linux-kernel - RE: [PATCH v2 3/4] mtd: devices: elm: Low power transition support

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Message-ID: <518397C60809E147AF5323E0420B992E3EA78558@DBDE01.ent.ti.com>
Date:	Tue, 19 Feb 2013 12:48:12 +0000
From:	"Philip, Avinash" <avinashphilip@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	"Hilman, Kevin" <khilman@...com>,
	"tony@...mide.com" <tony@...mide.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"artem.bityutskiy@...ux.intel.com" <artem.bityutskiy@...ux.intel.com>,
	"Mohammed, Afzal" <afzal@...com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"Nori, Sekhar" <nsekhar@...com>,
	"Hebbar, Gururaja" <gururaja.hebbar@...com>,
	"Hiremath, Vaibhav" <hvaibhav@...com>
Subject: RE: [PATCH v2 3/4] mtd: devices: elm: Low power transition support

On Wed, Feb 13, 2013 at 18:13:03, Russell King - ARM Linux wrote:
> On Wed, Feb 13, 2013 at 11:42:01AM +0000, Philip, Avinash wrote:
> > On Sat, Feb 09, 2013 at 15:52:44, Russell King - ARM Linux wrote:
> > > On Thu, Feb 07, 2013 at 06:06:57PM +0530, Philip Avinash wrote:
> > > > +static int elm_suspend(struct device *dev)
> > > > +{
> > > > +	struct elm_info *info = dev_get_drvdata(dev);
> > > > +	wait_queue_head_t wq;
> > > > +	DECLARE_WAITQUEUE(wait, current);
> > > > +
> > > > +	init_waitqueue_head(&wq);
> > > > +	while (1) {
> > > > +		/* Make sure that ELM not running */
> > > > +		if (info->idle) {
> > > > +			add_wait_queue(&wq, &wait);
> > > > +			schedule();
> > > > +			remove_wait_queue(&wq, &wait);
> > > > +		} else {
> > > > +			break;
> > > > +		}
> > > > +	}
> > > 
> > > The above code looks really wrong - it will just spin endlessly with the
> > > waitqueues doing nothing useful.  What are you trying to do here?
> > 
> > The intention of waitqeue is to make the suspend process really wait till
> > the ELM module finishes current activity. Although this type of protection
> > already achieved in mtd layer using nand_suspend(), this one is particularly
> > required for ELM module to really make sure that *any pending* corrections to
> > finish really before gone to suspend.
> 
> I don't think you understand what's going on with the above, and why the
> above is completely ineffective.
> 
> 1. Your wait queue head is declared on stack, and there's no references
>    to it outside of this function.  So _nothing_ can activate the wait
>    queue.
> 2. You're not changing the current thread's status away from TASK_RUNNING,
>    so schedule() will either return immediately or it will schedule another
>    task if it's time for this one to be preempted.
> 
> In other words, the above can be rewritten as:
> 
> 	while (info->idle)
> 		schedule();
> 
> and it will still have the same effect.
> 
> Now, if you want to be kinder to the system, then you should use a
> wait queue properly.  Put the waitqueue head in struct elm_info.  Use
> wait_event() here.  And call wake_up() where you set info->idle to
> false.

I understood the issue. Thanks for the detailed explanation.

It seems the entire mechanism of ELM module state check is not required.
The ELM suspend procedure initiated only after the current MTD transaction
finishes and is ensured in MTD class driver.

So I can simply disable ELM module in suspend without any check.

Thanks
Avinash

> 

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