Re: [PATCH v5 0/5] DMA mapping changes for SCSI core

[John Garry]

On 14/07/2022 04:10, Martin K. Petersen wrote:

Hi Martin,

> > So I set max hw sectors at this ‘opt’ mapping size to ensure that we
> > get no mappings which exceed this size. Indeed, I think max sectors is
> > 128Kb today for my host, which would be same as dma_opt_mapping_size()
> > value with an IOMMU enabled. And I find that only a small % of request
> > size may exceed this 128kb size, but it still has a big performance
> > impact.
> The purpose of the soft limit is to pick the appropriate I/O size
> (i.e. for best performance). The purpose of the hard limit is to ensure
> we don't submit something the hardware can't handle or describe.
>
> IOW, the hard limit is not about performance at all. The hard limit is
> mainly relevant for things that are way bigger than anything we'd issue
> as regular filesystem I/O such as multi-megabyte firmware images, etc.
>
> It's perfectly fine for firmware download performance to be
> "suboptimal". What is typically more important in that scenario is that
> the firmware image makes it inside a single I/O.

OK, fine. I've improved the next version such that the DMA mapping opt 
limit only affects the max_sectors default.

Thanks,
John