linux-kernel - Re: [RFC PATCH 4/6] PM / Runtime: Introduce flag can_power

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Message-Id: <201202212243.52788.rjw@sisk.pl>
Date:	Tue, 21 Feb 2012 22:43:52 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Zhang Rui <rui.zhang@...el.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Lin Ming <ming.m.lin@...el.com>,
	Jeff Garzik <jgarzik@...ox.com>, Tejun Heo <tj@...nel.org>,
	Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH 4/6] PM / Runtime: Introduce flag can_power_off

On Tuesday, February 21, 2012, Zhang Rui wrote:
> On 二, 2012-02-21 at 00:13 +0100, Rafael J. Wysocki wrote:
> > > So how to handle this case, say, for a device in the generic PM domain
> > > that supports 2 different low power state, D1 and D2.
> > > D2 is deeper than D1, and it is kind of cold power off with remote
> > > wakeup disabled. If the driver needs to runtime suspend the device with
> > > remote wakeup enabled, it should set the device to D1, but it can not
> > > set the RPM_SUSPEND?
> > 
> > The device is regarded as "suspended" if its bus type's (or PM domain's)
> > .runtime_suspend() callback has been executed and has returned 0 (success).
> > What the callback has actually done is not of any interest to the core.
> > 
> right.
> 
> > Now, the D1 and D2 case has to be handled by the bus (PM domain) and
> > driver.  In both cases the device will be regarded as "suspended" and the
> > core doesn't track the actual device state.
> > 
> 
> 
> > I think the problem here is that the PCI bus type's runtime PM callbacks
> > aren't very sophisticated (they just choose the lowest possible low-power
> > state and attempt to put the device into it) and I can see two possible
> > ways to address that.
> > 
> > First, you can modify pci_pm_runtime_suspend/_resume() to handle multiple
> > states (for example, to choose the target low-power state more intelligently
> > than they do right now).  Second, you can add a PM domain that will do what
> > you want from pci_pm_runtime_suspend/_resume() for a specific set of devices.
> > 
> But RPM_SUSPENDED is set by PM core after .runtime_suspend() being
> invoked, even if device is in D1 instead of D2, right?
> 
> So the problem is that, if a device in a generic power domain supports
> two low power state, one is compatible with generic power domain power
> off and another is not, how can the device driver pass this information
> to the generic power domain, i.e. how to runtime suspend a device while
> keep the generic power domain always on?

There are two "low-power" levels in the generic PM domains framework.  The
first one is the per-device low-power in which devices are put into their
individual (programmable) low-power states by the domain .dev_ops->stop()
callback.  The second one is when .stop() has been called for all devices,
so presumably all of them are in programmable low-power states and it's
possible to switch the entire domain off.  This is done by the domain
.power_off() callback.

It seems that the trick might be to make .dev_ops->stop() avoid turning off
power resources for the last suspending device in the domain and leave that
to domain .power_off().

Thanks,
Rafael
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