linux-kernel - [PATCH 3/4] scsi: core: Cap shost max_sectors according to DMA optimum mapping limits

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Date:   Fri, 20 May 2022 16:23:22 +0800
From:   John Garry <john.garry@...wei.com>
To:     <damien.lemoal@...nsource.wdc.com>, <joro@...tes.org>,
        <will@...nel.org>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <hch@....de>,
        <m.szyprowski@...sung.com>, <robin.murphy@....com>
CC:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <linux-scsi@...r.kernel.org>, <liyihang6@...ilicon.com>,
        <chenxiang66@...ilicon.com>, <thunder.leizhen@...wei.com>,
        John Garry <john.garry@...wei.com>
Subject: [PATCH 3/4] scsi: core: Cap shost max_sectors according to DMA optimum mapping limits

Streaming DMA mappings may be considerably slower when mappings go through
an IOMMU and the total mapping length is somewhat long. This is because the
IOMMU IOVA code allocates and free an IOVA for each mapping, which may
affect performance.

For performance reasons set the request_queue max_sectors from
dma_opt_mapping_size(), which knows this mapping limit.

In addition, the shost->max_sectors is repeatedly set for each sdev in
__scsi_init_queue(). This is unnecessary, so set once when adding the
host.

Signed-off-by: John Garry <john.garry@...wei.com>
---
 drivers/scsi/hosts.c    | 5 +++++
 drivers/scsi/scsi_lib.c | 4 ----
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index f69b77cbf538..a3ae6345473b 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -225,6 +225,11 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 	shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
 				   shost->can_queue);
 
+	if (dma_dev->dma_mask) {
+		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
+				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
+	}
+
 	error = scsi_init_sense_cache(shost);
 	if (error)
 		goto fail;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 8d18cc7e510e..2d43bb8799bd 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1884,10 +1884,6 @@ void __scsi_init_queue(struct Scsi_Host *shost, struct request_queue *q)
 		blk_queue_max_integrity_segments(q, shost->sg_prot_tablesize);
 	}
 
-	if (dev->dma_mask) {
-		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
-				dma_max_mapping_size(dev) >> SECTOR_SHIFT);
-	}
 	blk_queue_max_hw_sectors(q, shost->max_sectors);
 	blk_queue_segment_boundary(q, shost->dma_boundary);
 	dma_set_seg_boundary(dev, shost->dma_boundary);
-- 
2.26.2