David Brownell writes:
> Given that some hardware must return the dma addresses, why
> should it be a good thing to have an API that doesn't expose
> the notion of a reverse mapping? At this level -- not the lower
> level code touching hardware PTEs.
Because its' _very_ expensive on certain machines. You have to do
1 or more I/O accesses to get at the PTEs.
If you add this reverse notion to just one API (the dma pool one) then
people will complain (rightly) that there is not orthogonality in the
API since the other mapping functions do not provide it.
No, it is unacceptable.
Later,
David S. Miller
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