linux-kernel - Re: [PATCH v2 08/11] swiotlb-xen: introduce phys_to_dma/dma_to

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Date:   Mon, 8 Jun 2020 22:32:40 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>, jgross@...e.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        tamas@...engyel.com, roman@...eda.com,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [PATCH v2 08/11] swiotlb-xen: introduce phys_to_dma/dma_to_phys
 translations

On Mon, Jun 08, 2020 at 04:06:57PM -0700, Stefano Stabellini wrote:
> I understand what you are suggesting about having something like:
> 
>     xen_phys_to_dma(...)
>     {
>         phys_addr_t phys = xen_phys_to_bus(dev, paddr)
>         return phys_to_dma(phys);
>     }
> 
> I thought about it myself. I'll do it.

"something", yes. Except that I think the bus is a little confusing,
isn't it?  What is the Xen term for these addresses?  Also we probably
don't need the extra local variable.

> But I don't think I understood the comment about XEN_PFN_PHYS.

There is a comment above xen_phys_to_bus that it avoids using
XEN_PFN_PHYS because XEN_PFN_PHYS of the phys_addr_t vs dma_addr_t
mismatch.  But XEN_PFN_PHYS could just use a u64 instead of the
phys_addr_t and then we could use it.   Especially as XEN_PFN_PHYS
isn't actually used anywhere except in swiotlb-xen.c.  Or we could
remove XEN_PFN_PHYS enirely, as it isn't all that useful to start
with.