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

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Date:   Mon, 8 Jun 2020 16:06:57 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
cc:     Stefano Stabellini <sstabellini@...nel.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, 8 Jun 2020, Christoph Hellwig wrote:
> On Wed, Jun 03, 2020 at 03:22:44PM -0700, Stefano Stabellini wrote:
> > From: Stefano Stabellini <stefano.stabellini@...inx.com>
> > 
> > With some devices physical addresses are different than dma addresses.
> > To be able to deal with these cases, we need to call phys_to_dma on
> > physical addresses (including machine addresses in Xen terminology)
> > before returning them from xen_swiotlb_alloc_coherent and
> > xen_swiotlb_map_page.
> > 
> > We also need to convert dma addresses back to physical addresses using
> > dma_to_phys in xen_swiotlb_free_coherent and xen_swiotlb_unmap_page if
> > we want to do any operations on them.
> > 
> > Call dma_to_phys in is_xen_swiotlb_buffer.
> > Call phys_to_dma in xen_phys_to_bus.
> > Call dma_to_phys in xen_bus_to_phys.
> > 
> > Everything is taken care of by these changes except for
> > xen_swiotlb_alloc_coherent and xen_swiotlb_free_coherent, which need a
> > few explicit phys_to_dma/dma_to_phys calls.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
> > Tested-by: Corey Minyard <cminyard@...sta.com>
> > Tested-by: Roman Shaposhnik <roman@...eda.com>
> > ---
> > Changes in v2:
> > - improve commit message
> > ---
> >  drivers/xen/swiotlb-xen.c | 22 ++++++++++++----------
> >  1 file changed, 12 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index 0a6cb67f0fc4..60ef07440905 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -64,16 +64,16 @@ static inline dma_addr_t xen_phys_to_bus(struct device *dev, phys_addr_t paddr)
> >  
> >  	dma |= paddr & ~XEN_PAGE_MASK;
> >  
> > -	return dma;
> > +	return phys_to_dma(dev, dma);
> 
> So looking at this function:
> 
> The dma name for something passed to phys_to_dma is really
> weird.

Yeah, that is true, I am not sure why I chose that confusing name. I'll
rename it.


> The fact that the comments says don't use XEN_PFN_PHYS
> beause of the type mismatch while nothing but swiotlb-xen is the only
> user of XEN_PFN_PHYS is also weird.  I think XEN_PFN_PHYS needs to move
> to swiotlb-xen first, then use a hardcoded u64 for the size, and the
> split the function into a phys_to_xen_phys (or so) function where
> the result gets passed to phys_to_dma.

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.

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


> Similar for the reverse direction.

OK