[ofa-general] [PATCH RFC v3 1/2] RDMA/Core: MEM_MGT_EXTENSIONS support

[Steve Wise]

Roland Dreier wrote:
>  > I see. Just wondering, in the mlx4 case, is it a must to use dma
>  > consistent memory allocation or dma mapping would work too?
>
> dma mapping would work too but then handling the map/unmap becomes an
> issue.  I think it is way too complicated too add new verbs for
> map/unmap fastreg page list (in addition to the alloc/free fastreg page
> list that we are already adding) and force the consumer to do it.  And
> if we expect the low-level driver to do it, then the map is easy (can be
> done while posting the send) but the unmap is a pain -- it would have to
> be done inside poll_cq when reapind the completion, and the low-level
> driver would have to keep some complicated extra data structure to go
> back from the completion to the original fast reg page list structure.
>   

And certain platforms can fail map requests (like PPC64) because they 
have limited resources for dma mapping.  So then you'd fail a SQ work 
request when you might not want to...

Steve.