[PATCH] VT binding: fbcon: Update documentation

C. Attaching, Detaching and Unloading

It's possible to detach/attach the framebuffer console from the vt layer by

echoing anything to the following sysfs attributes found

/sys/class/graphics/fbcon.

Before going on on how to attach, detach and unload the framebuffer console, an

illustration of the dependencies may help.

	   attach - attach framebuffer console to vt layer

	   detach - detach framebuffer console to vt layer

The console layer, as with most subsystems, needs a driver that interfaces with

the hardware. Thus, in a VGA console:

If fbcon is detached from the vt layer, your boot console driver (which is

console ---> VGA driver ---> hardware.

Assuming the VGA driver can be unloaded, one must first unbind the VGA driver

from the console layer before unloading the driver.  The VGA driver cannot be

unloaded if it is still bound to the console layer. (See

Documentation/console/console.txt for more information).

This is more complicated in the case of the the framebuffer console (fbcon),

because fbcon is an intermediate layer between the console and the drivers:

console ---> fbcon ---> fbdev drivers ---> hardware

The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot

be unloaded if it's bound to the console layer.

So to unload the fbdev drivers, one must first unbind fbcon from the console,

then unbind the fbdev drivers from fbcon.  Fortunately, unbinding fbcon from

the console layer will automatically unbind framebuffer drivers from

fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from

fbcon.

So, how do we unbind fbcon from the console? Part of the answer is in

Documentation/console/console.txt. To summarize:

Echo the ID number of the 'frame buffer driver' to:

sys/class/tty/console/bind - attach framebuffer console to console layer

sys/class/tty/console/unbind - detach framebuffer console from console layer

If fbcon is detached from the console layer, your boot console driver (which is

usually VGA text mode) will take over.  A few drivers (rivafb and i810fb) will

restore VGA text mode for you.  With the rest, before detaching fbcon, you

must take a few additional steps to make sure that your VGA text mode is

5. Now to detach fbcon:

       vbetool vbestate restore < <vga state file> && \

       echo 1 > /sys/class/graphics/fbcon/detach

       'cat /sys/class/tty/console/backend' and take note of the ID

6. That's it, you're back to VGA mode. And if you compiled your drivers as

   modules, you can unload them at will.  So if you want to change your driver

   from xxxfb to yyyfb, you can do this:

The above is probably needed only once. Then:

	detach fbcon

	rmmod xxxfb

	modprobe yyyfb

       vbetool vbestate restore < <vga state file> && \

       echo <ID> > /sys/class/tty/console/unbind

	Of course, con2fbmap can do the same thing but will not work if xxxfb

	and yyyfb are not compatible (ie, cannot be loaded at the same time).

6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,

   you can unload it by 'rmmod fbcon'

7. To reattach fbcon:

       echo 1 > /sys/class/graphics/fbcon/attach

       echo <ID> > /sys/class/tty/console/bind

8. Once the framebuffer console is detached, and if it is compiled as a module,

the module can be unloaded with 'rmmod fbcon'.  This feature is great for

developers.

8. Once fbcon is unbound, all drivers registered to the system will also

become unbound.  This means that fbcon and individual framebuffer drivers

can be unloaded or reloaded at will. Reloading the drivers or fbcon will

automatically bind the console, fbcon and the drivers together. Unloading

all the drivers without unloading fbcon will make it impossible for the

console to bind fbcon.

Notes for vesafb users:

=======================

    c. Attach fbcon

        vbetool vbestate restore < <vesa state file> && \

	echo 1 > /sys/class/graphics/fbcon/attach

Variation 2:

    a. Before detaching fbcon, do:

	echo <ID> > /sys/class/tty/console/bind

       vbetool vbemode get

    c. Attach fbcon:

       vbetool vbemode set <mode number> && \

       echo 1 > /sys/class/graphics/fbcon/attach

	echo <ID> > /sys/class/tty/console/bind

--

Antonino Daplas <adaplas@pol.net>