Re: [RFC] Deprecate AGP GART support for Radeon/Nouveau/TTM
Am 20.05.20 um 18:18 schrieb Alex Deucher:
On Wed, May 20, 2020 at 10:43 AM 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
Since this is only a 32bit system I couldn't really test any OpenGL game
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.
We do support USWC on radeon, although I think we had separate flags
for cached and WC. That said we had a lot of problems with WC on 32
bit (see radeon_bo_create()). The other problem is that, at least on
the really old radeons, the PCI gart didn't support snooped and
unsnooped. It was always snooped. It wasn't until pcie that the gart
hw got support for both. For AGP, the expectation was that AGP
provided the uncached memory.
Oh, indeed. I didn't remembered that.
Interesting is that in this case I have no idea where the performance
difference is coming from.
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.
Works for me.
I will take that as an rb and commit at least the first patch.