On Fri, Nov 09, 2018 at 03:12:34PM -0800, David Miller wrote:

But patch #2 on the other hand, not so much.

I hate seeing values returned by reference, it adds cost especially
on cpus where all argments and return values fit in registers (we end
up forcing a stack slot and memory references).

And we don't need it here.

DMA addresses are like pointers, and therefore we can return errors and
valid success values in the same dma_addr_t just fine.  PTR_ERR() --> DMA_ERR(),
IS_PTR_ERR() --> IS_DMA_ERR, etc.

In the end this is an inline function, so with a decently smart
compiler the generated code shouldn't change too much.  The big problem
that prompted me to come up with this patch is that not handling failure
from dma_map* in a swiotlb setup can lead to grave data corruption, and
we have no easy way to force error checking on these return values.

I've added a few of the static typechecking suspect if they have a
better idea on how to make the return value of dma_map_single/pages
in a way that we get warnings if dma_mapping_error isn't called on them.
But I can't really think of a good way.