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

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Date:   Tue, 30 Jul 2019 08:26:51 +0200
From:   Christoph Hellwig <hch@....de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Christoph Hellwig <hch@....de>,
        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

On Mon, Jul 29, 2019 at 11:57:19AM +0200, Geert Uytterhoeven wrote:
> 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?

Most IOMMUs use 32-bit IOVAs, and thus we default to the 32-bit mask
because it is common and failsafe vs the normal linux assumptions.
However the ia64 SGI SN2 platform, and the powerpc IBM ebus
implementations seem to require a 64-bit mask already, so we keep that
behavior as is.