linux-kernel - Re: [PATCH 4/5] dma-mapping: provide a better default ->get_required

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Message-ID: <CAMuHMdUBPj8AVSuDwaBB_4gRD6k7vzo0WAFJEkTUbxSw31bzUg@mail.gmail.com>
Date:   Mon, 29 Jul 2019 11:57:19 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Takashi Iwai <tiwai@...e.de>,
        Robin Murphy <robin.murphy@....com>,
        Michal Simek <monstr@...str.eu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        linux-xtensa@...ux-xtensa.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] dma-mapping: provide a better default ->get_required_mask

Hi Christoph,

On Thu, Jul 25, 2019 at 8:35 AM Christoph Hellwig <hch@....de> wrote:
> Most dma_map_ops instances are IOMMUs that work perfectly fine in 32-bits
> of IOVA space, and the generic direct mapping code already provides its
> own routines that is intelligent based on the amount of memory actually
> present.  Wire up the dma-direct routine for the ARM direct mapping code
> as well, and otherwise default to the constant 32-bit mask.  This way
> we only need to override it for the occasional odd IOMMU that requires
> 64-bit IOVA support, or IOMMU drivers that are more efficient if they
> can fall back to the direct mapping.

As I know you like diving into cans of worms ;-)

Does 64-bit IOVA support actually work in general? Or only on 64-bit
platforms, due to dma_addr_t to unsigned long truncation on 32-bit?

https://lore.kernel.org/linux-renesas-soc/CAMuHMdWkQ918Y61tMJbHEu29AGLEyNwbvZbSBB-RRH7YYUNRcA@mail.gmail.com/

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds