[RFC] Deprecate AGP GART support for Radeon/Nouveau/TTM

[Christian König]

Am 13.05.20 um 13:03 schrieb Christian König:
> Unfortunately AGP is still to widely used as we could just drop support for using its GART.
>
> Not using the AGP GART also doesn't mean a loss in functionality since drivers will just fallback to the driver specific PCI GART.
>
> For now just deprecate the code and don't enable the AGP GART in TTM even when general AGP support is available.

So I've used an ancient system (32bit) to setup a test box for this.


The first GPU I could test is an RV280 (Radeon 9200 PRO) which is easily 
15 years old.

What happens in AGP mode is that glxgears shows artifacts during 
rendering on this system.

In PCI mode those rendering artifacts are gone and glxgears seems to 
draw everything correctly now.

Performance is obviously not comparable, cause in AGP we don't render 
all triangles correctly.


The second GPU I could test is an RV630 PRO (Radeon HD 2600 PRO AGP) 
which is more than 10 years old.

As far as I can tell this one works in both AGP and PCIe mode perfectly 
fine.

Since this is only a 32bit system I couldn't really test any OpenGL game 
that well.

But for glxgears switching from AGP to PCIe mode seems to result in a 
roughly 5% performance drop.

The surprising reason for this is not the better TLB performance, but 
the lack of USWC support for the PCIe GART in radeon.


So if anybody wants to get his hands dirty and squeeze a bit more 
performance out of the old hardware, porting USWC from amdgpu to radeon 
shouldn't be to much of a problem.


Summing it up I'm still leaning towards disabling AGP completely by 
default for radeon and deprecate it in TTM as well.

Thoughts? Especially Alex what do you think.

Regards,
Christian.