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

[Christoph Hellwig]

On Thu, Mar 11, 2021 at 04:31:36PM -0700, Logan Gunthorpe wrote:
>  	for_each_sg(sgl, sg, nents, i) {
> +		if (is_pci_p2pdma_page(sg_page(sg))) {
> +			if (sg_page(sg)->pgmap != pgmap) {
> +				pgmap = sg_page(sg)->pgmap;
> +				map = pci_p2pdma_dma_map_type(dev, pgmap);
> +				bus_off = pci_p2pdma_bus_offset(sg_page(sg));
> +			}
> +
> +			if (map < 0) {
> +				sg->dma_address = DMA_MAPPING_ERROR;
> +				ret = -EREMOTEIO;
> +				goto out_unmap;
> +			}
> +
> +			if (map) {
> +				sg->dma_address = sg_phys(sg) + sg->offset -
> +					bus_off;
> +				sg_dma_len(sg) = sg->length;
> +				sg_mark_pci_p2pdma(sg);
> +				continue;
> +			}
> +		}

This code needs to go into a separate noinline helper to reduce the impact
on the fast path.  Also as Robin noted the offset is already
accounted for in sg_phys.  We also don't ever set the dma_address in
the scatterlist to DMA_MAPPING_ERROR, that is just a return value
for the single entry mapping routines.