Re: use generic DMA mapping code in powerpc V4

[Rui Salvaterra]

On Mon, 10 Dec 2018 at 20:49, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> 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 ofâ 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. :/