netdev - Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern

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Date:	Mon, 7 Oct 2013 21:46:06 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Tejun Heo <tj@...nel.org>, Alexander Gordeev <agordeev@...hat.com>,
	<linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"Ralf Baechle" <ralf@...ux-mips.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Andy King <acking@...are.com>,
	"Jon Mason" <jon.mason@...el.com>,
	Matt Porter <mporter@...nel.crashing.org>,
	<linux-pci@...r.kernel.org>, <linux-mips@...ux-mips.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux390@...ibm.com>,
	<linux-s390@...r.kernel.org>, <x86@...nel.org>,
	<linux-ide@...r.kernel.org>, <iss_storagedev@...com>,
	<linux-nvme@...ts.infradead.org>, <linux-rdma@...r.kernel.org>,
	<netdev@...r.kernel.org>, <e1000-devel@...ts.sourceforge.net>,
	<linux-driver@...gic.com>,
	"Solarflare linux maintainers" <linux-net-drivers@...arflare.com>,
	"VMware, Inc." <pv-drivers@...are.com>,
	<linux-scsi@...r.kernel.org>
Subject: Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement
 pattern

On Tue, 2013-10-08 at 07:10 +1100, Benjamin Herrenschmidt wrote:
> On Mon, 2013-10-07 at 14:01 -0400, Tejun Heo wrote:
> > I don't think the same race condition would happen with the loop.  The
> > problem case is where multiple msi(x) allocation fails completely
> > because the global limit went down before inquiry and allocation.  In
> > the loop based interface, it'd retry with the lower number.
> > 
> > As long as the number of drivers which need this sort of adaptive
> > allocation isn't too high and the common cases can be made simple, I
> > don't think the "complex" part of interface is all that important.
> > Maybe we can have reserve / cancel type interface or just keep the
> > loop with more explicit function names (ie. try_enable or something
> > like that).
> 
> I'm thinking a better API overall might just have been to request
> individual MSI-X one by one :-)
> 
> We want to be able to request an MSI-X at runtime anyway ... if I want
> to dynamically add a queue to my network interface, I want it to be able
> to pop a new arbitrary MSI-X.

Yes, this would be very useful.

> And we don't want to lock drivers into contiguous MSI-X sets either.

I don't think there's any such limitation now.  The entries array passed
to pci_enable_msix() specifies which MSI-X vectors the driver wants to
enable.  It's usually filled with 0..nvec-1 in order, but not always.
And the IRQ numbers returned aren't usually contiguous either, on x86.

Ben.

> And for the cleanup ... well that's what the "pcim" functions are for,
> we can just make MSI-X variants.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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