[RFC PATCH v2 06/11] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg

[Christoph Hellwig]

On Fri, Mar 12, 2021 at 06:11:17PM +0000, Robin Murphy wrote:
> Sure, that's how things stand immediately after this patch. But then 
> someone comes along with the perfectly reasonable argument for returning 
> more expressive error information for regular mapping failures as well 
> (because sometimes those can be terminal too, as above), we start to get 
> divergent behaviour across architectures and random bits of old code subtly 
> breaking down the line. *That* is what makes me wary of making a 
> fundamental change to a long-standing "nonzero means success" interface...

Agreed.  IMHO dma_map_sg actually needs to be switched to return
unsigned to help root this out, going the other way is no helpful.