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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 29 Nov 2018 09:44:05 -0800
Message-ID: <CAHk-=wi2K7_+dgCbc-sAK9CST5Zh+pOuH7sVT9WW-kqs50HSzQ@mail.gmail.com>
Subject: Re: remove the ->mapping_error method from dma_map_ops V2
To:     Christoph Hellwig <hch@lst.de>
Cc:     Russell King - ARM Linux <linux@armlinux.org.uk>,
        linux-arch@vger.kernel.org, linux-ia64@vger.kernel.org,
        linux-parisc@vger.kernel.org, robin.murphy@arm.com,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        iommu@lists.linux-foundation.org, linux-alpha@vger.kernel.org,
        xen-devel@lists.xenproject.org,
        David Woodhouse <dwmw2@infradead.org>,
        linux-arm-kernel@lists.infradead.org
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On Thu, Nov 29, 2018 at 8:23 AM Christoph Hellwig <hch@lst.de> wrote:
>
> We can.  At least in theory.  The problem is that depending on the
> crazy mapping from physical and kernel virtual address to dma addresses
> these might be pages at pretty random places.  Look at fun like
> arch/x86/pci/sta2x11-fixup.c for how ugly these mappings could look.
>
> It also means that we might have setup swiotlb on just about every
> 32-bit architecture, even if it has no real addressing limit except for
> the one we imposed.

No. Really. If there's no iotlb, then you just mark that one page
reserved. It simply doesn't get used. It doesn't mean you suddenly
need a swiotlb.

If there *is* a iotlb, none of this should matter, because you'd just
never map anything into that page.

But whatever. It's independent from the patch series under discussion.
Make dma_mapping_error() at least return a real error (eg -EINVAL, or
whatever is the common error), and we can maybe do this later.

Or, better yet, plan on removing the single-page dma mappign entirely
at a later date, and make the issue moot.

              Linus