linux-kernel - Re: [PATCH v7 16/16] ARM: LPAE: Add the Kconfig entries

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Date:	Wed, 2 Nov 2011 17:21:42 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v7 16/16] ARM: LPAE: Add the Kconfig entries

On Wed, Aug 10, 2011 at 04:03:39PM +0100, Catalin Marinas wrote:
> +config ARCH_DMA_ADDR_T_64BIT
> +	def_bool ARM_LPAE
> +

I think this should be selected only when we have a DMA engine supporting
64-bit addresses.  Technically LPAE itself doesn't give us that assurance.

If you have this kind of a setup:

CPU <==++==> RAM
       ||
     IOMMU
       |
   DMA device

where == and || means >32-bit addressing, and | means 32-bit addressing.

In such a setup, having dma_addr_t be 64-bit is pointless because it
should never see 64-bit addresses (the DMA API should deal with the
IOMMU and provide a list of DMA addresses to be placed into RAM for
the DMA device which takes account of the mappings setup in the IOMMU.)

So, I think 64-bit dma_addr_t should be a property of the DMA devices
present in the system rather than whether the CPUs MMU can deal with
>32-bit addresses or not.
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