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From: Joerg Roedel <joro@8bytes.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Robin Murphy <robin.murphy@arm.com>,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	will@kernel.org, liyihang6@hisilicon.com
Subject: Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()
Date: Fri, 20 May 2022 10:51:57 +0200	[thread overview]
Message-ID: <YodWrRFircbmh1bN@8bytes.org> (raw)
In-Reply-To: <20220518131353.GB26019@lst.de>

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
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

WARNING: multiple messages have this Message-ID (diff)
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()
Date: Fri, 20 May 2022 10:51:57 +0200	[thread overview]
Message-ID: <YodWrRFircbmh1bN@8bytes.org> (raw)
In-Reply-To: <20220518131353.GB26019@lst.de>

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

  reply	other threads:[~2022-05-20  8:52 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-16 13:06 [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size() John Garry via iommu
2022-05-16 13:06 ` John Garry
2022-05-17  8:38 ` Christoph Hellwig
2022-05-17  8:38   ` Christoph Hellwig
2022-05-17  9:02   ` John Garry via iommu
2022-05-17  9:02     ` John Garry
2022-05-17  9:11     ` Christoph Hellwig
2022-05-17  9:11       ` Christoph Hellwig
2022-05-17 10:40 ` Robin Murphy
2022-05-17 10:40   ` Robin Murphy
2022-05-17 11:26   ` John Garry via iommu
2022-05-17 11:26     ` John Garry
2022-05-17 12:02     ` Robin Murphy
2022-05-17 12:02       ` Robin Murphy
2022-05-17 13:50       ` John Garry via iommu
2022-05-17 13:50         ` John Garry
2022-05-18 13:13       ` Christoph Hellwig
2022-05-18 13:13         ` Christoph Hellwig
2022-05-20  8:51         ` Joerg Roedel [this message]
2022-05-20  8:51           ` Joerg Roedel
2022-05-18 13:12   ` Christoph Hellwig
2022-05-18 13:12     ` Christoph Hellwig
2022-05-18 13:45     ` Robin Murphy
2022-05-18 13:45       ` Robin Murphy

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