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

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Message-ID: <CAHk-=wg5+rXK8dpjkSmK7tuz29VfxVWNEwmTVYB409mmyLkeew@mail.gmail.com>
Date:   Wed, 28 Nov 2018 08:47:05 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-arch@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>, linux@...linux.org.uk,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        xen-devel@...ts.xenproject.org, robin.murphy@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: remove the ->mapping_error method from dma_map_ops V2

On Tue, Nov 27, 2018 at 11:41 PM Christoph Hellwig <hch@....de> wrote:
>
> On Fri, Nov 23, 2018 at 07:55:11AM +0100, Christoph Hellwig wrote:
> > Well, I can tweak the last patch to return -EINVAL from dma_mapping_error
> > instead of the old 1 is as bool true.  The callers should all be fine,
> > although I'd have to audit them.  Still wouldn't help with being able to
> > return different errors.
>
> Any opinions?  I'd really like to make some forward progress on this
> series.

So I do think that yes, dma_mapping_error() should return an error
code, not 0/1.

But I was really hoping that the individual drivers themselves could
return error codes. Right now the patch-series has code like this:

      ret = needs_bounce(dev, dma_addr, size);
      if (ret < 0)
-         return ARM_MAPPING_ERROR;
+         return DMA_MAPPING_ERROR;

which while it all makes sense in the context of this patch-series, I
*really* think it would have been so much nicer to return the error
code 'ret' instead (which in this case is -E2BIG).

I don't think this is a huge deal, but ERR_PTR() has been hugely
successful elsewhere. And I'm not hugely convinced about all these
"any address can be valid" arguments. How the hell do you generate a
random dma address in the last page that isn't even page-aligned?

                     Linus