linux-kernel - Re: remove the ->mapping_error method from dma_map

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Message-ID: <20181129183132.GA30281@lst.de>
Date:   Thu, 29 Nov 2018 19:31:32 +0100
From:   Christoph Hellwig <hch@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        linux-arch@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org, robin.murphy@....com,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        David Woodhouse <dwmw2@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: remove the ->mapping_error method from dma_map_ops V2

On Thu, Nov 29, 2018 at 09:44:05AM -0800, Linus Torvalds wrote:
> 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.

Sure, we could just skip that page entirely based on dma_to_phys.

> 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.

Ok, I'll do that.

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

What would be the replacement?  Build a S/G list for every single page
mapping?  Not sure that would create a lot of happy campers..