Subject: [Linaro-acpi] [RFC] ACPI on arm64 TODO List

From: [email-masked] (Hanjun Guo)
Date: Thu, 15 Jan 2015 12:07:45 +0800

On 2015?01?14? 08:26, Al Stone wrote:
> On 01/13/2015 10:22 AM, Grant Likely wrote:
>> On Mon, Jan 12, 2015 at 7:40 PM, Arnd Bergmann <[email-masked]> wrote:
>>> On Monday 12 January 2015 12:00:31 Grant Likely wrote:
>>>> On Mon, Jan 12, 2015 at 10:21 AM, Arnd Bergmann <[email-masked]> wrote:
>>>>> On Saturday 10 January 2015 14:44:02 Grant Likely wrote:
>>>>>> On Wed, Dec 17, 2014 at 10:26 PM, Grant Likely <[email-masked]> wrote:
>>>>> This seems like a great fit for AML indeed, but I wonder what exactly
>>>>> we want to hotplug here, since everything I can think of wouldn't need
>>>>> AML support for the specific use case of SBSA compliant servers:
>>>>
>>>> [...]
>>>>
>>>> I've trimmed the specific examples here because I think that misses
>>>> the point. The point is that regardless of interface (either ACPI or
>>>> DT) there are always going to be cases where the data needs to change
>>>> at runtime. Not all platforms will need to change the CPU data, but
>>>> some will (say for a machine that detects a failed CPU and removes
>>>> it). Some PCI add-in boards will carry along with them additional data
>>>> that needs to be inserted into the ACPI namespace or DT. Some
>>>> platforms will have system level component (ie. non-PCI) that may not
>>>> always be accessible.
>>>
>>> Just to be sure I get this right: do you mean runtime or boot-time
>>> (re-)configuration for those?
>>
>> Both are important.
>>
>>>> ACPI has an interface baked in already for tying data changes to
>>>> events. DT currently needs platform specific support (which we can
>>>> improve on). I'm not even trying to argue for ACPI over DT in this
>>>> section, but I included it this document because it is one of the
>>>> reasons often given for choosing ACPI and I felt it required a more
>>>> nuanced discussion.
>>>
>>> I can definitely see the need for an architected interface for
>>> dynamic reconfiguration in cases like this, and I think the ACPI
>>> model actually does this better than the IBM Power hypervisor
>>> model, I just didn't see the need on servers as opposed to something
>>> like a laptop docking station to give a more obvious example I know
>>> from x86.
>
> I know of at least one server product (non-ARM) that uses the
> hot-plugging of CPUs and memory as a key feature, using the
> ACPI OSPM model.  Essentially, the customer buys a system with
> a number of slots and pays for filling one or more of them up
> front.  As the need for capacity increases, CPUs and/or RAM gets
> enabled; i.e., you have spare capacity that you buy as you need
> it.  If you use up all the CPUs and RAM you have, you buy more
> cards, fill the additional slots, and turn on what you need.  This
> is very akin to the virtual machine model, but done with real hardware
> instead.

There is another important user case for RAS, systems running critical
missions such as bank billing system, system like that need high
reliability that the machine can't be stopped.

So when error happened on hardware including CPU/memory DIMM on such
machines, we need to replace them at run-time.

>
> Whether or not this product is still being sold, I do not know.  I
> have not worked for that company for eight years, and they were just
> coming out as I left.  Regardless, this sort of hot-plug does make
> sense in the server world, and has been used in shipping products.

I think it still will be, Linux developers put lots of effort to
enable memory hotplug and computer node hotplug in the kernel [1], and
the code already merged into mainline.

[1]:
http://events.linuxfoundation.org/sites/events/files/lcjp13_chen.pdf

Thanks
Hanjun