AGP GART clarifications, please!

[Émeric MASCHINO]

2014-06-20 2:06 GMT+02:00 Dave Airlie <airlied at gmail.com>:
> So to run in AGP mode you need a chipset specific driver to manage the
> chipsets AGP GART and other features, that the GPU drivers can talk
> to.

Do the GPU drivers then talk differently to the graphics card when
there's a GART? I mean, are there different code paths in the GPU
drivers depending on the presence of GART or not? Or is the command
stream built by the DRI module exactly the same with or without GART?
Next, in the kernel DRM, are there different code paths taken
depending on the presence of GART or not? Or is it something
"transparently" (from the DRI module/kernel DRM point of view) managed
at the hardware level: I mean, the exact same data are sent to the
graphics card through the DRI module and kernel DRM, but depending on
the presence of GART or not, the data aren't handled the same at the
hardware level and are "intercepted" and managed differently (e.g.
reorganized) by the GART before being effectively delivered to the
graphics adapter?

> I'm not sure how best to measure a speed difference, it should be noticable with
> games and stuff, maybe not with plain desktop usage, anything that up
> or downloads
> lots of stuff or uses main RAM for textures.

So, my OpenSceneGraph datasets, old Quake II/III demos and
SPECviewperf 7.1.1 benchmark are too limited nowadays ;-) But they
were contemporary to my hardware, so should be representative of this
era.

Thanks Dave for this first batch of explanations,

     Émeric