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

["Michel Dänzer" <michel@daenzer.net>]

On 2020-05-11 10:51 p.m., Alex Deucher wrote:
> On Mon, May 11, 2020 at 4:25 PM Rui Salvaterra <rsalvaterra@gmail.com> wrote:
>> A segunda, 11/05/2020, 21:21, Alex Deucher <alexdeucher@gmail.com> escreveu:
>>>
>>> Note there is no loss of functionality here, at least on radeon
>>> hardware.  It just comes down to which MMU gets used for access to
>>> system memory, the AGP MMU on the chipset or the MMU built into the
>>> GPU.  On powerpc hardware, AGP has been particularly unstable, and
>>> IIRC, AGP has been disabled by default on radeon on powerpc for a
>>> while.
>>
>> So this basically just drops support for the AGP GART? What happens to the AGP signalling rates (beyond the base rate)?
> 
> I don't remember enough of the details, but I strongly doubt it was
> related to which MMU was used per se.  On r1xx/r2xx parts, AGP was
> effectively the non-snooped route to memory and the internal MMU only
> provided snooped (coherent) access to memory.  That and the limited
> TLB space are probably want limited performance in that case.  I don't
> recall what sort of TLBs the chipset GART tables provided.  On r3xx
> and newer the, on-chip MMU supported both snooped and unsnooped
> transactions and had more TLB space so the difference wasn't
> significant IIRC.

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).


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer
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