linux-kernel - Re: [RFC PATCHv3 3/4] x86/pci: Initial commit for new VMD device driver

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Message-ID: <20151104144905.GI13904@localhost.localdomain>
Date:	Wed, 4 Nov 2015 14:49:05 +0000
From:	Keith Busch <keith.busch@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-pci@...r.kernel.org, Jiang Liu <jiang.liu@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Bryan Veal <bryan.e.veal@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Martin Mares <mj@....cz>,
	Jon Derrick <jonathan.derrick@...el.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCHv3 3/4] x86/pci: Initial commit for new VMD device
 driver

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?
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