linux-kernel - Re: [PATCH v7 3/8] arm64: introduce is_device_dma

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Message-ID: <alpine.DEB.2.02.1411031101400.22875@kaball.uk.xensource.com>
Date:	Mon, 3 Nov 2014 11:10:18 +0000
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>,
	Catalin Marinas <Catalin.Marinas@....com>
Subject: Re: [PATCH v7 3/8] arm64: introduce is_device_dma_coherent

On Mon, 3 Nov 2014, Will Deacon wrote:
> On Mon, Nov 03, 2014 at 10:46:03AM +0000, Stefano Stabellini wrote:
> > On Mon, 27 Oct 2014, Stefano Stabellini wrote:
> > > Introduce a boolean flag and an accessor function to check whether a
> > > device is dma_coherent. Set the flag from set_arch_dma_coherent_ops.
> > > 
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > > Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> > > CC: will.deacon@....com
> > 
> > Will, Catalin,
> > are you OK with this patch?
> 
> It would be nicer if the dma_coherent flag didn't have to be duplicated by
> each architecture in dev_archdata. Is there any reason not to put it in the
> core code?

Yes, there is a reason for it: if I added a boolean dma_coherent flag in
struct device as Catalin initially suggested, what would be the default
for each architecture? Where would I set it for arch that don't use
device tree? It is not easy.

I thought it would be better to introduce is_device_dma_coherent only on
the architectures where it certainly makes sense to have it. In fact I
checked and arm and arm64 are the only architectures to define
set_arch_dma_coherent_ops at the moment. At that point if
is_device_dma_coherent becomes arch-specific, it makes sense to store
the flag in dev_archdata instead of struct device.
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