Re: [PATCH 6/7] Detaching fbcon: Add capability to attach/detach fbcon

Andrew Morton wrote:

On Tue, 06 Jun 2006 19:09:00 +0800
"Antonino A. Daplas" [off-list ref] wrote:

quoted

Add the ability to detach and attach the framebuffer console to and from
the vt layer. This is done by echo'ing any value to sysfs attributes located
in class/graphics/fbcon. The two attributes are:

      attach - bind fbcon to the vt layer
      detach - unbind fbcon from the vt layer

That's a bit unusual.  Normally a bind or unbind operation will operate
primarily upon the thing which is bound to, not upon the thing which is
bound.

I'm not against this.

I assume it makes sense this way, given the way the vt layer operates.  But
it rather rules out binding the fbcon layer to anything apart from the vt
layer (just thinking about it design-wise).

The vt-layer is different in that the device themselves bind to it, not
the other way around.  As I've told Jon, if someone change the vt-layer so
it behaves in a standard way, I'm all for it (that someone is probably me).

But thinking about it, it is probably not very difficult to change the
vt-layer.  We just make the vt-layer keep a list of registered drivers. And 
we already have /sys/class/tty/console, so I can put the attribute there
for bind and unbind.

ummm, expressing it differently:

	echo "bind fbcon" > /sys/whatever/some-vt-file
	echo "unbind fbcon" > /sys/whatever/some-vt-file

would be more typical usage.

quoted

Once fbcon is detached from the vt layer, fbcon can be unloaded if compiled as
a module. This feature is quite useful for developers who work on the
framebuffer or console subsystem. This is also useful for users who want to
go to text mode or graphics mode without having to reboot.

Do we have a place where this can be documented?

In Documentation/fb/fbcon.txt.

quoted

Directly unloading the fbcon module is not possible because the vt layer
increments the module reference count for all bound consoles.  Detaching fbcon
decrements the module reference count to zero so unloading becomes possible.

Right.  So the vt layer should be told to let go of the fbcon layer.  It's
a vt operation, not an fbcon one, yes?

Yes. I'll take a stab at doing it this way. 

Tony