linux-kernel - Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev

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Message-ID: <20160427155019.GE20646@e104818-lin.cambridge.arm.com>
Date:	Wed, 27 Apr 2016 16:50:19 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Felipe Balbi <balbi@...nel.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	linux-usb@...r.kernel.org, Sekhar Nori <nsekhar@...com>,
	linux-kernel@...r.kernel.org,
	David Fisher <david.fisher1@...opsys.com>,
	"Thang Q. Nguyen" <tqnguyen@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent
 dev

On Wed, Apr 27, 2016 at 04:11:17PM +0200, Arnd Bergmann wrote:
> On Wednesday 27 April 2016 14:59:00 Catalin Marinas wrote:
> > 
> > I would be in favour of a dma_inherit() function as well. We could hack
> > something up in the arch code (like below) but I would rather prefer an
> > explicit dma_inherit() call by drivers creating such devices.
> > 
> > diff --git a/arch/arm64/include/asm/dma-mapping.h b/arch/arm64/include/asm/dma-mapping.h
> > index ba437f090a74..ea6fb9b0e8fa 100644
> > --- a/arch/arm64/include/asm/dma-mapping.h
> > +++ b/arch/arm64/include/asm/dma-mapping.h
> > @@ -29,8 +29,11 @@ extern struct dma_map_ops dummy_dma_ops;
> >  
> >  static inline struct dma_map_ops *__generic_dma_ops(struct device *dev)
> >  {
> > -       if (dev && dev->archdata.dma_ops)
> > -               return dev->archdata.dma_ops;
> > +       while (dev) {
> > +               if (dev->archdata.dma_ops)
> > +                       return dev->archdata.dma_ops;
> > +               dev = dev->parent;
> > +       }
> 
> I think this would be a very bad idea: we don't want to have random
> devices be able to perform DMA just because their parent devices
> have been set up that way.

I agree, it's a big hack. It would be nice to have a simpler way to do
this in driver code rather than explicitly calling
of_dma_configure/arch_setup_dma_ops as per the original patch in this
thread.

-- 
Catalin