Date:	Wed, 28 Sep 2011 20:42:14 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH] sysfs: add per pci device msi[x] irq listing (v3)

On Wed, Sep 28, 2011 at 04:18:55PM -0600, Bjorn Helgaas wrote:
> On Thu, Sep 22, 2011 at 8:32 AM, Neil Horman <nhorman@...driver.com> wrote:
> >
> > On Thu, Sep 22, 2011 at 07:54:28AM -0600, Matthew Wilcox wrote:
> > > On Mon, Sep 19, 2011 at 11:47:15AM -0400, Neil Horman wrote:
> > > > So a while back, I wanted to provide a way for irqbalance (and other apps) to
> > > > definitively map irqs to devices, which, for msi[x] irqs is currently not really
> > > > possible in user space.  My first attempt wen't not so well:
> > > > https://lkml.org/lkml/2011/4/21/308
> > > >
> > > > It was plauged by the same issues that prior attempts were, namely that it
> > > > violated the one-file-one-value sysfs rule.  I wandered off but have recently
> > > > come back to this.  I've got a new implementation here that exports a new
> > > > subdirectory for every pci device,  called msi_irqs.  This subdirectory contanis
> > > > a variable number of numbered subdirectories, in which the number represents an
> > > > msi irq.  Each numbered subdirectory contains attributes for that irq, which
> > > > currently is only the mode it is operating in (msi vs. msix).  I think fits
> > > > within the constraints sysfs requires, and will allow irqbalance to properly map
> > > > msi irqs to devices without having to rely on rickety, best guess methods like
> > > > interface name matching.
> > >
> > > This approach feels like building bigger rockets instead of a space
> > > elevator :-)
> > >
> > In which case your comments make me think that you're trying to build the
> > Death Star instead of buying more tie fighters :)
> > https://docs.google.com/viewer?url=http://www.dau.mil/pubscats/ATL%20Docs/Sep-Oct11/Ward.pdf
> >
> > > What we need is to allow device drivers to ask for per-CPU interrupts,
> > > and implement them in terms of MSI-X.  I've made a couple of stabs at
> > > implementing this, but haven't got anything working yet.  It would solve
> > Yes, IIRC you were trying to do this the first time I proposed this:
> > https://lkml.org/lkml/2011/4/21/315
> >
> > > a number of problems:
> > >
> > Thats great, I don't see how this precludes what I'm trying to do here.  All
> > this patch does is expose a definitive relationship between msi irqs and the pci
> > devices that allocate them.  The kernel internal model used to allocate msi
> > interrupts can change, the kobject creation and removal just has to change with
> > it (presumably to create and destroy the msi irq kobjects when the individual
> > irqs are allocated/freed, rather than in a batch).  I don't see why we should
> > block enhancements to the existing msi implementation until you get new model
> > sorted, especially when this feature works equally well, despite the model we
> > use internally.
> 
> Matthew, I don't understand this issue well enough to know whether
> Neil's patch would get in the way of your planned enhancements, or
> whether it would be baggage we won't want to maintain forever.  As far
> as I can tell, the patch exposes an (IRQ -> device) mapping, which
> would still be meaningful even with per-CPU interrupts.  Can you
> educate me?
> 
Thats my view on the subject, to which I think I commented.  Matthews
enhancements are perfectly reasonable, but they're orthogonal to these changes.
Regardless of the way they're allocated (matthews changes), theres still an
association between the irq and the device (my changes)

> Neil, why do you propose doing this just for MSI IRQs?  I would think
> it'd be useful information for *all* IRQs, regardless of type, and
> that exposing the mapping for all IRQs would make it easier for tools.
> 
Because legacy (non-msi) irqs are already ostensibly exposed via
/proc/bus/pci/devices/.../irq.  So non-msi irqs are already covered.

> Also, you have a nice long changelog, but in five years, the details
> about previous attempts and changes between v1/v2/v3 will be useless.
> Can you replace it with a short summary of what the patch *does*,
> maybe something along the lines of the
> Documentation/ABI/testing/sysfs-bus-pci update?  (BTW, that update
> contains a couple typos -- "set subdirectories," "vecotor")
> 
Yeah, I can repost a v4 with a more concise changelog.  I'll post it tomorrow.

Thanks!
NEil

> Bjorn
> 
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