Re: [PATCH v1 16/16] dma-mapping: Disallow .map_sg operations from returning zero on error

[Robin Murphy]

On 2021-07-16 07:33, Christoph Hellwig wrote:
> On Thu, Jul 15, 2021 at 10:45:44AM -0600, Logan Gunthorpe wrote:
> > @@ -194,6 +194,8 @@ static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
> >   	else
> >   		ents = ops->map_sg(dev, sg, nents, dir, attrs);
> >   
> > +	WARN_ON_ONCE(ents == 0);
>
> Turns this into a negative error code while we're at it, just to keep
> the callers sane?

Right, by this point returning the 0 would pass through 
dma_map_sg_attrs() OK, but AFAICS dma_map_sgtable() would now get 
confused and return success but with sgt->nents = 0. Coercing it to an 
error code (which dma_map_sg_attrs() would then just change right back) 
seems sensible for the sake of easy robustness.

Robin.