Composite really slow

[Eric Anholt]

On Sun, 2004-11-21 at 14:17, Dave Airlie wrote:
> > 
> > It's all open source.  Someone just needs to find the time to port KAA
> > and the related driver bits to xorg.  Unfortunately, it's not a
> > trivial task.
> > 
> 
> This may be a stupid question and let me clarify I've no idea about
> XAA or KAA and have never looked at either of them, but can XAA not be
> extended (incompatilbity maybe?) to be better at doing
> render/composite stuff, I don't like the idea of rewriting drivers
> just because some interface is not up to the job....

Basically no, in my opinion.  XAA wants you to have single rectangular
space for offscreen memory at a specific bitdepth.  Rendering to
offscreen pixmaps just means that you've got a 'y' value past the height
of the screen -- when your driver is called on to do some rendering, you
don't have information about the pixmap/window being drawn to.

This is totally not what we want for render.  For render, we *need* to
be storing pixmaps of different bitdepth in offscreen memory (right now
we upload them every single time, which is why XAA Render accel sucks
for everything but text).  So, if we're storing non-screen-format
pixmaps in offscreen memory, and we need to be able to do core rendering
to them, we need to modify the interfaces to give the driver's core
rendering code information about the depth, and we can't use a unified
rectangular offscreen because an x/y/w/h doesn't make sense when the bpp
differs.

If we just throw out the rectangular framebuffer, memory management
becomes much easier.  And we can avoid the hacks we've got to deal with
the fact that cards frequently have more memory than they can address in
2d operations if the starting offset is the beginning of framebuffer.

This is the main thing that's important about KAA, in my opinion -- it
starts from the right basis, that being linear memory management.  It
also offers a simpler and smaller driver interface, which I think is
also a feature.

-- 
Eric Anholt                                eta at lclark.edu          
http://people.freebsd.org/~anholt/         anholt at FreeBSD.org
           Thank goodness for the 22nd Amendment