[PATCH 2/9] block: add scatterlist-less DMA mapping helpers

[Leon Romanovsky]

On Tue, Jun 10, 2025 at 07:06:40AM +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.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  block/blk-mq-dma.c         | 162 +++++++++++++++++++++++++++++++++++++
>  include/linux/blk-mq-dma.h |  63 +++++++++++++++
>  2 files changed, 225 insertions(+)
>  create mode 100644 include/linux/blk-mq-dma.h
> 

Thanks,
Reviewed-by: Leon Romanovsky <leonro at nvidia.com>