Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device

On 24/03/2020 04:22, Christoph Hellwig wrote:

On Mon, Mar 23, 2020 at 09:07:38PM +0530, Aneesh Kumar K.V wrote:

quoted

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.


-- 
Alexey