linux-kernel - Re: [PATCH 5/7] I/OAT: Add support for MSI and MSI-X

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Date:	Fri, 20 Jul 2007 12:05:24 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
Cc:	"Nelson, Shannon" <shannon.nelson@...el.com>,
	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
	<davem@...emloft.net>, <jeff@...zik.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"Leech, Christopher" <christopher.leech@...el.com>
Subject: Re: [PATCH 5/7] I/OAT: Add support for MSI and MSI-X

 > 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.
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