ACPI-Based System Device Hot-Plugging Framework
Written by Michael Larabel in Hardware on 7 October 2012 at 10:56 AM EDT. Add A Comment
For several months there have been developers at Huawei and elsewhere working on an ACPI-based system device hot-plug framework for the Linux kernel.

Now up to its third revision, this hot-plugging framework written against the Advanced Configuration and Power Interface (ACPI) specification continues to advance.

As far as what this hot-plug framework means, Huawei's Jiang Liu describes it as:
Modern high-end server may support advanced hotplug features for system devices, including physical processor, memory board, IO extension board and/or computer node. The ACPI specifications have provided standard interfaces between firmware and OS to support device hotplug at runtime. This patch series provide an ACPI based hotplug framework to support system device hotplug at runtime, which will replace current existing ACPI device driver based CPU/memory/CONTAINER hotplug mechanism.

The new ACPI system device hotplug framework has following design goals:

1) Provide a mechanism to detect hotplug slots by checking ACPI _EJ0 method, ACPI PRCT (platform RAS capabilities table) and other platform specific mechanisms.
2) Unify the way to enumerate ACPI based hotplug slots. All hotplug slots will be enumerated by the enumeration driver, instead of by ACPI device drivers.
3) Unify the way to handle ACPI hotplug events. All ACPI hotplug events for system devices will be handled by a generic ACPI hotplug driver, instead of handled by ACPI device drivers.
4) Solve dependencies among hotplug slots. You need first to remove the memory device before removing a physical processor if a hotpluggable memory device is connected to a hotpluggable physical processor.
5) Provide better error handling and recover.
6) Provide interface to cancel ongoing hotplug operations. It may take a very long time to remove a memory device, so provide interface to cancel the operation. This is important especially for memory hot-removal.
7) Support new RAS features, such as socket/memory migration.
8) Provide better user interfaces to the hotplug functionalities.
9) Trigger hotplug events/operations by software. This feature is useful for hardware fault management and power saving.

The new ACPI based hotplug framework is modelled after PCI hotplug architecture, which uses a hotplug driver to manage hotplug operations and different device drivers to manage individual devices.
The latest v3 patches can be found on the kernel mailing list and will hopefully be merged into a future Linux kernel release. The third version of these kernel patches fix minor bugs, refine the code, implement fake slot enumator for testing purposes, enhance the ACPI container driver for the new framework, enhance the ACPI processor driver for the new framework, and resolve dependencies among slots.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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