netdev - Re: [RFC 1/2] PCI-Express Non-Transparent Bridge Support

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Message-ID: <20120716175505.GF9598@jonmason-lab>
Date:	Mon, 16 Jul 2012 10:55:06 -0700
From:	Jon Mason <jon.mason@...el.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-pci@...r.kernel.org, Dave Jiang <dave.jiang@...el.com>
Subject: Re: [RFC 1/2] PCI-Express Non-Transparent Bridge Support

On Sun, Jul 15, 2012 at 05:19:21PM -0700, Greg KH wrote:
> On Sun, Jul 15, 2012 at 04:55:48PM -0700, Jon Mason wrote:
> > On Sat, Jul 14, 2012 at 10:10:15AM -0700, Greg KH wrote:
> > > On Fri, Jul 13, 2012 at 02:44:59PM -0700, Jon Mason wrote:
> > > > +static int max_num_cbs = 2;
> > > > +module_param(max_num_cbs, uint, 0644);
> > > > +MODULE_PARM_DESC(max_num_cbs, "Maximum number of NTB transport connections");
> > > > +
> > > > +static bool no_msix;
> > > > +module_param(no_msix, bool, 0644);
> > > > +MODULE_PARM_DESC(no_msix, "Do not allow MSI-X interrupts to be selected");
> > > 
> > > How would a user, or a distro, know to set these options?  Why are they
> > > even options at all?
> > 
> > Good question.  There is actually a potential benefit to disabling
> > MSI-X.  The NTB device on one of our platforms only has 3 MSI-X
> > vectors.  In the current driver design, that would limit them to 3
> > client/virtual devices.  However, there are 15bits in the ISR that can
> > be used for the same purpose.  So, if you disable MSI-X, you can have
> > 15 instead of 3.  
> 
> But again, how would a user, or a distro, know to set these?  Where is
> the documentation describing it?  Why really have these options at all
> and not just fix the platform issues (only 3 MSI-X vectors?  Really?)

I believe we'll want multiple clients (or have multiqueue Ethernet).  I'm happy to add something to /Documentation to describe it and why it would be useful, or I can remove it and re-introduce it when I add multiqueue Ethernet.

3 MSI-X vectors (plus one for PCI-E link up/down) on Xeon NTB, and 33 for Atom NTB.  Yeah, really.

> 
> thanks,
> 
> greg k-h
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