linux-kernel - Re: [RFC][Update 2][PATCH 1/4] ACPI / PM: Export power states of ACPI devices via sysfs

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Date:	Tue, 22 Jan 2013 01:48:44 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 1/4] ACPI / PM: Export power states of ACPI devices via sysfs

On Monday, January 21, 2013 03:08:04 PM Greg Kroah-Hartman wrote:
> On Mon, Jan 21, 2013 at 11:59:12PM +0100, Rafael J. Wysocki wrote:
> > On Monday, January 21, 2013 02:26:47 PM Greg Kroah-Hartman wrote:
> > > On Mon, Jan 21, 2013 at 11:27:32PM +0100, Rafael J. Wysocki wrote:
> > > > On Monday, January 21, 2013 12:53:05 PM Greg Kroah-Hartman wrote:
> > > > > On Mon, Jan 21, 2013 at 02:04:32PM +0100, Rafael J. Wysocki wrote:
> > > > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > > > 
> > > > > > Make it possible to retrieve the current power state of a device with
> > > > > > ACPI power management from user space via sysfs by adding a new
> > > > > > attribute power_state to the sysfs directory associated with the
> > > > > > struct acpi_device object representing the device's ACPI node.
> > > > > > 
> > > > > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > > > ---
> > > > > >  Documentation/ABI/testing/sysfs-devices-power_state |   21 ++++++++++++++
> > > > > >  drivers/acpi/scan.c                                 |   29 +++++++++++++++++++-
> > > > > >  2 files changed, 49 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > Index: linux-pm/drivers/acpi/scan.c
> > > > > > ===================================================================
> > > > > > --- linux-pm.orig/drivers/acpi/scan.c
> > > > > > +++ linux-pm/drivers/acpi/scan.c
> > > > > > @@ -178,6 +178,23 @@ err_out:
> > > > > >  }
> > > > > >  EXPORT_SYMBOL(acpi_bus_hot_remove_device);
> > > > > >  
> > > > > > +static ssize_t power_state_show(struct device *dev,
> > > > > > +				struct device_attribute *attr, char *buf)
> > > > > > +{
> > > > > > +	struct acpi_device *adev = to_acpi_device(dev);
> > > > > > +	int state;
> > > > > > +	int ret;
> > > > > > +
> > > > > > +	ret = acpi_device_get_power(adev, &state);
> > > > > > +	if (ret)
> > > > > > +		return ret;
> > > > > > +
> > > > > > +	return sprintf(buf, "%s %s\n", acpi_power_state_string(state),
> > > > > > +		       acpi_power_state_string(adev->power.state));
> > > > > > +}
> > > > > 
> > > > > You are showing 2 different things here in a single sysfs file, which is
> > > > > really frowned apon.  Any chance to split this up into two different
> > > > > sysfs files instead?
> > > > 
> > > > Well, I can, but I'm not sure how to call the other one.  "sw_power_state"
> > > > perhaps?
> > > 
> > > I don't know, as I'm not quite sure what it is supposed to represent :)
> > 
> > The first one is power state as read using _PSC or inferred from power
> > resources on/off configuration.  That's easy.
> > 
> > Now, if power resources are shared between two or more devices, it is possible
> > that one device will be in, say, D3hot from the software point of view, but its
> > real ("physical") power state will be different, because the other devices
> > still keep the shared resource "on".  In that case, if the "software" power
> > state of a device is lower-power (higher-number) than its real power state,
> > we know that that particular device doesn't prevent the shared resource from
> > being turned off.  This is good to know, I think. :-)
> 
> I agree that it's good to know, just what to call it.  I think you just
> named it with "real_power_state", right?

Yes, I think I'll just call them "power_state" and "real_power_state".  That
should be clear enough.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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