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

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