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

[Alex Deucher <alexdeucher@gmail.com>]

On Mon, May 11, 2020 at 4:25 PM Rui Salvaterra <rsalvaterra@gmail.com> wrote:
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> A segunda, 11/05/2020, 21:21, Alex Deucher <alexdeucher@gmail.com> escreveu:
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>> 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.
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> 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.

Alex
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