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

[Alex Deucher]

On Tue, May 12, 2020 at 1:02 PM Rui Salvaterra <rsalvaterra at gmail.com> wrote:
>
> On Tue, 12 May 2020 at 17:38, Daniel Vetter <daniel at 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.

Alex