linux-kernel - Re: [PATCH 16/67] powerpc: rename dma_direct_ to dma_nommu

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Message-ID: <87h8s3cvel.fsf@concordia.ellerman.id.au>
Date:   Wed, 03 Jan 2018 17:24:02 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
        linux-ia64@...r.kernel.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
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        linux-s390 <linux-s390@...r.kernel.org>,
        linux-c6x-dev@...ux-c6x.org,
        "open list\:QUALCOMM HEXAGON..." <linux-hexagon@...r.kernel.org>,
        "the arch\/x86 maintainers" <x86@...nel.org>,
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        adi-buildroot-devel@...ts.sourceforge.net,
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Subject: Re: [PATCH 16/67] powerpc: rename dma_direct_ to dma_nommu_

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> On Tue, Jan 2, 2018 at 10:45 AM, Michael Ellerman <mpe@...erman.id.au> wrote:
>> Christoph Hellwig <hch@....de> writes:
>>
>>> We want to use the dma_direct_ namespace for a generic implementation,
>>> so rename powerpc to the second best choice: dma_nommu_.
>>
>> I'm not a fan of "nommu". Some of the users of direct ops *are* using an
>> IOMMU, they're just setting up a 1:1 mapping once at init time, rather
>> than mapping dynamically.
>>
>> Though I don't have a good idea for a better name, maybe "1to1",
>> "linear", "premapped" ?
>
> "identity"?

I think that would be wrong, but thanks for trying to help :)

The address on the device side is sometimes (often?) offset from the CPU
address. So eg. the device can DMA to RAM address 0x0 using address
0x800000000000000.

Identity would imply 0 == 0 etc.

I think "bijective" is the correct term, but that's probably a bit
esoteric.

cheers