linux-kernel - Re: [PATCH v2 5/7] ARM: dma: Use dma_pfn

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Date:	Mon, 21 Apr 2014 09:38:01 -0400
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 v2 5/7] ARM: dma: Use dma_pfn_offset for dma address translation

On Saturday 19 April 2014 03:43 PM, Russell King - ARM Linux wrote:
> On Sat, Apr 19, 2014 at 10:32:50AM -0400, Santosh Shilimkar wrote:
>> From: Grygorii Strashko <grygorii.strashko@...com>
>>
>> In most of cases DMA addresses can be performed using offset value of
>>  Bus address space relatively to physical address space as following:
>>
>> PFN->DMA:
>>  __pfn_to_phys(pfn + [-]dma_pfn_offset)
>>
>> DMA->PFN:
>>  __phys_to_pfn(dma_addr) + [-]dma_pfn_offset
>>
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Cc: Russell King <linux@....linux.org.uk>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: Olof Johansson <olof@...om.net>
>> Cc: Grant Likely <grant.likely@...aro.org>
>> Cc: Rob Herring <robh+dt@...nel.org>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Linus Walleij <linus.walleij@...aro.org>
>> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
>> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@...com>
>> ---
>>  arch/arm/include/asm/dma-mapping.h |   17 +++++++++++++----
>>  1 file changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/dma-mapping.h b/arch/arm/include/asm/dma-mapping.h
>> index e701a4d..8c12149 100644
>> --- a/arch/arm/include/asm/dma-mapping.h
>> +++ b/arch/arm/include/asm/dma-mapping.h
>> @@ -58,22 +58,31 @@ 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_ERROR_CODE;
>> +	return (dma_addr_t)__pfn_to_bus(pfn - dev->dma_pfn_offset);
> 
> How do ISA devices (iow, those which pass a NULL device) work with this?
> This looks to me like it ends up breaking some drivers.
> 
> I've also seen some drivers (such as the Freescale FEC driver) which
> perform DMA coherent allocations with a NULL device - technically, that's
> a bug in the driver, but the above change will cause them to regress.
> 
Good point. We can keep the NULL case working as well...

static inline dma_addr_t pfn_to_dma(struct device *dev, unsigned long 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 will update the patch accordingly.

Regards,
Santosh
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