Re: [RFC] Remove AGP support from Radeon/Nouveau/TTM
On Wed, 13 May 2020 at 04:21, Alex Deucher <alexdeucher@gmail.com> wrote:
>
> On Tue, May 12, 2020 at 1:02 PM Rui Salvaterra <rsalvaterra@gmail.com> wrote:
> >
> > On Tue, 12 May 2020 at 17:38, Daniel Vetter <daniel@ffwll.ch> wrote:
> > >
> > > Otherwise all agree, agp is a mighty mess and essentially just
> > > crapshot outside of x86. It kinda worked for the much more static
> > > allocations for dri1, but with in-kernel memory managers all the cache
> > > flushing issues showed up big time and it all fell to pieces. Plus a
> > > lot of these host chipset back then where designed for the rather
> > > static windows gpu managers, so even on x86 the coherency issues for
> > > agp mode when used together with ttm or something else really dynamic
> > > is pretty bad because the hw just doesn't really cope and has all
> > > kinds of flushing troubles and races. I think the later agp chipsets
> > > were better.
> >
> > That was rather insightful, thanks. I was starting to doubt my own
> > memory, as I was almost sure I never had any hangs with AGP on PowerPC
> > before KMS was a thing. But even on x86, I distinctly remember never
> > being able to get sideband addressing working with any AGP cards, my
> > system would randomly hang too.
> > I'm starting to believe AGP was shoehorned into PCI the same way VLB
> > was shoehorned into ISA (and for the same reason). History repeats
> > itself… :)
>
> Pre-KMS, the kernel just allocated a static relatively small (e.g., 8
> MB) AGP buffer which never changed. In that case, things were
> somewhat more reliable.
This is why the AGP hw on Macs has issues I believe. It was designed
and only tested around the one static early allocation, I'm not sure
OSX ever did dynamic.
When it went dynamic I think the AGP bits had some problems with
coherency of the GART tables that we never figured out.
Dave.
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