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

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Message-ID: <CAErSpo5bPvaATcptz3wGdLWOG-1thHKCHDK5D8K=D9wy+NDomA@mail.gmail.com>
Date:	Fri, 9 May 2014 14:19:44 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	"linux-doc@...r.kernel.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" <linux-kernel@...r.kernel.org>,
	James Bottomley <jbottomley@...allels.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments

On Fri, May 9, 2014 at 1:52 PM, Arnd Bergmann <arnd@...db.de> wrote:
> 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.

Oh, good point.  I don't work that high up in the stack, so thanks for
reminding me of that.

Bjorn
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