Re: [RFC] Remove AGP support from Radeon/Nouveau/TTM

[Alex Deucher <alexdeucher@gmail.com>]

On Tue, May 12, 2020 at 5:40 AM Karoly Balogh (Charlie/SGR)
<charlie@scenergy.dfmk.hu> wrote:
>
> Hi,
>
> On Tue, 12 May 2020, Rui Salvaterra wrote:
>
> > > FWIW, on my last-generation PowerBook with RV350 (IIRC), there was a
> > > big performance difference between AGP and PCI GART. The latter was
> > > sort of usable for normal desktop operation, but not so much for
> > > OpenGL apps (which were usable with AGP).
> >
> > I never really understood what were the issues with AGP on PowerPC
> > (well, Apple, the only ones I've tested) machines. I mean, did OS X also
> > disable AGP entirely, or did it have workarounds somewhere else on the
> > stack nobody was able to figure out?
>
> I don't know about OS X, but I doubt there is a major/blocker hardware
> issue, at least not one which affects every AGP machine.
>
> MorphOS' own Radeon driver uses the AGP facilities to some degree on all
> AGP PowerPC Macs supported by that OS, which is from PMac AGP Graphics
> (3,1) all the way up to the AGP G5 (7,3), including the various portables
> and the Mac mini G4. For example it can utilize it to stream video data
> directly from mainboard RAM, so you don't have to copy it with the CPU,
> allowing reasonably good 720p h264 video playback on most systems above
> the 1Ghz mark with the native MPlayer port. I'm sure the 3D part of the
> driver also use it to some degree, given the performance improvement we
> experienced when the AGP support was enabled (initially the system was
> running without it), but to which extent I can't say.

The problem is AGP doesn't support CPU cache snooping.  Technically
PCI must support coherent device access to system memory.  Unsnooped
access is an optional feature and some platforms may not support it at
all.  Unfortunately, AGP required unsnooped access.  x8t generally
provides a way to do this, but other platforms, not so much.  I don't
recall to what extent PowerPC supported this.  The Linux DMA API
doesn't really have a way to get uncached memory for DMA so there is
that too.  Windows and Mac may provide a way to do this depending on
the platforms.  What probably should have been done on AGP boards was
to use both the AGP GART and the device GART.  The former for uncached
memory (if the platform supported it) and the latter for cached
memory.  That never happened.

Alex
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