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

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Message-ID: <20181119135459.GB16334@lst.de>
Date:   Mon, 19 Nov 2018 14:55:00 +0100
From:   Christoph Hellwig <hch@....de>
To:     David Miller <davem@...emloft.net>
Cc:     hch@....de, iommu@...ts.linux-foundation.org,
        torvalds@...ux-foundation.org, linux-alpha@...r.kernel.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Julia Lawall <Julia.Lawall@...6.fr>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [RFC] remove the ->mapping_error method from dma_map_ops

On Fri, Nov 09, 2018 at 03:12:34PM -0800, David Miller wrote:
> But patch #2 on the other hand, not so much.
> 
> I hate seeing values returned by reference, it adds cost especially
> on cpus where all argments and return values fit in registers (we end
> up forcing a stack slot and memory references).
> 
> And we don't need it here.
> 
> DMA addresses are like pointers, and therefore we can return errors and
> valid success values in the same dma_addr_t just fine.  PTR_ERR() --> DMA_ERR(),
> IS_PTR_ERR() --> IS_DMA_ERR, etc.

In the end this is an inline function, so with a decently smart
compiler the generated code shouldn't change too much.  The big problem
that prompted me to come up with this patch is that not handling failure
from dma_map* in a swiotlb setup can lead to grave data corruption, and
we have no easy way to force error checking on these return values.

I've added a few of the static typechecking suspect if they have a
better idea on how to make the return value of dma_map_single/pages
in a way that we get warnings if dma_mapping_error isn't called on them.
But I can't really think of a good way.