[PATCHv3 5/6] ARM: imx: Add initial support for Freescale LS1021A

Jingchang Lu jingchang.lu at freescale.com
Thu Sep 11 20:17:44 PDT 2014

>-----Original Message-----
>From: Arnd Bergmann [mailto:arnd at arndb.de]
>Sent: Thursday, September 11, 2014 6:45 PM
>To: Lu Jingchang-B35083
>Cc: linux-arm-kernel at lists.infradead.org; Guo Shawn-R65073;
>devicetree at vger.kernel.org
>Subject: Re: [PATCHv3 5/6] ARM: imx: Add initial support for Freescale
>On Thursday 11 September 2014 09:53:56 Jingchang Lu wrote:
>> >From: Arnd Bergmann [mailto:arnd at arndb.de]
>> >
>> >Ok, I see. The actual point of dma_zone_size however is slightly
>> >different, and I think you should not use it like this. We normally
>> >only use ZONE_DMA if there are devices that have a limitation smaller
>> >than 4GB, and that appear to be the case for your system.
>> >
>> >The message you quote is only present in arch/powerpc, so I'm not
>> >sure what symptoms you are actually seeing. Please try removing the
>> >dma_zone_size setting for your platform and report if it works or
>> >what the symptom is if it does not work with the latest kernel.
>> >
>> >We definitely need to get this to work out of the box without a
>> >dma_zone_size hack.
>> >
>> >Can you describe what the memory layout is of your platform? Can you
>> >have RAM installed above the 4GB physical address boundary? If you
>> >can, are there any devices that are unable to perform DMA into that
>> >memory without the use of an IOMMU?
>> >
>> Our first LS1021A support is based on kernel-3.12, where when LPAE
>> enabled it will compare the device's coherent_dma_mask with
>> arm_dma_limit, which is 64-bit 0xffffffffffffffff without
>> CONFIG_ZONE_DMA, with CONFIG_ZONE_DMA can limit the comparison and avoid
>the warning.
>> All device can address 32-bit address space on LS1021A, With you
>> comment above, I remove the dma_zone_size and only reserve the
>> CONFIG_ZONE_DMA On kernel-3.12, finding all works well, so I will remove
>the dma_zone_size setting on 3.12. Thanks.
>Ok, good.
>> I have a look on latest kernel, finding get_coherent_dma_mask() has
>> limited the mask to (u64)DMA_BIT_MASK(32), does this mean the
>> CONFIG_ZONE_DMA is not needed for me here? Thanks.
>You only need to enable CONFIG_ZONE_DMA in the case of RAM above the 4GB
>boundary. I would suggest you do the same as some of the other platforms
>by adding
>	select ZONE_DMA if ARM_LPAE
>into your Kconfig file. If LPAE is disabled, you know that you won't need
>	Arnd

I will do that. Thanks.

Best Regards,

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