Re: Reading from /dev/fb0 very slow?

[Pavel Machek]

Hi!

> > I did some benchmarks, and my framebuffer is *way* faster when writing
> > than when reading:
> 
> That is quite normal.

Unfortunately, at least X and few framebuffer modes can not survive
that too well.

vesafb ypan mode attempts to do video-to-video copy, which is slow.
Xserver attempts that, too. 

> > That is 6 times slower! This is also very visible in X, where moving
> > regions is expensive, while just drawing regions is fast. For example
> > gnome-terminal is *way* faster *with* transparent background option.
> > 
> > Any idea why such assymetry? [This is toshiba 4030cdt with vesafb and
> > 2.4.5]
> 
> Writes to a PCI device can be queued or posted. Reads from a PCI device for
> obvious reasons have to stall the CPU until the data returns. 

But they can't be posted indifinitely, right? I'm copying whole
framebuffer at a time, I do not believe PCI has enough buffers to
cache *that*. [Or is it using some kind of burst mode it can not use
for reading? That does not give a sense, you can cache reads, too....]

-- 
The best software in life is free (not shareware)!		Pavel
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