From mboxrd@z Thu Jan  1 00:00:00 1970
From: Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>
Subject: Re: [PATCH] of: Set the DMA mask to 64 bits on ARM LPAE systems
Date: Thu, 25 Apr 2013 18:58:19 +0100
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To: Laura Abbott <lauraa-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
Cc: Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, "devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org" <devicetree-discuss-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org>, "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" <linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org>
List-Id: devicetree@vger.kernel.org

On Thu, Apr 25, 2013 at 10:09:57AM -0700, Laura Abbott wrote:
> I thought about this as well but in arch/arm/mm/mm.h:
>
> #ifdef CONFIG_ZONE_DMA
> extern phys_addr_t arm_dma_limit;
> #else
> #define arm_dma_limit ((phys_addr_t)~0)
> #endif
>
> arm_dma_limit is explicitly cast to phys_addr_t, which means that  
> arm_dma_limit will be always be sizeof(phys_addr_t) regardless of  
> sizeof(dma_addr_t).

This is the size of the DMA zone, which controls the _minimum_ DMA mask
that can be used in the system.  DMA masks must always be greater than
this limit.