[Intel-gfx] Meaning of "fbpercrtc=1" option?

Tobias Hain tobias.hain at gmx.de
Wed Oct 7 10:14:37 CEST 2009

Hello there,

I'm using KMS on the latest ubuntu 9.10 bits. My notebook 1280 x 800 is docked 
to an external 1920 x 1200 display.

I'm facing two inconvenieces:

i) During boot both the internal LVDS and external DVI display show the same 
console content. It's a clone,where only the upper left part of the bigger 
1920 x 1200 display is used for 1280 x 800 output. Much space of the external 
display is unusable

ii) Once X has fired up and I can switch with KMS Crtl + Alt + Fn between the 
console and X. The console still only uses a fraction of the bigger display, 
but switching works fine. However the console output on the LVDS internal 
display doesn't turn OFF once switched to X again. That means: While the 
external display shows X, the LVDS shows a stalled console session (backlight 

I resolved both issues by booting with "fbpercrtc=1" option, which I found in 
the source code. It boots fine with a huge 1920 x 1200 console only on the 
external screen and switching between X and console leaves the internal LVDS 
turned off. That's the way I want to use it.

However I don't know what this option actually does:
. Is it safe to use? Or less/untested code?
. Does it consume more memory - maybe because it allocates memory for two 
framebuffer consoles? Is there an option to e.g. force enabling just one fb - 
the bigger one?
. Can the LVDS and DVI framebuffer console used indepenently? dmesg shows me 
fb0/fb1 names?

It's not directly related, but the intel X driver uses
(II) intel(0): Using fuzzy aspect match for initial modes
whose meaning keith described in his blog. I would like to see an override for 
this one in xorg.conf. Nobody else?

For both KMS fb console and X initial mode I'd like to tell my system: Use the 
biggest display only.


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