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

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Date:   Thu, 9 Jun 2022 18:54:01 +0100
From:   John Garry <john.garry@...wei.com>
To:     Bart Van Assche <bvanassche@....org>,
        <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 09/06/2022 18:18, Bart Van Assche wrote:
>>>
>>> SCSI host bus adapters that support 64-bit DMA may support much 
>>> larger transfer sizes than 128 KiB.
>>
>> Indeed, and that is my problem today, as my storage controller is 
>> generating DMA mapping lengths which exceeds 128K and they slow 
>> everything down.
>>
>> If you say that SRP enjoys best peformance with larger transfers then 
>> can you please test this with an IOMMU enabled (iommu group type DMA 
>> or DMA-FQ)?
> 
> Hmm ... what exactly do you want me to test? Do you perhaps want me to 
> measure how much performance drops with an IOMMU enabled? 

Yes, I would like to know of any performance change with an IOMMU 
enabled and then with an IOMMU enabled and including my series.

> I don't have 
> access anymore to the SRP setup I referred to in my previous email. But 
> I do have access to devices that boot from UFS storage. For these 
> devices we need to transfer 2 MiB per request to achieve full bandwidth.

ok, but do you have a system where the UFS host controller is behind an 
IOMMU? I had the impression that UFS controllers would be mostly found 
in embedded systems and IOMMUs are not as common on there.

Thanks,
John