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

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Date:	Fri, 20 Jul 2007 12:19:03 -0700
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	"Roland Dreier" <rdreier@...co.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

> 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?

I might have caused some confusion.  You had asked if any drivers
support MSI but not MSI-X, so I threw 2 drivers out there that currently
support both, and why we support MSI for compatibility.

> 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.

MSI-X doesn't make much sense if you have 1 Rx queue on your NIC, since
1 vector essentially acts like MSI.  In the case of why MSI-X could
fail, I have had it fail when I misconfigured my driver and didn't ask
for enough vectors for what I was assigning, so the driver disabled the
multiple Rx queues, and fell back to MSI.

I hope this helps.

-PJ
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