[Xorg] Xfbdev acceleration without offscreen pixmaps?

[Andy Ritger]

On Tue, 29 Jun 2004, Keith Packard wrote:

> 
> Around 14 o'clock on Jun 29, Michael Frey wrote:
> 
> > One item -- because I do not have much memory on my video card --
> Using 
> > the composite extension is very slow.  I disabled it and the server 
> > screams.
> 
> Life is hard for those of us with small video cards; you'll see us 
> clustered 'round the 512Mb video cards with envy in our eyes.


Please be careful when counting on large vidmem configurations for
Composite support.  As the amount of video memory increases, less
of it will be mappable for direct CPU accesses (due to exhausting
address space, SBIOS compatibility issues, etc).

In our experiments, we found many SBIOSes failed to even boot with
a graphics board strapped for 512 Mb of video memory, even with
low system memory populations.  And of course, as the amount of 
video memory + system memory configurations increase, mapping all 
memory will quickly exhaust a 32 bit address space. 
 
For >= 512Mb video cards, we're most likely going to only strap 
them at some smaller size (a likely config will be 512 real/256
strapped).  The rest of vidmem will of course be accessible via 
the GPU, but not accessible directly by the CPU.
 
This most likely doesn't have a huge impact on Composite, but it
will mean that we can't count on sw rendering to all redirected
windows (or, limit redirected windows to the mappable region of
vidmem).  Of course, the solution will be for large vidmem drivers
to add paths such that all rendering is done via the GPU.

Just FYI.

Thanks,
- Andy



> If you have an AGP card, and some documentation, it's not at all 
> inconceivable to use main memory from the accelerator to make Composite 
> scream as well.
> 
> Glad it's working.
> 
> -keith
> 
> 
> 
>