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

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Message-ID: <20070424204208.GD27911@linux-os.sc.intel.com>
Date:	Tue, 24 Apr 2007 13:42:08 -0700
From:	Ashok Raj <ashok.raj@...el.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Ashok Raj <ashok.raj@...el.com>, 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 Tue, Apr 24, 2007 at 09:28:11PM +0200, Andi Kleen wrote:
> 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

Some of the early DMAR's dont handle zero-length-reads as required. hardware
that supports it correctly will advertise via its capabilities.

We could remove the cmdline option since it should not be really required.
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