[PATCH V3] of: Set the DMA mask to 64 bits when dma_addr_t is 64-bits

[Laura Abbott]

On 7/3/2013 7:15 AM, Ming Lei wrote:
> On Sat, Apr 27, 2013 at 5:32 AM, Rob Herring <robherring2 at gmail.com> wrote:
>> On 04/26/2013 03:31 PM, Laura Abbott wrote:
>>> Currently, of_platform_device_create_pdata always sets the
>>> coherent DMA mask to 32 bits. On ARM systems without CONFIG_ZONE_DMA,
>>> arm_dma_limit gets set to ~0 or 0xFFFFFFFFFFFFFFFF on LPAE based
>>> systems. Since arm_dma_limit represents the smallest dma_mask
>>> on the system, the default of 32 bits prevents any dma_coherent
>>> allocation from succeeding unless clients manually set the
>>> dma mask first. Rather than make every client on an LPAE system set
>>> the mask manually, account for the size of dma_addr_t when setting
>>> the coherent mask.
>>>
>>> Signed-off-by: Laura Abbott <lauraa at codeaurora.org>
>>> ---
>>>   drivers/of/platform.c |    2 +-
>>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
>>> index 0970505..5f0ba94 100644
>>> --- a/drivers/of/platform.c
>>> +++ b/drivers/of/platform.c
>>> @@ -214,7 +214,7 @@ struct platform_device *of_platform_device_create_pdata(
>>>   #if defined(CONFIG_MICROBLAZE)
>>>        dev->archdata.dma_mask = 0xffffffffUL;
>>>   #endif
>>> -     dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
>>> +     dev->dev.coherent_dma_mask = DMA_BIT_MASK(sizeof(dma_addr_t) * 8);
>>
>> This is going to change the mask from 32 to 64 bits on 64-bit powerpc
>> and others possibly. Maybe it doesn't matter. I think it doesn't, but
>> I'm not sure enough to apply for 3.10. So I'll queue it for 3.11.
>
> Without the patch, LPAE enabled board may not boot at all, but looks
> it still isn't in -next tree.
>
> But I am wondering if it is a correct approach, because enabling LPAE
> doesn't mean the I/O devices can support DMA to/from 64bit address, and
> it is very probably devices can't do it at all.
>

The problem is the way the arm_dma_limit is set up, all dma allocations 
are currently broken regardless of if the actual device supports 64-bit 
addresses or not.

> Another way is to always set arm_dma_limit as 0xFFFFFFFF when
> CONFIG_ZONE_DMA is unset, and let driver override device's dma
> mask if the device supports 64bit DMA.
>

I previously asked about the arm_dma_limit and was told that the current 
behavior is the correct approach (see 
https://lists.ozlabs.org/pipermail/devicetree-discuss/2013-April/032729.html 
and 
https://lists.ozlabs.org/pipermail/devicetree-discuss/2013-April/032690.html) 


The point here is to set the mask to something reasonable such that 
allocations can succeed by default. If devices can't use part of the 
memory space they can adjust the limit accordingly.

Thanks,
Laura

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