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

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Date:   Thu, 22 Nov 2018 17:07:15 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, linux-arch@...r.kernel.org,
        linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>, joro@...tes.org,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, jdmason@...zu.us,
        xen-devel@...ts.xenproject.org, robin.murphy@....com,
        linux-arm-kernel@...ts.infradead.org, m.szyprowski@...sung.com
Subject: Re: remove the ->mapping_error method from dma_map_ops V2

On Thu, Nov 22, 2018 at 08:50:47AM -0800, Linus Torvalds wrote:
> On Thu, Nov 22, 2018 at 6:03 AM Christoph Hellwig <hch@....de> wrote:
> >
> > The 0 return doesn't work for direct mappings that have ram at address
> > zero and a lot of IOMMUs that start allocating bus space from address
> > zero, so we can't consolidate on that, but I think we can move everyone
> > to all-Fs, which the patch here does.
> 
> Hmm. Maybe not limit it to just all ones, but actually use the
> (standard, for the kernel) IS_ERR_VALUE()?
> 
> That basically reserves the last 4095 values in an unsigned long for
> error values.
> 
> Then those functions could actually return *what* error they
> encountered, using just plain
> 
>         return -ENOMEM;
> 
> or whatever?

Linus,

I'm afraid that won't work very well - 32 bit platforms with 64-bit
addresses (LPAE) would have dma_addr_t as a 64-bit value, which
wouldn't fit into an unsigned long.

IS_ERR_VALUE() would cast a 64-bit DMA address down to a 32-bit
pointer (effectively masking with 0xffffffff).  It would have the
effect of making (eg) 0xXXXXXXXX_fffffVVV an error, where XXXXXXXX
are any of the top 32-bits of a 64-bit bus address, and VVV is the
error code value.

That could be a problem if you hit it in several places throughout
your available RAM... we'd have to mark every top page of RAM in
a naturally aligned 4GB as unusable, as well as block the top page
in natually aligned 4GB blocks from IOMMUs... and then what about
buses that have weird offsets...

So, I don't think the IS_ERR_VALUE() would work very well.

-- 
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