linux-kernel - Re: [PATCH v3 3/4] scsi: core: Cap shost max_sectors according to DMA optimum mapping limits

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Date:   Wed, 8 Jun 2022 10:33:26 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     John Garry <john.garry@...wei.com>,
        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
Subject: Re: [PATCH v3 3/4] scsi: core: Cap shost max_sectors according to DMA
 optimum mapping limits

On 6/6/22 02:30, John Garry wrote:
> +	if (dma_dev->dma_mask) {
> +		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
> +				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
> +	}

Since IOVA_RANGE_CACHE_MAX_SIZE = 6 this limits max_sectors to 2**6 * 
PAGE_SIZE or 256 KiB if the page size is 4 KiB. I think that's too 
small. Some (SRP) storage arrays require much larger transfers to 
achieve optimal performance.

Thanks,

Bart.