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

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Date:   Thu, 14 Jul 2022 08:52:15 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
CC:     Christoph Hellwig <hch@....de>, <damien.lemoal@...nsource.wdc.com>,
        <joro@...tes.org>, <will@...nel.org>, <jejb@...ux.ibm.com>,
        <m.szyprowski@...sung.com>, <robin.murphy@....com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-ide@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <iommu@...ts.linux.dev>, <linux-scsi@...r.kernel.org>,
        <linuxarm@...wei.com>
Subject: Re: [PATCH v5 0/5] DMA mapping changes for SCSI core

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