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

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Date:   Mon, 10 Dec 2018 17:04:46 +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 Sat, 8 Dec 2018 at 17:17, Christoph Hellwig <hch@....de> wrote:
>
> On Sun, Dec 02, 2018 at 05:11:02PM +1100, Benjamin Herrenschmidt wrote:
> > Talking of which ... Christoph, not sure if we can do something about
> > this at the DMA API level or keep hacks but some adapters such as the
> > nVidia GPUs have a HW hack we can use to work around their limitations
> > in that case.
> >
> > They have a register that can program a fixed value for the top bits
> > that they don't support.
> >
> > This works fine for any linear mapping with an offset, provided they
> > can program the offset in that register and they have enough DMA range
> > to cover all memory from that offset.
> >
> > I can probably get the info about this from them so we can exploit it
> > in nouveau.
>
> I think we can expose the direct mapping offset if people care enough,
> we just have to be very careful designing the API.  I'll happily leave
> that to those that actually want to use it, but I'll gladly help
> reviewing it.

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.

Thanks,

Rui