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Date:   Fri, 20 May 2022 10:51:57 +0200
From:   Joerg Roedel <joro@8bytes.org>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        John Garry <john.garry@huawei.com>, will@kernel.org,
        m.szyprowski@samsung.com, chenxiang66@hisilicon.com,
        thunder.leizhen@huawei.com, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, liyihang6@hisilicon.com
Subject: Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()
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On Wed, May 18, 2022 at 03:13:53PM +0200, Christoph Hellwig wrote:
> On Tue, May 17, 2022 at 01:02:00PM +0100, Robin Murphy wrote:
> >> So how to inform the SCSI driver of this caching limit then so that it may 
> >> limit the SGL length?
> >
> > Driver-specific mechanism; block-layer-specific mechanism; redefine this 
> > whole API to something like dma_opt_mapping_size(), as a limit above which 
> > mappings might become less efficient or start to fail (callback to my 
> > thoughts on [1] as well, I suppose); many options. Just not imposing a 
> > ridiculously low *maximum* on everyone wherein mapping calls "should not be 
> > larger than the returned value" when that's clearly bollocks.
> 
> Well, for swiotlb it is a hard limit.  So if we want to go down that
> route we need two APIs, one for the optimal size and one for the
> hard limit.

I agree with Robin, and if it really helps some drivers I am all for
doing a dma_opt_mapping_size() instead. Limiting DMA mapping sizes to
make drivers perform better gets a clear NAK from my side.

Regards,

	Joerg