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

[Ming Lei]

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.

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.

Thanks,
-- 
Ming Lei