linux-kernel - Re: [PATCH] apds9802als: add runtime PM support

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Date:	Tue, 26 Oct 2010 09:51:45 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Hong Liu <hong.liu@...el.com>
cc:	Alan Cox <alan@...ux.intel.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH] apds9802als: add runtime PM support

On Tue, 26 Oct 2010, Hong Liu wrote:

> > > diff --git a/drivers/misc/apds9802als.c b/drivers/misc/apds9802als.c
> > > index fbe4960..15f9436 100644
> > > --- a/drivers/misc/apds9802als.c
> > > +++ b/drivers/misc/apds9802als.c
> > 
> > > @@ -265,13 +274,34 @@ static int apds9802als_suspend(struct i2c_client *client, pm_message_t mesg)
> > >  
> > >  static int apds9802als_resume(struct i2c_client *client)
> > >  {
> > > -	struct als_data *data = i2c_get_clientdata(client);
> > > +	als_set_default_config(client);
> > > +
> > > +	pm_runtime_get(&client->dev);
> > > +	pm_runtime_put(&client->dev);
> > > +	return 0;
> > > +}
> > 
> 
> Hi, Alan
> 
> Thanks for your review.
> 
> > This almost certainly does not do what you think.
> > 
> > pm_runtime_get() will increment the device's usage count and queue an 
> > asynchronous resume request.  However, since the PM workqueue is frozen 
> > during system sleep transitions, the device will remain suspended.
> > 
> > The pm_runtime_put() will decrement the usage count again, but since
> > there is already an async resume on the queue it will not queue an
> > async suspend.  The final result will be that when tasks are unfrozen,
> > the device will finally be resumed -- long after it should have been.
> > 
> > It looks like what you want to do here is simply call
> > apds9802als_runtime_resume() directly.  
> 
> You mean apds9802als_runtime_suspend()?

This is about the function listed above: apds9802als_resume().  It
should call apds9802als_runtime_resume() directly.  I don't see any
reason why you would want it to call apds9802als_runtime_suspend() --
having a resume function that actually suspends the device doesn't make
any sense.

> I want to put the device into
> runtime suspended state, can I just call pm_runtime_suspend() directly?

Why do you want apds9802als_resume() to put the device into a suspended 
state?  Shouldn't it _resume_ the device?

> > And according to the advice in
> > Documentation/power/runtime_pm.txt section 6, you should also call

Have you read that document?

> > 	pm_runtime_disable(dev);
> > 	pm_runtime_set_active(dev);
> > 	pm_runtime_enable(dev);
> > 
> > You probably also do not want the asynchronous calls to 
> > pm_runtime_get() and pm_runtime_put() in apds9802als_probe().  A more 
> > common sequence is:
> > 
> > 	pm_runtime_set_active(dev);
> > 	pm_runtime_enable(dev);
> 
> I want to put the device into runtime suspend state after probe, so
> pm_runtime_suspend() after these two calls is OK?

No.  Try doing what I said.  You should find that the device _does_ get
runtime-suspended when the probe function returns.  Assuming you have
defined a proper apds9802als_runtime_idle() function.

Alan Stern

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