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

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Date:	Wed, 27 Apr 2016 18:04:27 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Catalin Marinas <catalin.marinas@....com>,
	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>
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev

On Wednesday 27 April 2016 16:50:19 Catalin Marinas wrote:
> 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.
> 


I haven't followed the entire discussion, but what's wrong with passing
around a pointer to a 'struct device *hwdev' that represents the physical
device that does the DMA?

	Arnd