linux-kernel - Re: [PATCH v5 4/5] scsi: scsi_transport_sas: Cap shost max

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Message-ID: <2e6475e5-4899-1e3a-1418-918b9510ec6d@opensource.wdc.com>
Date:   Fri, 1 Jul 2022 08:49:36 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     John Garry <john.garry@...wei.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,
        iommu@...ts.linux.dev, linux-scsi@...r.kernel.org,
        linuxarm@...wei.com
Subject: Re: [PATCH v5 4/5] scsi: scsi_transport_sas: Cap shost max_sectors
 according to DMA optimal limit

On 6/30/22 21:08, 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@...wei.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..1b45248748e0 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) {
> +		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
> +				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
> +	}

Hmm... shost->max_sectors becomes the max_hw_sectors limit for the block
dev. So using dma_max_mapping_size(dma_dev) for that limit makes sense.
Shouldn't dma_opt_mapping_size(dma_dev) be used to limit only the default
"soft" limit (queue max_sectors limit) instead of the hard limit ?

> +
>  	return 0;
>  }
>  
-- 
Damien Le Moal
Western Digital Research