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Date: Wed, 4 Nov 2015 14:49:05 +0000
From: Keith Busch <keith.busch@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
	linux-pci@vger.kernel.org, Jiang Liu <jiang.liu@linux.intel.com>,
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	Jon Derrick <jonathan.derrick@intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCHv3 3/4] x86/pci: Initial commit for new VMD device
 driver
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On Tue, Nov 03, 2015 at 12:42:02PM +0100, Thomas Gleixner wrote:
> On Tue, 3 Nov 2015, Keith Busch wrote:
> > > > +	msi_irqdomain = pci_msi_create_irq_domain(NULL, &pci_chained_msi_domain_info,
> > > > +						  vmd_irqdomain);
> 
> But that parent limitation does not matter simply because your
> msi_irqdomain does not follow down the hierarchy in the allocation
> path.
> 
> So we can avoid the vmd_irqdomain creation completely. It's just
> wasting memory and has no value at all. Creating the msi domain with a
> NULL parent is possible.

I'm having trouble following the hierarchy and didn't understand the
connection between the parent and msi comain. It's still new to me,
but I don't think a NULL parent is allowable with msi domains:

 pci_msi_setup_msi_irqs()
  pci_msi_domain_alloc_irqs()
   msi_domain_alloc_irqs()
    __irq_domain_alloc_irqs()
     irq_domain_alloc_irqs_recursive()
      msi_domain_alloc()
       irq_domain_alloc_irqs_parent()

The last call returns -ENOSYS since there parent is NULL. Was the
intension to allow no parent, or do I still need to allocate one to
achieve the desired chaining?