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Date:	Mon, 02 Mar 2009 10:46:18 +1100
From:	Michael Ellerman <michael@...erman.id.au>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Grant Grundler <grundler@...isc-linux.org>,
	linux-pci@...r.kernel.org, jbarnes@...tuousgeek.org,
	linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 1/6] Rewrite MSI-HOWTO

On Fri, 2009-02-27 at 05:14 -0700, Matthew Wilcox wrote:
> On Thu, Feb 26, 2009 at 11:15:25PM -0700, Grant Grundler wrote:
> > ...
> > > +3. Why use MSIs?
> > > +
> > > +There are three reasons why using MSIs can give an advantage over
> > > +traditional pin-based interrupts.
> > ...
> > > +PCI devices can only support a single pin-based interrupt per function.
> > 
> > Related to this is a 4th reason: distribute workload across CPUs
> > and enables construction of efficient, multi-queue devices.
> > Care to mention that?
> 
> That's true for MSI-X, but not for MSIs in general.  Workload is already
> distributed across CPUs with round-robin interrupts.  I'm inclined to
> leave out this level of detail.
> 
> > > +The MSI-X capability is much more flexible than the MSI capability.
> > > +It supports up to 2048 interrupts, each of which can be separately
> > > +assigned.
> > 
> > Nothing describes "assignment" below or what is meant by "assigned".
> > My guess is you wanted to differentiate MSIX from MSI with:
> >   ... and each MSIX can be directed at a different CPU.

So might each MSI, depending on the hardware.

> > > +5.4. Finding why MSIs are disabled on a device
> > > +
> > > +From the above three sections, you can see that there are many reasons
> > > +why MSIs may not be enabled for a given device.  Your first step should
> > > +be to examine your dmesg carefully to determine whether MSIs are enabled
> > > +for your machine.  You should also check your .config to be sure you
> > > +have enabled CONFIG_PCI_MSI.
> > 
> > Should mention "fgrep MSI /proc/interrupts" to see if any devices have
> > MSI in use?
> 
> Yes, you're right.

That won't work on (some) powerpc machines at least .. because MSIs just
get routed into the regular PIC.

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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