For the problem when using swiotlb

[Ding Tianhong]

On 2014/11/18 2:09, Catalin Marinas wrote:
> On Mon, Nov 17, 2014 at 12:18:42PM +0000, Arnd Bergmann wrote:
>> On Monday 17 November 2014 19:56:27 Ding Tianhong wrote:
>>>         The commit 3690951fc6d42f3a0903987677d0e592c49dd8db(arm64: Use swiotlb late initialisation)
>>> switches the DMA mapping code to swiotlb_tlb_late_init_with_default_size(), this will occur a problem
>>> when I run the scsi stress tests, the message as below:
>>>
>>>         sas_controller b1000000.sas: swiotlb buffer is full (sz: 65536 bytes)..
>>>         DMA: Out of SW-IOMMU space for 65536 bytes at device b1000000.sas
>>>
>>> The reason is that the swiotlb_tlb_late_init_with_default_size() could only alloc 16M memory for DMA-mapping,
>>> and the param in cmdline "swiotlb=xxx" is useless because the get_free_pages() only use the buddy to assigned a
>>> maximum memory of 16M(The MAX_ORDER is 13 for 4k pages), obviously 16M is too small in many scenes, but
>>> the swiotlb_init() which could reserved a bigger memory as wished could work well for most drivers.
>>>
>>> I could not get a better way to fix this problem except to revert this patch, so could you please give me some
>>> advise and help me, thanks very much.
>>
>> In general, you should not need to use swiotlb for most devices, in
>> particular for high-performance devices like network or block.
>>
>> Please make sure that you have set up the dma-ranges properties in
>> your DT properly to allow 64-bit DMA if the device supports it.
> 
> That's the problem indeed, the DMA API ends up using swiotlb bounce
> buffers because the physical address of the pages passed to (or
> allocated by) the driver are beyond 32-bit limit (which is the default
> dma mask).
> 

Thanks everyone, I think I found the way to fix it, need to enable DMA_CMA, to reserve a big memory
for CMA and set coherent mask for dev, then dma_alloc and dma_mapping will not use the swiotlb until
the memory out of mask or swiotlb_force is enabled.

If I still understand uncorrectly, please inform me.

Regards
Ding