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[PATCH] VT binding: fbcon: Update documentation

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Update Documentation/fb/fbcon.txt to reflect the following changes:

1. Simple illustration of the binding of the console down to individual
   framebuffer drivers

2. Usage of userspace tools to help with recovery of text console

3. How to use the attributes in /sys/class/tty/console to unload fbcon and
   the framebuffer drivers

Signed-off-by: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Antonino A. Daplas 2006-06-26 00:27:10 -07:00 committed by Linus Torvalds
parent e614b18dce
commit 418d1ce618
1 changed files with 51 additions and 24 deletions
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75
Documentation/fb/fbcon.txt
View file

@ -150,14 +150,42 @@ C. Boot options
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
@ -183,27 +211,26 @@ restored properly. The following is one of the several methods that you can do:
5. Now to detach fbcon:
'cat /sys/class/tty/console/backend' and take note of the ID
The above is probably needed only once. Then:
vbetool vbestate restore < <vga state file> && \
echo 1 > /sys/class/graphics/fbcon/detach
echo <ID> > /sys/class/tty/console/unbind
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:
detach fbcon
rmmod xxxfb
modprobe yyyfb
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:
=======================
@ -227,11 +254,11 @@ Variation 1:
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
@ -242,7 +269,7 @@ Variation 2:
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>