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

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Message-ID: <CALjTZvZNNj7L6MWg=xdA31xbfwW_8gej5iUPXqz4Xg55EQUYSA@mail.gmail.com>
Date:   Mon, 10 Dec 2018 20:03:49 +0000
From:   Rui Salvaterra <rsalvaterra@...il.com>
To:     hch@....de
Cc:     benh@...nel.crashing.org, 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 19:33, Christoph Hellwig <hch@....de> wrote:
>
> On Mon, Dec 10, 2018 at 05:04:46PM +0000, Rui Salvaterra wrote:
> > Hi, Christoph and Ben,
> >
> > It just came to my mind (and this is most likely a stupid question,
> > but still)… Is there any possibility of these changes having an
> > (positive) effect on the long-standing problem of Power Mac machines
> > with AGP graphics cards (which have to be limited to PCI transfers,
> > otherwise they'll hang, due to coherence issues)? If so, I have a G4
> > machine where I'd gladly test them.
>
> These patches themselves are not going to affect that directly.
> But IFF the problem really is that the AGP needs to be treated as not
> cache coherent (I have no idea if that is true) the generic direct
> mapping code has full support for a per-device coherent flag, so
> support for a non-coherent AGP slot could be implemented relatively
> simply.

Thanks for the insight, Christoph. Well, the problem[1] is real, and
it's been known for a long time, though I can't be sure if it's *only*
a coherence issue. If someone who knows the hardware manages to cook
up a patch (as hacky is it may be), I'll definitely fire up old my G4
laptop to test it! :)

[1] https://bugs.freedesktop.org/show_bug.cgi?id=95017