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From:   Rui Salvaterra <rsalvaterra@gmail.com>
Date:   Tue, 11 Dec 2018 09:37:02 +0000
Message-ID: <CALjTZvZ1Ud4P_XqB23Yq=815VyF=UbbT8YCFjTaPN=5tORWrtQ@mail.gmail.com>
Subject: Re: use generic DMA mapping code in powerpc V4
To:     benh@kernel.crashing.org
Cc:     hch@lst.de, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
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On Mon, 10 Dec 2018 at 20:49, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
>

[snip]

>
> AGP is a gigantic nightmare :-) It's not just cache coherency issues
> (some implementations are coherent, some aren't, Apple's is ... weird).
>
> Apple has all sort of bugs, and Darwin source code only sheds light on
> some of them. Some implementation can only read, not write I think, for
> example. There are issues with transfers crossing some boundaries I
> beleive, but it's all unclear.
>
> Apple makes this work with a combination of hacks in the AGP "driver"
> and the closed source GPU driver, which we don't see.
>
> I have given up trying to make that stuff work reliably a decade ago :)
>
> Cheers,
> Ben.

That's what I was afraid of=E2=80=A6 what a mess. At least now I have a
definitive answer from one of the original authors of the code, thanks
a lot, Ben. :)
I have an unresearched belief that AGP support was hacked in the Mac
series as an afterthought (weren't they supposed to be PCI/PCI-X
only?), and your explanation surely seems to corroborate. :/