[PATCH 2/8] block: add scatterlist-less DMA mapping helpers

[Keith Busch]

On Mon, Jun 23, 2025 at 04:12:24PM +0200, Christoph Hellwig wrote:
> Add a new blk_rq_dma_map / blk_rq_dma_unmap pair that does away with
> the wasteful scatterlist structure.  Instead it uses the mapping iterator
> to either add segments to the IOVA for IOMMU operations, or just maps
> them one by one for the direct mapping.  For the IOMMU case instead of
> a scatterlist with an entry for each segment, only a single [dma_addr,len]
> pair needs to be stored for processing a request, and for the direct
> mapping the per-segment allocation shrinks from
> [page,offset,len,dma_addr,dma_len] to just [dma_addr,len].
> 
> One big difference to the scatterlist API, which could be considered
> downside, is that the IOVA collapsing only works when the driver sets
> a virt_boundary that matches the IOMMU granule.  For NVMe this is done
> already so it works perfectly.

Looks good.

Reviewed-by: Keith Busch <kbusch at kernel.org>