Re: [PATCH v6 5/6] scsi: scsi_transport_sas: Cap shost opt_sectors according to DMA optimal limit

Damien Le Moal
2022-07-18T10:48:06Z

On 7/14/22 20:15, John Garry wrote:
> 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.
>
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
> drivers/scsi/scsi_transport_sas.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/scsi/scsi_transport_sas.c b/drivers/scsi/scsi_transport_sas.c
> index 12bff64dade6..2f88c61216ee 100644
> --- a/drivers/scsi/scsi_transport_sas.c
> +++ b/drivers/scsi/scsi_transport_sas.c
> @@ -225,6 +225,7 @@ static int sas_host_setup(struct transport_container *tc, struct device *dev,
> {
> struct Scsi_Host *shost = dev_to_shost(dev);
> struct sas_host_attrs *sas_host = to_sas_host_attrs(shost);
> + struct device *dma_dev = shost->dma_dev;
>
> INIT_LIST_HEAD(&sas_host->rphy_list);
> mutex_init(&sas_host->lock);
> @@ -236,6 +237,11 @@ static int sas_host_setup(struct transport_container *tc, struct device *dev,
> dev_printk(KERN_ERR, dev, "fail to a bsg device %d\n",
> shost->host_no);
>
> + if (dma_dev->dma_mask) {
> + shost->opt_sectors = min_t(unsigned int, shost->max_sectors,
> + dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
> + }
> +
> return 0;
> }
>

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>

--
Damien Le Moal
Western Digital Research