Sunday, May 16, 2010

Fedora 10+ Kernel Modesetting (KMS)

Boring backstory: I tend to buy computers in sets, so currently, my three primary R&D machines are HPs: a set of twins, and a more powerful third. That one is now loaded with ESX, so my main Linux desktop is one of the twins. Last week, its hard drive died and I just got the replacement. Now to the real story...

I'd been running Fedora 6 or 8 to work with Xen. That project has been finished for several months, so when I installed the new drive, I loaded 11. I found, however, that I could not get the Gnome desktop to run at better than 1024x768. I had run 11 before without problems using the same monitor at 1600x1200-- but that was on "third", who is now running ESX.

I checked the twin out, and the card should have been able to run 1280x1024. I could get system-config-display to specify 1280, but the desktop would always drop to 1024. After investigating the problem, I found the culprit was KMS, or kernel modesetting. (Yes, its one word.)

The idea is that the kernel, who owns all the hardware anyway, will decide the best resolution, and the software will do as it is told. Unfortunately, it works with Intel, plays nice with nVidia, but there are a few issues with ATI. Turns our, third was nVidia, and the twins are ATI.

A feature that is closely tied to KMS is the new boot progress screen called "Plymouth". Without KMS, Plymouth is just a three color progress bar. With KMS, its a blue sun projecting solar flares. For these ATI Radeon machines, no Plymouth. This is because KMS isn't reverse compatible. As a result, Gnome looked to the kernel for the correct resolution, kernel said "don't know", and so the desktop could not be made to exceed 1024.

In the end, the solution ended up being to add a Grub argument:
Still no Plymouth, but when Gnome asks the kernel for the correct resolution, the response is "decide yourself". Worked for me. Other possibilities, any one of the following:
radeon modeset=0
radeon modeset=1

Good reading:
Plymouth Graphical Boot
How To Enable Graphical Boot with Plymouth

1 comment:

  1. On a side note... This is the first time Compiz (FNA: Beryl) has worked with the native video drivers.