radeon, apertures & memory mapping

[Ville Syrjälä]

On Sun, Mar 13, 2005 at 11:04:59PM +1100, Benjamin Herrenschmidt wrote:
> > 
> > I must be missing something something obvious because I don't quite 
> > understand what major drawbacks there are with the non-overlapping mode. 
> > As I see it you get at least the same amount of CPU accessible memory as 
> > you get in the overlapping mode.
> 
> Yes, you do, but that means that if the apertures are configured such
> that the entire VRAM fits in a single aperture, then you just can't use
> the second aperture at all. Which means you can't have separate swapper
> setting for both apertures, and thus, can't let two independant
> processes access the video memory with different bit depth, at least on
> big endian machines unless you do trickery, and play with the swapper
> before each access.

Ok so the problem is byte swapping. Looking at atyfb for example it uses 
the "big-endian" aperture on big-endian systems and selects the byte 
swapping method according to the bit depth. If that really means that all 
host access to the aperture gets byte swapped then I don't see how the 
current situation can work correctly for DirectFB. Offscreen surfaces can 
use any bit depth and so their bytes could be swapped incorrectly. Makes 
me wish I had a PPC box alongside the x86 one.

-- 
Ville Syrjälä
syrjala at sci.fi
http://www.sci.fi/~syrjala/