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

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Message-ID: <9452408.uEOOA4q1YW@wuerfel>
Date:	Wed, 27 Apr 2016 16:11:17 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Catalin Marinas <catalin.marinas@....com>
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 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.

	Arnd