Tearing screen even with VSyncing on NVIDIA ION

I've got a home media machine which I to watch MythTV and various online video streaming services. It's been annoying me for a while (possibly since I updated to Ubuntu 11.04) that there was tearing on the screen. It looked exactly like it wasn't vsyncing correctly, but I had vsyncing enabled in all the players I was trying.

The machine I'm using is an Acer Revo with NVIDIA ION chipset, so it isn't the most powerful system. Searching the internet for 'tearing' turned up results which were mostly about people not being able to play 720p or 1080p videos. However that certainly wasn't my issue as I'd done all of the following:

• Giving the graphics card 512MB of RAM (called UMA in the BIOS setting).
• Enabling VDPAU acceleration.
• Disabling the compiz window manager (selecting "Ubuntu classic (no effects)" as the session).
• Setting export __GL_SYNC_TO_VBLANK=1 and ignoring issues with vsync occurring for the wrong display.
• Checking the refresh rate was correctly setup with nvidia-settings utility.
• Updating to the latest NVIDIA drivers

But none of this helped. I then realised that all rendering to the screen was suffering from this, not just video playback. I wondered if the ION couldn't handle the memory bandwidth, but turning down the resolution didn't help. I then thought that maybe it was my TV being a bit broken with HDMI, but testing that would require finding a VGA cable or moving a monitor downstairs, so I didn't try that.

I even got to the point of thinking about installing Windows XP to see if it had the same issue and, if it didn't, giving up on Ubuntu. However after one last search I discovered a post on the XBMC forum. It contained the wisdom of adding the following to /etc/X11/xorg.conf:

Section "Extensions"
    Option         "Composite" "Disable"
EndSection

So I went ahead and did that, restarted X and all tearing was gone. I don't know where the bug is in the system, but I'm just glad of the ability in Linux to control things like this.