linux-kernel - Re: [PATCH 1/2] dma-mapping: Add dma_addr_is_phys

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Message-ID: <20191015073104.GA32252@lst.de>
Date:   Tue, 15 Oct 2019 09:31:04 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     David Gibson <david@...son.dropbear.id.au>,
        Ram Pai <linuxram@...ibm.com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...abs.org,
        mdroth@...ux.vnet.ibm.com, aik@...ux.ibm.com, paul.burton@...s.com,
        b.zolnierkie@...sung.com, m.szyprowski@...sung.com, hch@....de,
        jasowang@...hat.com, andmike@...ibm.com, sukadev@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/2] dma-mapping: Add dma_addr_is_phys_addr()

On Mon, Oct 14, 2019 at 11:29:24AM +0100, Robin Murphy wrote:
>> However, I would like to see the commit message (and maybe the inline
>> comments) expanded a bit on what the distinction here is about.  Some
>> of the text from the next patch would be suitable, about DMA addresses
>> usually being in a different address space but not in the case of
>> bounce buffering.
>
> Right, this needs a much tighter definition. "DMA address happens to be a 
> valid physical address" is true of various IOMMU setups too, but I can't 
> believe it's meaningful in such cases.
>
> If what you actually want is "DMA is direct or SWIOTLB" - i.e. "DMA address 
> is physical address of DMA data (not necessarily the original buffer)" - 
> wouldn't dma_is_direct() suffice?

It would.  But drivers have absolutely no business knowing any of this.