[vdr] PCI fun (RGB/PAL over VGA at variable frame rate)

Artur Skawina art_k at o2.pl
Sun Aug 17 03:41:18 CEST 2008


Gavin Hamill wrote:
> Over the last days, Thomas and I have been trying to sort out why my 
> nearly-identical machine couldn't run his VGA sync patches properly.
> 
> The key difference is my Radeon 7000VE is PCI, whilst his is AGP. I 
> tried the PCI Radeon in two old Pentium-3 era machines, and on my modern 
> Pentium D930 desktop, all with the same behaviour - fullscreen video 
> over PCI causes huge CPU usage in the Xorg process, even when using xv 
> 'acceleration'.
> 
> When I switch the PCI Radeon for a PCI Express X300 (the very lowest 'X' 
> series you can get), everything is glorious: Xorg CPU use is barely 1%.
> 
> Unfortunately I don't have any machines with both AGP and PCI on which 
> I can try the same OS image but we both think it's safe to conclude that 
> PCI is just unsuitable for this task.

PCI in general should be perfectly fine, for SDTV at least. 
While displaying SDTV (vdrsxfe) I see ~20% cpu use for X on AGP, ~44% on PCI
(same machine, different heads, AGP is MGA450, PCI is MGA200).

The huge difference is likely due to something else, like
- display (X) driver (but even drivers which just memcpy the video
   data to the (xv) framebuffer should work on a modern machine)
- PCI chipset (eg I had a VIA-based mobo, and it couldn't even keep up
   with SDTV on a PCI head, swapping the mobo for one w/ a real chipset
   made all problems suddenly disappear...)

You could probably do some setpci tweaks to improve PCI throughput, but
I doubt the gain would be enough (I'd expect 10% improvement or so).

artur



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