linux-kernel - Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping

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Date:   Fri, 20 May 2022 10:51:57 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>,
        John Garry <john.garry@...wei.com>, will@...nel.org,
        m.szyprowski@...sung.com, chenxiang66@...ilicon.com,
        thunder.leizhen@...wei.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, liyihang6@...ilicon.com
Subject: Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()

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