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
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next prev parent reply other threads:[~2022-05-20 8:52 UTC|newest]
Thread overview: 12+ 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
2022-05-17 8:38 ` Christoph Hellwig
2022-05-17 9:02 ` John Garry via iommu
2022-05-17 9:11 ` Christoph Hellwig
2022-05-17 10:40 ` Robin Murphy
2022-05-17 11:26 ` John Garry via iommu
2022-05-17 12:02 ` Robin Murphy
2022-05-17 13:50 ` John Garry via iommu
2022-05-18 13:13 ` Christoph Hellwig
2022-05-20 8:51 ` Joerg Roedel [this message]
2022-05-18 13:12 ` Christoph Hellwig
2022-05-18 13:45 ` Robin Murphy
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