linux-kernel - Re: [patch 2.6.28-rc2] atmel_serial: keep clock off when it's not needed

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Date:	Tue, 28 Oct 2008 10:08:12 -0700
From:	David Brownell <david-b@...bell.net>
To:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Nicolas Ferre <nicolas.ferre@....atmel.com>,
	Andrew Victor <linux@...im.org.za>
Subject: Re: [patch 2.6.28-rc2] atmel_serial: keep clock off when it's not needed

On Tuesday 28 October 2008, Haavard Skinnemoen wrote:
> David Brownell <david-b@...bell.net> wrote:
> > On Tuesday 28 October 2008, Haavard Skinnemoen wrote:
> > > David Brownell <david-b@...bell.net> wrote:
> > > > From: David Brownell <dbrownell@...rs.sourceforge.net>
> > > > 
> > > > The atmel_serial driver is mismanaging its clock by leaving it on
> > > > at all times ... the whole point of clock management is to leave
> > > > it off unless it's actively needed, which conserves power!!
> > > > 
> > > > Signed-off-by: David Brownell <dbrownell@...rs.sourceforge.net>
> > > 
> > > Hmm...the only remaining clk_enable() is in atmel_serial_pm(). Is that
> > > really enough?
> > > 
> > > It looks like the serial core calls ->pm() to power the port up before
> > > doing anything that might touch the registers, but I can't see that the
> > > console layer does the same thing...
> > 
> > I verified it on AT91, where the console is normally DBGU and the
> > other USARTs do get an open().
> > 
> > Didn't verify on AVR32, since 2.6.28-rc can't see the root FS because
> > of that NOR flash problem.  Though I suppose I can try it on an older
> > kernel.
> > 
> > As a rule the boot loader will be using that USART, and thus will
> > have enabled its clock.  :)
> 
> Hmm. But in the cases when "normally" and "as a rule" don't apply, the
> board will lock up solid with no console output since it will wait
> forever for the TXRDY bit to be set...
> 
> I'd like to know for sure that it cannot happen, please.

I'd just like to know that this bug is fixed.  Do you have a
better fix?  (And a board broken by this patch, to verify it?)

- Dave

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