Re: Support for serial console on legacy free machines

["Stuart MacDonald" <stuartm@connecttech.com>]

From: "Russell King" <rmk@arm.linux.org.uk>
> No.  Console initialisation is done early, before PCI is setup.  This

I figured that out eventually. :-)

> means that the serial driver is relying on a static array of IO port
> addresses.  At this time, the serial driver hasn't probed any ports at
> all, so it doesn't really know what does and doesn't exist.

Correct.

> If, for example, you specify your console on ttyS25, (and you have
> support for >=32 ports compiled in) I wonder what happens?  I can
> see one of two things happening:
>
> 1. Kernel locks up waiting for the non-existent "transmitter" to become
>    ready.
> 2. Kernel continues blindly writing to a non-existent port without
>    locking up and you get no messages at all.

Well, the io address would be... <checks asm/serial.h>
 { 0, BASE_BAUD, 0x148, 12, BOCA_FLAGS }, /* ttyS25 */
on i386 builds. I guess it would depend on what is actually at
0x148. Nothing good would come of it if you didn't have a uart
there, I'm sure.

> Now, this static table is updated after PCI and PNP initialisation, when
> the ports are actually probed.  Your ttyS25 may now change port address
> under the serial console!  I wonder what baud rate the messages come out
> at?  75 baud? ;(

Correct. That entry would be overwritten by whatever dynamic ports
were discovered. Of course, if you only had one 8 port card, it'd
remain the same. But if you had enough 8 port cards, you'd get a serial
port. I think the effect would be that that serial port suddenly becomes
the console, at 9600 8n1. Hm.

> The more I think about this, the more that I think we need to get rid
> of this early console initialisation.  I think Linus really wants early
> console initialisation though, and to be honest, its an extremely useful
> debugging tool for those pesky non-boots with blank displays.

There seems to be a serial console interface; perhaps we need
a general console interface that other code can make use of.
That might pave the way for an ethernet console, or a usb console,
etc.

..Stu