linux-kernel - Re: [PATCH] USB: improve ehci_watchdog's side effect in CPU power management

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Message-Id: <1223462420.2845.11.camel@yangyi-dev>
Date:	Wed, 08 Oct 2008 18:40:20 +0800
From:	Yi Yang <yi.y.yang@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, dbrownell@...rs.sourceforge.net,
	gregkh@...e.de, linux-usb@...r.kernel.org
Subject: Re: [PATCH] USB: improve ehci_watchdog's side effect in CPU power
	management

On Thu, 2008-10-02 at 16:56 -0700, Andrew Morton wrote:
> On Thu, 25 Sep 2008 17:25:44 +0800
> Yi Yang <yi.y.yang@...el.com> wrote:
> 
> > ehci_watchdog will wake up CPU very frequently so that CPU
> > stays at C3 very short, average residence time is about 50
> > ms on Aspire One, but we expect it should be about 1 second
> > or more, so this kind of periodic timer is very bad for power
> > saving.
> > 
> > We can't remove this timer because of some bad USB controller
> > chipset, but at least we should reduce its side effect to as
> > possible as low.
> > 
> > This patch can make CPU stay at C3 longer, average residence time
> > is about twice as long as original. 
> > 
> > Please consider to apply it, thanks
> > 
> > Signed-off-by: Yi Yang <yi.y.yang@...el.com>
> > ---
> >  ehci.h |   12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/usb/host/ehci.h b/drivers/usb/host/ehci.h
> > index 5799298..9d530d9 100644
> > --- a/drivers/usb/host/ehci.h
> > +++ b/drivers/usb/host/ehci.h
> > @@ -181,14 +181,16 @@ timer_action (struct ehci_hcd *ehci, enum ehci_timer_action action)
> >  	 * the async ring; just the I/O watchdog.  Note that if a
> >  	 * SHRINK were pending, OFF would never be requested.
> >  	 */
> > -	if (timer_pending(&ehci->watchdog)
> > -			&& ((BIT(TIMER_ASYNC_SHRINK) | BIT(TIMER_ASYNC_OFF))
> > -				& ehci->actions))
> > -		return;
> > +	enum ehci_timer_action oldactions = ehci->actions;
> >  
> >  	if (!test_and_set_bit (action, &ehci->actions)) {
> >  		unsigned long t;
> >  
> > +		if (timer_pending(&ehci->watchdog)
> > +			&& ((BIT(TIMER_ASYNC_SHRINK) | BIT(TIMER_ASYNC_OFF))
> > +				& oldactions))
> > +			return;
> > +
> >  		switch (action) {
> >  		case TIMER_IO_WATCHDOG:
> >  			t = EHCI_IO_JIFFIES;
> > @@ -204,7 +206,7 @@ timer_action (struct ehci_hcd *ehci, enum ehci_timer_action action)
> >  			t = DIV_ROUND_UP(EHCI_SHRINK_FRAMES * HZ, 1000) + 1;
> >  			break;
> >  		}
> > -		mod_timer(&ehci->watchdog, t + jiffies);
> > +		mod_timer(&ehci->watchdog, round_jiffies(t + jiffies));
> >  	}
> >  }
> >  
> 
> <looks>
> 
> <regrets it>
> 
> 
> Why does this:
> 
> 	t = DIV_ROUND_UP(EHCI_SHRINK_FRAMES * HZ, 1000) + 1;
> 
> add "1000" to a jiffies value when it doesn't know what HZ is?  It'll
> be adding anywhere from one second up to ten seconds to the timeout
> interval depending upon compile-time options.
I don't know why original author did so.

> 
> I suspect s/1000/HZ/ would improve things here.  Or just delete it -
> doesn't the subsequent round_jiffies() do the same thing, only better? 
> This code needs help, I suspect.
If 1000 is substituted with HZ, this computation is meaningless, we can
invoke EHCI_SHRINK_FRAMES directly.

I think the best way to do with this bad case is to remove such a
watchdog completely unless we detect a bad usb chipset which will lost
data frame if we don't start watchdog timer.
> 
> 
> Also, do we really need to inline this large function into at least
> five callsites?
> 

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