H. Peter Anvin wrote:
>> Bjorn Helgaas wrote:

>> > +#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.
> 
> Wow.  I bow before your storehouse of x86 arcana :-)
> 
> I guess you're referring to the "BIOS data area," which I'd never
> heard of before (but fortunately, Google knows).

You'll want to google for Ralph Brown ...
if your back allowes bowing that much.

> What would you think about doing this only for COM1 and COM2?  The
> only real value for doing this in the first place is so "console=ttyS0"
> always goes to COM1, even if we don't have SERIAL_PORT_DFNS.  User-
> space ought to use some sort of udev magic if it cares about persistent
> naming.

Since the first four COM ports are magic, and since using the BIOS port
numbers will move the non-legacy ports anyway, you should use all up to
four stored port numbers.

BTW1: These addresses may be used to detect ports on non-standard addresses,
but unfortunately they don't tell the IRQ.

BTW2: When I submitted a patch using the BIOS data area, I was told that it
might not exist on systems booting from non-PC firmware. This claim was not
yet backed with any knowledge, nor did anybody suggest a way to detect this
situation.

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