Re: [PATCH 5/5] VT binding: Add new doc file describing the feature

[Randy.Dunlap]

On Sun, 11 Jun 2006 08:21:13 +0800 Antonino A. Daplas wrote:

> Jon Smirl wrote:
> > On 6/10/06, Antonino A. Daplas <adaplas@xxxxxxxxx> wrote:
> >> > I may be looking at the problem a little differently. I see the
> >> > drivers like fb, vga, etc as registering with the console and saying
> >> > they are capable of providing console services. I then see the console
> >> > system as opening one of the registered devices. A driver is free to
> >> > register/unregister whenever it wants to as long as it isn't open by
> >> > the console system. Console can only open one driver at a time.
> >>
> >> No, this isn't true.  You can have multiple console drivers active,
> >> that's why you have a first and last parameter in take_over_console().
> >> Thus at boot time, the system driver will take consoles 0 - 63.
> >> Later on when a driver loads, it can take over consoles 0 - 7, leaving
> >> consoles 8 - 63 to the system driver.
> >>
> >> To put it another way, console drivers can register for consoles 0 - 63,
> >> but the user may choose to use it only for consoles 0 - 7.
> >>
> >> This is another reason for the system driver, it makes the unbinding
> >> behavior predictable.  Without a system driver, guessing which driver
> >> replaces the just unbound one may become just a tad bit confusing for
> >> the typical user.
> > 
> > I find the whole console/tty layer to be quite confusing to talk
> > about. I am mixing up console as in where printk goes and console the
> > video card login device. The part about making everything equal was
> > directed towards the printk output device.
> > 
> 
> Sorry about that, I probably should use VT or VC for the main topic
> of this thread, and plain 'console' for where the printk output goes.
> 
> This illustration might help, though I'm not sure if it's entirely
> correct. The main topic of this thread deals with the VT branch only.
> 
> Console
>    :
>    :-----:-----------:--- etc
>    :     :           :
>    VT    Serial      Net
>    :
>    :--------:-------:-----etc
>    :        :       : 
>    vgacon   fbcon   newport_con
>    :        :       :
>    :        :       :
>    hardware fbdev   hardware
>             :
>             :
>             hardware
> 
> 
> > I see now that you can have tty0-7 assigned to a different console
> > driver than tty8-63.
> > Why do I want to do this?
> 
> Multi-head.  I can have vgacon on the primary card for tty0-7,
> fbcon on the secondary card for tty8-16.
> 
> > Why do we need 64 predefined tty devices?
> 
> I don't know, but no one's stopping you to redefine MAX_NR_CONSOLES to
> a lower or higher number.
> 
> > 
> > Googling around the only example I could find was someone with a VGA
> > card and a Hercules card. They setup 8 consoles on each card.
> > 
> >> > Over time it would nice if these all merged to a single
> >> > interchangeable interface. I would really like to be able to
> >> > dynamically switch to serial/net while debugging a video driver. Is
> >> > there some fundamental reason why these can't be merged?
> >>
> >> It's already possible to redirect the system messages to two different
> >> console classes, ie with the boot parameter:
> >>
> >> console=tty0,ttyS0 /* direct output to VT and serial console */
> >>
> >> And you can already choose the console you want by adjusting
> >> /etc/inittab.
> > 
> > How can I change where printk are going at run-time? I didn't know you
> > could do that.
> 
> I really don't know.  Maybe we have some kind of entry in /proc somewhere?

If not, it should be in /sys(fs) (if being added).

fwiw, I have a patch to list all registered consoles:
http://www.xenotime.net/linux/patches/consoles-list.patch


---
~Randy
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