Bug 16375

This bug is about the issue where playing some videos causes X to eat 100% CPU for no apparent reason. It was known early that the cause was nouveau spinning on DMA notifiers, but it wasn't really known why. After an evening of debugging, I was able to explore the problem.

The issue occurs when PutImage() is called at a faster rate than images are transferred by the hardware. I.e. you get a backlog of frames you want to DMA. So the driver is doing exactly what it was supposed to do, it waits for a DMA notifier to become available.

Now the bad assumption here was that waiting for a DMA notifier was a rare occurance. Unfortunately it can be provoked under quite normal circumstances:

1. Input video is a NTSC video at 30 FPS

2. Monitor is running at 60 Hz

3. The video is interlaced, and the deinterlacer is one that outputs an image each field, not each frame, effectively doubling the frame rate.

4. The driver is waiting for vsync between each frame to avoid tearing.

The result is a Xv stream at 60 Hz, which has very good odds of drowning the (vsync-locked) DMA engine.

So you can't really solve the basic problem of the player wanting to output frames faster than is possible (again, because of the vsync restriction). What can be done is to make sure that waiting for a DMA notifier is done without hogging the CPU.

For comparison, the radeon engine seems to do a blocking DRM call as its means of throttling. The driver uses a different design, but from what I can tell the throttling point is where it allocates FIFO space for the Xv image.



Related to this issue is the fact that you can get incorrect frame orders on screen (the output tends to flicker a bit). This is an inherent effect of the racy design of libXv (and possibly the Xv protocol itself). When using XvShmPutImage(), there is no notifier that the server has had time to process the image. If it takes to long, your player will start overwriting the buffer with the next frame. With double buffring players (like mplayer), this will in most cases have the effect of frames getting displayed in the wrong order.