linux-kernel - Re: Enable arm_global_timer for Zynq brakes boot

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Date:	Tue, 30 Jul 2013 15:14:43 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	Stephen Boyd <sboyd@...eaurora.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stuart Menefy <stuart.menefy@...com>,
	Russell King <linux@....linux.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: Enable arm_global_timer for Zynq brakes boot

On Tue, Jul 30, 2013 at 10:47:15AM +0200, Daniel Lezcano wrote:
> On 07/30/2013 02:03 AM, Sören Brinkmann wrote:
> > Hi Daniel,
> > 
> > On Mon, Jul 29, 2013 at 02:51:49PM +0200, Daniel Lezcano wrote:
> > (snip)
> >>
> >> the CPUIDLE_FLAG_TIMER_STOP flag tells the cpuidle framework the local
> >> timer will be stopped when entering to the idle state. In this case, the
> >> cpuidle framework will call clockevents_notify(ENTER) and switches to a
> >> broadcast timer and will call clockevents_notify(EXIT) when exiting the
> >> idle state, switching the local timer back in use.
> > 
> > I've been thinking about this, trying to understand how this makes my
> > boot attempts on Zynq hang. IIUC, the wrongly provided TIMER_STOP flag
> > would make the timer core switch to a broadcast device even though it
> > wouldn't be necessary. But shouldn't it still work? It sounds like we do
> > something useless, but nothing wrong in a sense that it should result in
> > breakage. I guess I'm missing something obvious. This timer system will
> > always remain a mystery to me.
> > 
> > Actually this more or less leads to the question: What is this
> > 'broadcast timer'. I guess that is some clockevent device which is
> > common to all cores? (that would be the cadence_ttc for Zynq). Is the
> > hang pointing to some issue with that driver?
> 
> If you look at the /proc/timer_list, which timer is used for broadcasting ?

In case of a vanilla kernel and with my patches for enabling the global
timer (I removed the wrongly set flag from the C2 state + adding the DT
fragment to use the GT), this is what I see (full output from timer_list
attached):
	Tick Device: mode:     1
	Broadcast device
	Clock Event Device: xttcps_clockevent

And the local timer seems to be the arm twd timer (also in both cases).

I don't think I can gather this information for the actual broken case,
but AFAIK, there shouldn't be any other timer be capable of this for
Zynq.

	Sören


View attachment "timer_list" of type "text/plain" (4105 bytes)