linux-kernel - Re: [PATCH v3 5/7] ARM: dma: Use dma_pfn

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Date:	Fri, 2 May 2014 15:58:52 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org,
	Grygorii Strashko <grygorii.strashko@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v3 5/7] ARM: dma: Use dma_pfn_offset for dma address
	translation

On Thu, Apr 24, 2014 at 11:30:05AM -0400, Santosh Shilimkar wrote:
> diff --git a/arch/arm/include/asm/dma-mapping.h b/arch/arm/include/asm/dma-mapping.h
> index e701a4d..424fda9 100644
> --- a/arch/arm/include/asm/dma-mapping.h
> +++ b/arch/arm/include/asm/dma-mapping.h
> @@ -58,22 +58,35 @@ static inline int dma_set_mask(struct device *dev, u64 mask)
>  #ifndef __arch_pfn_to_dma
>  static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long pfn)
>  {
> -	return (dma_addr_t)__pfn_to_bus(pfn);
> +	if (!dev)
> +		return (dma_addr_t)__pfn_to_bus(pfn);
> +	else
> +		return (dma_addr_t)__pfn_to_bus(pfn - dev->dma_pfn_offset);

I really don't trust gcc to do this right, so I think it would be better
to make life easier on the compiler:

	if (dev)
		pfn -= dev->dma_pfn_offset;
	return (dma_addr_t)__pfn_to_bus(pfn);

>  static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
>  {
> -	return __bus_to_pfn(addr);
> +	if (!dev)
> +		return __bus_to_pfn(addr);
> +	else
> +		return __bus_to_pfn(addr) + dev->dma_pfn_offset;

and:

	unsigned long pfn = __bus_to_pfn(addr);

	if (dev)
		pfn += dev->dma_pfn_offset;

	return pfn;

>  }
>  
>  static inline void *dma_to_virt(struct device *dev, dma_addr_t addr)
>  {
> -	return (void *)__bus_to_virt((unsigned long)addr);
> +	if (!dev)
> +		return (void *)__bus_to_virt((unsigned long)addr);
> +	else
> +		return (void *)__bus_to_virt(__pfn_to_bus(dma_to_pfn(dev, addr)));

This is quite horrendous.  There's easier ways to do this...  I assume
you haven't looked at the assembler resulting from this at all with
stuff like the p2v patching enabled?

>  }
>  
>  static inline dma_addr_t virt_to_dma(struct device *dev, void *addr)
>  {
> -	return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
> +	if (!dev)
> +		return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
> +	else
> +		return pfn_to_dma(dev,
> +				__bus_to_pfn(__virt_to_bus((unsigned long)(addr))));

Same here.

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