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

[Bart Van Assche]

On 6/9/22 01:00, John Garry wrote:
> On 08/06/2022 22:07, Bart Van Assche wrote:
> > On 6/8/22 10:50, John Garry wrote:
> > > Please note that this limit only applies if we have an IOMMU enabled 
> > > for the scsi host dma device. Otherwise we are limited by dma direct 
> > > or swiotlb max mapping size, as before.
> >
> > 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? 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.

Thanks,

Bart.