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

[John Garry]

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