linux-kernel - Re: [Intel IOMMU][patch 4/8] Supporting Zero Length Reads in Intel IOMMU.

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Message-Id: <200704242128.12057.ak@suse.de>
Date:	Tue, 24 Apr 2007 21:28:11 +0200
From:	Andi Kleen <ak@...e.de>
To:	Ashok Raj <ashok.raj@...el.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org, gregkh@...e.de,
	muli@...ibm.com, asit.k.mallick@...el.com,
	suresh.b.siddha@...el.com, anil.s.keshavamurthy@...el.com,
	arjan@...ux.intel.com, shaohua.li@...el.com
Subject: Re: [Intel IOMMU][patch 4/8] Supporting Zero Length Reads in Intel IOMMU.

On Tuesday 24 April 2007 08:03:03 Ashok Raj wrote:
> PCI specs permit zero length reads (ZLR) even if the mapping for that region 
> is write only. Support for this feature is indicated by the presence of a bit 
> in the DMAR capability. If a particular DMAR does not support this capability
> we map write-only regions as read-write.
> 
> This option can also provides a workaround for some drivers that request
> a write-only mapping when they really should request a read-write.
> (We ran into one such case in eepro100.c in handling rx_ring_dma)

Better just fix the drivers instead of adding such hacks

-Andi
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