[PATCH 14/15] nvme-pci: optimize mapping single segment requests using SGLs

[Christoph Hellwig]

If the controller supports SGLs we can take another short cut for single
segment request, given that we can always map those without another
indirection structure, and thus don't need to create a scatterlist
structure.

Signed-off-by: Christoph Hellwig <h...@lst.de>
---
 drivers/nvme/host/pci.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 47fc4d653961..38869f59c296 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -819,6 +819,23 @@ static blk_status_t nvme_setup_prp_simple(struct nvme_dev 
*dev,
        return 0;
 }
 
+static blk_status_t nvme_setup_sgl_simple(struct nvme_dev *dev,
+               struct request *req, struct nvme_rw_command *cmnd,
+               struct bio_vec *bv)
+{
+       struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
+
+       iod->first_dma = dma_map_bvec(dev->dev, bv, rq_dma_dir(req), 0);
+       if (dma_mapping_error(dev->dev, iod->first_dma))
+               return BLK_STS_RESOURCE;
+       iod->dma_len = bv->bv_len;
+
+       cmnd->dptr.sgl.addr = cpu_to_le64(iod->first_dma);
+       cmnd->dptr.sgl.length = cpu_to_le32(iod->dma_len);
+       cmnd->dptr.sgl.type = NVME_SGL_FMT_DATA_DESC << 4;
+       return 0;
+}
+
 static blk_status_t nvme_map_data(struct nvme_dev *dev, struct request *req,
                struct nvme_command *cmnd)
 {
@@ -836,6 +853,11 @@ static blk_status_t nvme_map_data(struct nvme_dev *dev, 
struct request *req,
                        if (bv.bv_offset + bv.bv_len <= dev->ctrl.page_size * 2)
                                return nvme_setup_prp_simple(dev, req,
                                                             &cmnd->rw, &bv);
+
+                       if (iod->nvmeq->qid &&
+                           dev->ctrl.sgls & ((1 << 0) | (1 << 1)))
+                               return nvme_setup_sgl_simple(dev, req,
+                                                            &cmnd->rw, &bv);
                }
        }
 
-- 
2.20.1