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

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Date:	Fri, 3 Oct 2008 10:51:49 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	yi.y.yang@...el.com, <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

Responding just Andrew's comments, disregarding whether or not the 
patch itself is worthwhile...

On Thu, 2 Oct 2008, Andrew Morton wrote:

> <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.

Look again.  The macro doesn't _add_ 1000 to a jiffies value; it 
_divides_ the value by 1000.  This is because EHCI_SHRINK_FRAMES is in 
milliseconds.

However this could be changed to

	t = msecs_to_jiffies(EHCI_SHRINK_FRAMES) + 1;

even though that would involve more runtime code.

> 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.

That subsequent round_jiffies() is most likely a mistake.

> Also, do we really need to inline this large function into at least
> five callsites?

I agree; this function should not be inline.

Alan Stern

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