linux-kernel - Re: [RFC][Update 2][PATCH 2/4] ACPI / PM: Expose reference count values of ACPI power resources

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Date:	Mon, 21 Jan 2013 14:33:43 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Kristen C. Accardi" <kristen.c.accardi@...el.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [RFC][Update 2][PATCH 2/4] ACPI / PM: Expose reference count
 values of ACPI power resources

On Mon, Jan 21, 2013 at 11:35:51PM +0100, Rafael J. Wysocki wrote:
> On Monday, January 21, 2013 12:53:57 PM Greg Kroah-Hartman wrote:
> > On Mon, Jan 21, 2013 at 02:05:47PM +0100, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > 
> > > Since ACPI power resources are going to be used more extensively on
> > > new hardware platforms, it becomes necessary for user space (powertop
> > > in particular) to observe some properties of those resources for
> > > diagnostics purposes.
> > > 
> > > For this reason, export the reference counts of ACPI power resources
> > > to user space by adding a new reference_count attribute to the sysfs
> > > directory representing each power resource.  The value read from
> > > that attribute represents the number of devices using the power
> > > resource at the given time.  If that value is 0, it meas that the
> > > power resource is not used and therefore it has been turned off.
> > 
> > Why does userspace need to know a reference count?  Is it so that if it
> > is not 0, it can work to try to lower it to 0?  Or something else?
> 
> Yes, this information is needed to say if (1) the power resource is in use
> and (2) how many users there are at the moment, so that we can go and look
> if it really has to be in use.
> 
> Perhaps I can just expose the "in use"/"not in use" information.  I don't
> think it will be much less convenient, because we should scan all of the
> possible users anyway in case they are coming and going frequently.
> 
> So should I do that and rename the attribute to "resource_in_use" (or something
> similar)?

That makes a bit more sense to do, thanks.

greg k-h
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