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

[Alex Deucher]

On Fri, May 22, 2020 at 6:41 AM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
>
> Am 20.05.20 um 18:18 schrieb Alex Deucher:
> > On Wed, May 20, 2020 at 10:43 AM Christian König
> > <ckoenig.leichtzumerken at gmail.com> wrote:
> >> 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.
> > 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.

Yeah, Reviewed-by: Alex Deucher <alexander.deucher at amd.com>

>
> Thanks,
> Christian.
>
> >
> > Alex
>