Alexey Kardashevskiy <a...@ozlabs.ru> writes:
> On 24/03/2020 04:22, Christoph Hellwig wrote:
>> On Mon, Mar 23, 2020 at 09:07:38PM +0530, Aneesh Kumar K.V wrote:
>>>
>>> This is what I was trying, but considering I am new to DMA subsystem, I
>>> am not sure I got all the details correct. The idea is to look at the
>>> cpu addr and see if that can be used in direct map fashion(is
>>> bus_dma_limit the right restriction here?) if not fallback to dynamic
>>> IOMMU mapping.
>>
>> I don't think we can throw all these complications into the dma
>> mapping code. At some point I also wonder what the point is,
>> especially for scatterlist mappings, where the iommu can coalesce.
>
> This is for persistent memory which you can DMA to/from but yet it does
> not appear in the system as a normal memory and therefore requires
> special handling anyway (O_DIRECT or DAX, I do not know the exact
> mechanics). All other devices in the system should just run as usual,
> i.e. use 1:1 mapping if possible.
This is O_DIRECT with a user buffer that is actually mmap from a dax
mounted file system.
What we really need is something that will falback to iommu_map_page
based on dma_addr. ie. Something equivalent to current
dma_direct_map_page(), but instead of fallback to swiotlb_map page we
should fallback to iommu_map_page().
Something like?
dma_addr_t dma_direct_map_page(struct device *dev, struct page *page,
unsigned long offset, size_t size, enum dma_data_direction dir,
unsigned long attrs)
{
phys_addr_t phys = page_to_phys(page) + offset;
dma_addr_t dma_addr = phys_to_dma(dev, phys);
if (unlikely(!dma_capable(dev, dma_addr, size, true))) {
return iommu_map(dev, phys, size, dir, attrs);
return DMA_MAPPING_ERROR;
}
....
...
-aneesh
