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Subject: Re: use generic DMA mapping code in powerpc V4
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     Rui Salvaterra <rsalvaterra@gmail.com>, hch@lst.de
Cc:     linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
Date:   Sun, 02 Dec 2018 17:11:02 +1100
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On Fri, 2018-11-30 at 11:44 +0000, Rui Salvaterra wrote:
> Thanks for the quick response! I applied it on top of your
> powerpc-dma.4 branch and retested.
> I'm not seeing nouveau complaining anymore (I'm not using X11 or any
> DE, though).
> In any case and FWIW, this series is
> 
> Tested-by: Rui Salvaterra <rsalvaterra@gmail.com>

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.

Cheers,
Ben.

> Thanks,
> Rui