RE: [PATCH v4 0/6] ACPI: Add _OST support for ACPI hotplug

[Moore, Robert]

It does not make any difference. Essentially, a get_handle is performed by evaluate_object anyway.



>-----Original Message-----
>From: Toshi Kani [mailto:toshi.kani@xxxxxx]
>Sent: Tuesday, May 29, 2012 4:43 PM
>To: shuahkhan@xxxxxxxxx
>Cc: Moore, Robert; lenb@xxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx;
>bhelgaas@xxxxxxxxxx; liuj97@xxxxxxxxx; andi@xxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx
>Subject: RE: [PATCH v4 0/6] ACPI: Add _OST support for ACPI hotplug
>
>On Tue, 2012-05-29 at 16:44 -0600, Shuah Khan wrote:
>> On Tue, 2012-05-29 at 22:27 +0000, Moore, Robert wrote:
>> > > > 2. Calling acpi_get_handle() on _OST prior to executing the method.
>> > > > This will ensure that this method only gets run if it is present
>> > > under
>> > > > the device in question. Coupled with what is already outlined in #1
>> > > > above, now _OST gets executed only when it is defined under the
>> > > device object.
>> > > > Example case in the existing code, please see
>> > > acpi_processor_ppc_ost()
>> > > > implementation.
>> > >
>> > > Yes, I did look at acpi_processor_ppc_ost() when I implemented the
>> > > function.  I believe calling acpi_get_handle() is redundant since
>> > > acpi_ns_get_node() is called within acpi_evaluate_object() as well.
>> > > acpi_evaluate_object() simply returns with AE_NOT_FOUND when _OST
>> > > method does not exist.
>> > >
>> >
>> > This is correct. If _OST does not exist, AE_NOT_FOUND will be returned
>from evaluate_object.
>>
>> Yes that is correct from the ACPI Spec and implementation point of view.
>> My thinking is that a call to acpi_get_handle() might not penalize the
>> OS as much as acpi_evaluate_object() would on systems that don't
>> actually implement _OST. In other words, acpi_get_handle() might not go
>> as deep as acpi_evaluate_object() would go into the ACPI layer, hence
>> might be a safer measure on platforms that don't actually implement this
>> optional method under all devices included in this patch set.
>>
>
>I do not think we need to worry about it.  The code difference is not
>that much, and this _OST path is limited to ACPI hotplug operations,
>which are infrequent events.  Automatic workload balancing can make
>frequent use of the operations, but is not frequent enough to make any
>difference here.  I think simpler code works fine.
>
>Thanks,
>-Toshi

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