[PATCH v1 08/17] dma-mapping: add a dma_need_unmap helper

[Christoph Hellwig]

On Thu, Oct 31, 2024 at 09:18:11PM +0000, Robin Murphy wrote:
>>   +/**
>> + * dma_need_unmap - does this device need dma_unmap_* operations
>> + * @dev: device to check
>> + *
>> + * If this function returns %false, drivers can skip calling dma_unmap_* after
>> + * finishing an I/O.  This function must be called after all mappings that might
>> + * need to be unmapped have been performed.
>
> In terms of the unmap call itself, why don't we just use dma_skip_sync to 
> short-cut dma_direct_unmap_*() and make sure it's as cheap as possible?
>
> In terms of not having to unmap implying not having to store addresses at 
> all, it doesn't seem super-useful when you still have to store them for 
> long enough to find out that you don't :/

I don't fully understand the comment, mostly because the way I read the
two sentences appear to contradict each other.

Bypassing dma_direct_unmap_ is not the important part, because it already
is pretty cheap.  Storing the addresses is not.

That being said now that we never check need_unmap in the iova path
it might make sense to not have a separate helper, but it needs to
be exposed and documented.