Subject: [PATCH 1/2] arm64: dma-mapping: implement dma_get_sgtable()

From: [email-masked] (Robin Murphy)
Date: Mon, 20 Jul 2015 18:00:57 +0100

Hi Will,

On 20/07/15 17:36, Will Deacon wrote:
> On Fri, Jul 17, 2015 at 04:58:21PM +0100, Robin Murphy wrote:
>> The default dma_common_get_sgtable() implementation relies on the CPU
>> address of the buffer being a regular lowmem address. This is not always
>> the case on arm64, since allocations from the various DMA pools may have
>> remapped vmalloc addresses, rendering the use of virt_to_page() invalid.
>>
>> Fix this by providing our own implementation based on the fact that we
>> can safely derive a physical address from the DMA address in both cases.
>>
>> CC: Jon Medhurst <[email-masked]>
>> Signed-off-by: Robin Murphy <[email-masked]>
>> ---
>>   arch/arm64/mm/dma-mapping.c | 14 ++++++++++++++
>>   1 file changed, 14 insertions(+)
>>
>> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
>> index d16a1ce..4b9b600 100644
>> --- a/arch/arm64/mm/dma-mapping.c
>> +++ b/arch/arm64/mm/dma-mapping.c
>> @@ -337,10 +337,24 @@ static int __swiotlb_mmap(struct device *dev,
>>   	return __dma_common_mmap(dev, vma, cpu_addr, dma_addr, size);
>>   }
>>
>> +int __swiotlb_get_sgtable(struct device *dev, struct sg_table *sgt,
>> +			  void *cpu_addr, dma_addr_t handle, size_t size,
>> +			  struct dma_attrs *attrs)
>> +{
>> +	int ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
>> +
>> +	if (!ret)
>> +		sg_set_page(sgt->sgl, phys_to_page(dma_to_phys(dev, handle)),
>> +			    PAGE_ALIGN(size), 0);
>> +
>> +	return ret;
>> +}
>
> Any reason not to do this in dma_common_get_sgtable?

Summarising the discussion over at [1], most architectures seem to 
depend on dma_common_get_sgtable, but only a handful implement 
dma_to_phys (plus this approach seems to match the original intent). 
There doesn't seem to be a nice solution for doing this in common code 
without a big cross-architecture patch, and it's somewhat questionable 
how widely this is actually needed.

Robin.

[1]:http://thread.gmane.org/gmane.linux.kernel/1998795

> Will