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scsi: sd: allow max_sectors be capped at DMA optimal size limit
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. New member Scsi_Host.opt_sectors is added, which is the optimal host max_sectors, and use this value to cap the request queue max_sectors when set. It could be considered to have request queues io_opt value initially set at Scsi_Host.opt_sectors in __scsi_init_queue(), but that is not really the purpose of io_opt. Finally, even though Scsi_Host.opt_sectors value should never be greater than the request queue max_hw_sectors value, continue to limit to this value for safety. Signed-off-by: John Garry <john.garry@huawei.com> Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com> Acked-by: Martin K. Petersen <martin.petersen@oracle.com> Signed-off-by: Christoph Hellwig <hch@lst.de>
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@ -3296,6 +3296,8 @@ static int sd_revalidate_disk(struct gendisk *disk)
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(sector_t)BLK_DEF_MAX_SECTORS);
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}
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rw_max = min_not_zero(rw_max, sdp->host->opt_sectors);
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/* Do not exceed controller limit */
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rw_max = min(rw_max, queue_max_hw_sectors(q));
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@ -607,6 +607,7 @@ struct Scsi_Host {
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short unsigned int sg_tablesize;
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short unsigned int sg_prot_tablesize;
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unsigned int max_sectors;
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unsigned int opt_sectors;
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unsigned int max_segment_size;
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unsigned long dma_boundary;
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unsigned long virt_boundary_mask;
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