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To: "Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@intel.com>
Cc: "Nelson, Shannon" <shannon.nelson@intel.com>, <akpm@linux-foundation.org>,
       <linux-kernel@vger.kernel.org>, <davem@davemloft.net>,
       <jeff@garzik.org>, "Williams, Dan J" <dan.j.williams@intel.com>,
       "Leech, Christopher" <christopher.leech@intel.com>
Subject: Re: [PATCH 5/7] I/OAT: Add support for MSI and MSI-X
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From: Roland Dreier <rdreier@cisco.com>
Date: Fri, 20 Jul 2007 12:05:24 -0700
In-Reply-To: <D5C1322C3E673F459512FB59E0DDC32903445CD4@orsmsx414.amr.corp.intel.com> (Waskiewicz Jr's message of "Fri, 20 Jul 2007 11:09:03 -0700")
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 > Both igb (recently posted) and ixgbe (also recently posted) support both
 > MSI and MSI-X.  Right now when we try to request MSI-X vectors, if we
 > fail to acquire what we've asked for, we fall back to MSI support.  If
 > MSI fails to initialize, we fall back to legacy interrupts.  So it needs
 > to be there in case MSI-X allocation fails for the NIC driver.

Hmm, I see I don't understand what this driver is doing.  What is a
"struct ioatdma_device"?  Is this driver requesting interrupts that
come from the NIC or the IOAT DMA engine?

Anyway, if the NICs support MSI-X, is there any chance of failing to
get one MSI-X vectors but then succeeding in getting MSI enabled?
How could that happen?  I don't see what falling back to MSI buys you
beyond more code.

 - R.