linux-kernel - Re: [PATCH v3 4/7] of: configure the platform device dma parameters

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Date:	Thu, 29 May 2014 16:04:49 -0400
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	Grant Likely <grant.likely@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Rob Herring <robh+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v3 4/7] of: configure the platform device dma parameters

On Thursday 29 May 2014 03:24 PM, Arnd Bergmann wrote:
> On Thursday 29 May 2014 10:08:10 Santosh Shilimkar wrote:
>> On Thursday 29 May 2014 10:01 AM, Linus Walleij wrote:
>>> On Wed, May 28, 2014 at 4:04 PM, Santosh Shilimkar
>>> <santosh.shilimkar@...com> wrote:
>>>> On Wednesday 28 May 2014 09:32 AM, Linus Walleij wrote:
>>>
>>>>>> I suspect what you want is
>>>>>>
>>>>>>         dma-ranges = <0x80000000 0 0x80000000>;
>>>>>>
>>>>>> to translate dma_addr_t 0x80000000-0xffffffff to phys_addr_t 0x0-0x7fffffff
>>>>>> rather than phys_addr_t 0x800000000-0x87fffffff.
>>>>>
>>>> Interesting. Where does the ROM address space resides on integrator then considering
>>>> address 0 is used for DMA.
>>>
>>> The ROM is at physical address 0x20000000, don't ask me
>>> why 
>>>
>>> The RAM is typically at 0x00000000-0x0fffffff, on up to four parallell
>>> tiles, i.e. up to four completely independent CPUs are booted off the
>>> same ROM and using a set of shared peripherals.
>>>
>> The reason I asked the question because most of the ARM SOC I came across
>> aren't using the RAM phys address 0 and thought was because of boot architecture
>> with ROM occupying that address with reset vector starting at address 0.
>>
>> That was one of the main reason we had description on max_*pfn on ARM w.r.t
>> to other acrhes.
>>
>> Will corner ARM guys to understand bit more about it in some conference 
> 
> If this is anything like the versatile express, the reason it works is probably
> because there is another microcontroller in the system that does the bootstrap
> and is able to load code into RAM before turning on the main CPU.
> 
That make sense now. Thanks for those extra bits.

Regards,
Santosh

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