linux-kernel - RE: [PATCH v4 0/6] ACPI: Add

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Date:	Tue, 29 May 2012 16:44:56 -0600
From:	Shuah Khan <shuahkhan@...il.com>
To:	"Moore, Robert" <robert.moore@...el.com>
Cc:	shuahkhan@...il.com, Toshi Kani <toshi.kani@...com>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"liuj97@...il.com" <liuj97@...il.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 0/6] ACPI: Add _OST support for ACPI hotplug

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.

-- Shuah


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