linux-kernel - Re: [PATCH v4 3/7] [RFC] arm/arm64: introduce is_dma

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Message-ID: <alpine.DEB.2.02.1410131358490.17038@kaball.uk.xensource.com>
Date:	Mon, 13 Oct 2014 15:43:18 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Will Deacon <will.deacon@....com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Catalin Marinas <Catalin.Marinas@....com>
Subject: Re: [PATCH v4 3/7] [RFC] arm/arm64: introduce is_dma_coherent

On Mon, 13 Oct 2014, Will Deacon wrote:
> On Mon, Oct 13, 2014 at 12:16:14PM +0100, Stefano Stabellini wrote:
> > On Fri, 10 Oct 2014, Stefano Stabellini wrote:
> > > On Fri, 10 Oct 2014, Will Deacon wrote:
> > > > On Fri, Oct 10, 2014 at 12:51:44PM +0100, Stefano Stabellini wrote:
> > > > > Introduce a function to check whether a device is dma coherent.
> > > > > 
> > > > > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > > > > CC: linux@....linux.org.uk
> > > > > CC: catalin.marinas@....com
> > > > > CC: will.deacon@....com
> > > > > CC: linux-arm-kernel@...ts.infradead.org
> > > > > ---
> > > > >  arch/arm/include/asm/dma-mapping.h   |    6 ++++++
> > > > >  arch/arm64/include/asm/dma-mapping.h |    5 +++++
> > > > >  2 files changed, 11 insertions(+)
> > > > > 
> > > > > diff --git a/arch/arm/include/asm/dma-mapping.h b/arch/arm/include/asm/dma-mapping.h
> > > > > index c45b61a..bededbb 100644
> > > > > --- a/arch/arm/include/asm/dma-mapping.h
> > > > > +++ b/arch/arm/include/asm/dma-mapping.h
> > > > > @@ -126,6 +126,12 @@ static inline int set_arch_dma_coherent_ops(struct device *dev)
> > > > >  	set_dma_ops(dev, &arm_coherent_dma_ops);
> > > > >  	return 0;
> > > > >  }
> > > > > +
> > > > > +static inline bool is_dma_coherent(struct device *dev)
> > > > > +{
> > > > > +    return (__generic_dma_ops(dev) == &arm_coherent_dma_ops);
> > > > > +}
> > > > 
> > > > Hmm, what about the IOMMU ops?
> > > 
> > > Maybe I should check __generic_dma_ops(dev) != &arm_dma_ops?
> > > Do you have any better suggestions?
> > 
> > ping?
> 
> Without any clear idea about why this is needed or how it's used, I don't
> have any better ideas.

Thanks for the quick reply.

It is used in dom0 to figure out whether the device is not coherent: in
that case Dom0 is going to issue an hypercall and Xen is going to
execute the required cache maintenance operations on Dom0's behalf.

In practice for this use case iommu_dma_ops is not a problem, as the
device is never going to appear as being behind an SMMU in dom0.
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