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Subject: RE: [(repost) git Patch 1/1] avoid IRQ0 ioapic pin collision
Date: Thu, 4 May 2006 01:07:29 -0400
Message-ID: <CFF307C98FEABE47A452B27C06B85BB656C800@hdsmsx411.amr.corp.intel.com>
Thread-Topic: [(repost) git Patch 1/1] avoid IRQ0 ioapic pin collision
Thread-index: AcZtsT6vgePsEi1vT3SuA0EDhIDZnQAAVFawAF4YQWA=
From: "Brown, Len" <len.brown@intel.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Protasevich, Natalie" <Natalie.Protasevich@UNISYS.com>,
       "Andi Kleen" <ak@suse.de>, <sergio@sergiomb.no-ip.org>,
       "Kimball Murray" <kimball.murray@gmail.com>,
       <linux-kernel@vger.kernel.org>, <akpm@digeo.com>, <kmurray@redhat.com>,
       <linux-acpi@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1726
Lines: 44

>>> We should never have had a problem with un-connected interrupt lines
>>> consuming vectors, as the vectors are handed out at run-time
>>> only when interrupts are requested by drivers.
>>
>>Incorrect.  By being requested by drivers I assume you mean by
>>request_irq.  assign_irq_vector is called long before request_irq
>>and in fact there is currently not a call from request_irq to
>>assign_irq_vector.  Look where assign_irq_vector is called in 
>io_apic.c

Hmmm, on Natalie's ping, I looked at this again.

setup_IO_APIC_irqs() actually consumes only 16 vectors.
This is because it only sets up the IRQs for pins
found with find_irq_entry(), which searches mp_irqs[]
which at this point contains only the legacy identify mappings.

The PNP code then takes a swing at things, and it registers
a handful of gsi's, but they are all duplicates of the legacy
mappings already set-up, so no additional vectors are consumed.

So on a big system, the large quantity of vectors will not be consumed
at
IOAPIC initialization time, but later at device probe time.
Not via request_irq(), but via pci_enable_device():

pci_enable_device()
 pci_enable_device_bars()
  pcibios_enable_device()
   pcibios_enable_irq() -> acpi_pci_irq_enable()
    acpi_register_gsi()
     mp_register_gsi()
      io_apic_set_pci_routing()
       entry.vector = assign_irq_vector(irq)

So except for the legacy IRQs, we are already allocating the vectors
on-demand, and that doesn't need to be fixed.

-Len
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