Linux, KMS, 3D, ATI, Oh My!

Yesterday I realized that accelerated 3D rendering no longer worked on my ancient ATI R200 based card. After some experimentation, I discovered that ATI accelerated 3D graphics now requires Kernel Mode Setting (KMS) to be enabled. This was a huge problem because Xv, the X accelerated video rendering system, has diagonal "lightning strike" tearing when used with KMS. I really wanted 3D back, though, so what to do?

Today, I was bored with work, so I decided to play with the ATI R600 based card which I bought way back in the spring. Initially it was very disappointing. The X server crashed almost immediately which was a regression from earlier tests: not a good sign!

Examining the system logs indicated that the problem appeared to be in the kernel, not the X server. I've been running a 2.6.34 kernel from Debian experimental and it so happens that a 2.6.36-rc6 kernel was available. Installing the newer version fixed the X server crash! Yay!

Accelerated 3D rendering now works on the R600 card but what about the tearing in Xv? Frankly, it was a bigger concern than 3D. If Xv didn't work, I would have to go back to the R200 card, which still supported an Xv mode that worked without KMS, and all this effort would have been for naught. So it was a great relief to discover that the Xv tearing problem has been fixed on the R600 card. Double Yay!

It's not often that one accomplishes anything while avoiding work, so I'm quite pleased with today's efforts. It's been a good day!

Disabling Radeon KMS

Kernel Mode Setting (KMS) for the ATI Radeon RV280 chip is not ready for primetime yet so i decided to disable KMS and revert to User Mode Setting (UMS) for now. KMS is the future but it is too immature on the Radeon hardware. Now, how do I disable KMS?

First I changed /etc/modprobe.d/radeon-kms.conf to

option radeon modeset=0

Then I ran update-initramfs -u to copy the change into the boot RAM filesystem, and rebooted. For some reason that doesn't disable KMS. Hmm. Figure out why or find another way? Door number two please.

Next, I added the kernel parameter nomodeset in /boot/grub/menu.lst. Run update-grub and reboot. Nope. KMS is still enabled. Hmm. Figure out why or find another way? Door number two please.

Next I replaced the nomodeset kernel parameter with radeon.modeset=0, again in /boot/grub/menu.lst. Run update-grub and reboot. Ding ding ding, we have a winner! Phew! Pant, pant!

It should be abundantly clear that KMS in the Radeon driver is a work-in-progress when disabling it takes that much effort. ^_^

Kernel Mode Setting

It looks like Kernel Mode Setting (KMS) for ATI chipsets has arrived on my Debian sid desktop system. As usual, I only realized this was happening after mplayer started complaining about the system being to slow. Errors in the X server logs revealed that indeed the Direct Rendering Infrastructure (DRI) was not working. Without DRI hardware acceleration, mplayer is definitely going to have trouble playing 720p video with soft subtitles.

This article indicated that the X server version was OK but I needed the latest Debian kernel package (2.6.32-10), which did indeed fix the DRI error and made mplayer much happier. However, it is not perfect. There are now artifacts appearing in the video during fast pan or tilt scenes. Even in an xterm, rapid scrolling results in noticable discontinuities.

The article mentions needing new microcode for the graphics chip. I haven't yet discovered if such microcode actually exists for the ancient R280 chipset on my ATI 9250 card. Or the problem may lie elsewhere. More digging is required. ^_^