On Sun, 2 Dec 2001, Billy Biggs wrote:
> > > Why does it take so long to copy the data to the framebuffer? Can't
> > > we use DMA here? Does it really take that long to just copy 512k?
> >
> > It's a little more than that because the driver is using 4:2:2
> > internally. Copying the way it is doing you can't get much more than
> > 160 MB/sec and uses the CPU the whole time.
>
> Hrm, I hope it doesn't just double each chroma scanline. I'm in fear
> now. Wish that was documented somewhere, I would have written a better
> chroma upsampler sooner.
It just doubles each chroma line. That's all the nv driver does.
I'm not sure there's any preprocessing that would make the result more
accurate. Fact of the matter is most hardware can't do planar YCbCr
formats due to the high bandwidth requirements. I don't know of anybody
doing packed 4:2:0. Oh, I guess 3dfx does, but it still takes up
2 bytes per pixel and their conversion looks really bad. They
don't seem to being doing CCIR 601 so it looks all washed out.
> > DMA won't make the transfer go any faster (it will probably be slower
> > unless you're using 2x+ AGP), but it won't eat the CPU. The only
> > drivers that do this are NVIDIA's binary drivers and supposedly some
> > experimental ATI drivers that some people are working on. Maybe the
> > i810 drivers do too, not that it would help, since the bandwidth is
> > probably the same writing to video ram or the framebuffer.
>
> I still think that using DMA is essential for all drivers if the
> images doesn't require some conversion. I guess I'd be worried about
> drivers which require scanlines to be 32bit aligned. Too bad the Xv API
> doesn't allow for that. (or am I on crack again)
I'm not sure what you mean. Pretty much everything always needs
to be 32 bit aligned. mpeg scanlines are always multiples of 16 pixels
anyhow.
Mark.
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