linux-kernel - Re: [PATCH v2 0/7] of: setup dma parameters using dma-ranges and dma-coherent

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Date:	Mon, 21 Apr 2014 11:36:04 -0400
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Rob Herring <robherring2@...il.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Russell King <linux@....linux.org.uk>,
	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>,
	Grygorii Strashko <grygorii.strashko@...com>
Subject: Re: [PATCH v2 0/7] of: setup dma parameters using dma-ranges and
 dma-coherent

On Monday 21 April 2014 11:35 AM, Rob Herring wrote:
> On Mon, Apr 21, 2014 at 10:13 AM, Santosh Shilimkar
> <santosh.shilimkar@...com> wrote:
>> Hi Rob,
>>
>> On Monday 21 April 2014 10:37 AM, Rob Herring wrote:
>>> On Sat, Apr 19, 2014 at 9:32 AM, Santosh Shilimkar
>>> <santosh.shilimkar@...com> wrote:
>>>> Here is an updated version of [2] based on discussion. Series introduces
>>>> support for setting up dma parameters based on device tree properties
>>>> like 'dma-ranges' and 'dma-coherent' and also update to ARM 32 bit port.
>>>> Earlier version of the same series is here [1].
>>>>
>>>> The 'dma-ranges' helps to take care of few DMAable system memory restrictions
>>>> by use of dma_pfn_offset which we maintain now per device. Arch code then
>>>> uses it for dma address translations for such cases. We update the
>>>> dma_pfn_offset accordingly during DT the device creation process.The
>>>> 'dma-coherent' property is used to setup arch's coherent dma_ops.
>>>>
>>>> After some off-list discussion with RMK and Arnd, I have now dropped the
>>>> controversial dma_mask setup code from the series which actually isn't blocking
>>>> me as such. Considering rest of the parts of the series are already aligned,
>>>> am hoping to get this version merged for 3.16 merge window.
>>>
>>> Can you briefly describe what the h/w looks like in terms of addresses
>>> for the problem you are trying to solve? Something like: Cpu view of
>>> RAM is X to Y address, X corresponds to DMA address Z. Max DMA address
>>> is ?
>>>
>> Let me try with say 8 GB RAM example
>>
>> CPU view of memory : 0x0000 0008 0000 0000 to 0x0000 000a 0000 0000
>>
>> From above memory range, first 2 GB of memory has an alias 32 bit
>> view in the hardware. Hardware internally map the address issued within
>> that first 2 GB to same memory.
>>
>> DMA view of first 2 GB [ 0x0000 0008 0000 0000 to 0x0000 0008 7fff ffff]
>> is : 0x8000 0000 to 0xffff fffff.
> 
> Are you setting ZONE_DMA to be 2GB so allocations stay within DMAable
> memory and that is enough that you don't need to set DMA masks?
> 
Yes. Thats right.

Regards,
Santosh

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