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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 29 Nov 2018 10:53:32 -0800
Message-ID: <CAHk-=wjjmtZAAGvB8c6hyKvABPHNTfE+b+eHu7iktdogDfTgMw@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 10:31 AM Christoph Hellwig <hch@lst.de> wrote:
>
> > 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..

It's what we ended up doing with some other cases, and it didn't
really end up hurting as much as I thought it would.

I'm thinking of the vfs functions that end up turning "buf, len" into

        struct iovec iov = { .iov_base = (void __user *)buf, .iov_len = len };

and then passing it around as a single-entry iov instead (not even
that - they end up being an iov_iter, which is not just the iov, but
the whole "what _kind_ of iov" indirection)

Maybe a very similar model could be used for just simplifying the core
dma mapping setup: sure, people will want to do single-area dma, but
how bad would it be to just turn them into single-entry SG lists on
stack, and then the dma-maping internally would just always see that?

Most of the high-performance IO is already using SG lists anyway, no?
Disk/networking/whatever.

But just an idea. And the "map_sg()" error handling isn't actually any
better, I think. It returns zero on error, no? So it's not improving
the error handling.

The whole dma-mapping layer seems full of those kinds of "inspired
error handling choices" ;)

                  Linus