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    SubjectRE: [PATCH v4 0/6] ACPI: Add _OST support for ACPI hotplug
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    It does not make any difference. Essentially, a get_handle is performed by evaluate_object anyway.



    >-----Original Message-----
    >From: Toshi Kani [mailto:toshi.kani@hp.com]
    >Sent: Tuesday, May 29, 2012 4:43 PM
    >To: shuahkhan@gmail.com
    >Cc: Moore, Robert; lenb@kernel.org; linux-acpi@vger.kernel.org;
    >bhelgaas@google.com; liuj97@gmail.com; andi@firstfloor.org; linux-
    >kernel@vger.kernel.org
    >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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