linux-kernel - RE: [PATCH V3 1/3] x86/ioapic: Support hot-removal of IOAPICs present during boot

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Message-Id: <1466579612-20417-1-git-send-email-rui.y.wang@intel.com>
Date:	Wed, 22 Jun 2016 15:13:32 +0800
From:	Rui Wang <rui.y.wang@...el.com>
To:	helgaas@...nel.org
Cc:	tglx@...utronix.de, rjw@...ysocki.net, tony.luck@...el.com,
	bhelgaas@...gle.com, linux-acpi@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	rui.y.wang@...el.com
Subject: RE: [PATCH V3 1/3] x86/ioapic: Support hot-removal of IOAPICs present during boot

On Friday, June 17, 2016 1:10 AM, Bjorn Helgaas wrote:
> ioapic_insert_resources() is x86-specific, but I'm not sure why; it seems
> like it does things that should be applicable to ia64 as well.
> 
> acpi_ioapic_add() is not x86-specific, and it is called from
> acpi_pci_root_add() for the hot-add case.  You're adding an x86-xpecific call
> in pci_assign_unassigned_resources().  Why should the hot-add case be for
> all arches, but the boot-time case only for x86?

Hi Bjorn,

It turns out that IOAPIC hotplug has not been pursued on ia64. There were
demos showing CPU sockets online/offline on ia64 but the CPUs had no IIO,
thus no IOAPIC hotplug.

So to answer the first question:
ioapic_insert_resources() is x86-specific because it's inserting what has
been setup in io_apic_init_mappings() which, through mpc_ioapic_addr(), is
capable of handling both the static case (acpi_parse_ioapic(), etc.) and the
hotplug case (acpi_ioapic_add()). But on ia64, there's only the static
case through acpi_parse_iosapic(), no need for the hotplug case yet.

To answer the second question:
acpi_ioapic_add() is in effect x86-specific, because it's an empty function
when CONFIG_ACPI_HOTPLUG_IOAPIC isn't defined. And CONFIG_ACPI_HOTPLUG_IOAPIC
depends on CONFIG_X86_IO_APIC (see drivers/acpi/Kconfig). This was introduced
in c183619b6 (x86/irq, ACPI: Implement ACPI driver to support IOAPIC hotplug).
That commit shows the dependency.

I'll send a newer version with comments explaining these.

Thanks
Rui