linux-kernel - Re: use generic DMA mapping code in powerpc V4

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Message-ID: <CALjTZvZ1Ud4P_XqB23Yq=815VyF=UbbT8YCFjTaPN=5tORWrtQ@mail.gmail.com>
Date:   Tue, 11 Dec 2018 09:37:02 +0000
From:   Rui Salvaterra <rsalvaterra@...il.com>
To:     benh@...nel.crashing.org
Cc:     hch@....de, linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: use generic DMA mapping code in powerpc V4

On Mon, 10 Dec 2018 at 20:49, Benjamin Herrenschmidt
<benh@...nel.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… 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. :/