linux-kernel - Re: [PATCH v2 4/5] iommu: Use dma_addr

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Message-ID: <9450765.GaoRYW154J@wuerfel>
Date:	Fri, 09 May 2014 21:52:08 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	David Woodhouse <dwmw2@...radead.org>, linux-doc@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joerg Roedel <joro@...tes.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	linux-kernel@...r.kernel.org,
	James Bottomley <jbottomley@...allels.com>,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments

On Friday 09 May 2014 09:32:26 Bjorn Helgaas wrote:
> > There are people who care deeply about the performance of IOMMU API
> > map/unmap. It isn't used *just* for virtual machines any more. See
> > drivers/infiniband/hw/usnic/usnic_uiom.c for example.
> 
> Of course we should care about IOMMU API performance.  We should also
> care about interface consistency, and it seems there's a tradeoff in
> this case.  I said "relatively" because I expect map/unmap to be less
> frequent than read/write operations that use the mapping.  I don't
> know anything about infiniband, so maybe that assumption is false
> there.

In most drivers using the streaming DMA API, every mapping is used
exactly once. Think of network or block drivers: they rarely send
the same data twice to the device, and it usually comes from or
goes to some user space buffer.

	Arnd
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