Re: [PATCHv3 01/10] clocksource: add generic dummy timer driver

From: Stephen Boyd <sboyd@codeaurora.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: "linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-msm@vger.kernel.org" <linux-arm-msm@vger.kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCHv3 01/10] clocksource: add generic dummy timer driver
Date: Mon, 25 Mar 2013 09:49:18 -0700	[thread overview]
Message-ID: <5150800E.2000905@codeaurora.org> (raw)
In-Reply-To: <20130322180305.GB1436@e106331-lin.cambridge.arm.com>

On 03/22/13 11:03, Mark Rutland wrote:
> On Thu, Mar 21, 2013 at 06:13:17PM +0000, Stephen Boyd wrote:
>> On 03/21/13 11:09, Mark Rutland wrote:
>>> Hi Stephen,
>>>
>>> I've just been trying to test the dummy timer, and realised it's broken, as it
>>> registers a cpu notifier from a device_initcall (after SMP's been brought up),
>>> and doesn't ensure all active CPUs have been set up. Evidently no-one else has
>>> attempted to test it thus far, and I'm not able to throughly test it at the
>>> moment.
>> Would it be sufficient to register as a pre-smp initcall?
> I've looked a bit further into the problem, and I believe using early_initcall
> will make it work as well as the current arm-specific dummy timers work with a
> rating of 100 (this means the recent patch lowering the rating broke tc2 as
> explained below).

Yes, moving to early_initcall() should make the dummy timer equivalent
to the code that is already in the arm directory. It sounds like there
is a problem with mainline though?

>
> I've spent the last few hours trying to get the dummy_timer driver working on
> tc2 with the sp804 as the broadcast source (with architected timer support
> disabled). It turns out that having dummy timer's rating so low means that it
> won't be selected as the tick device on cpu0 in preference to the sp804, and
> thus won't push the sp804 out of that spot (allowing it to become the broadcast
> source). This leads to boot stalling.

I'm not following here. Why would we want to remove sp804 from the tick
duty?

>
> Jumping the dummy_timer's rating up would fix this, but that doesn't seem
> great. Registering the dummy before all other clocks would also fix this (I
> tried calling dummy_timer_register from time_init), but I can't see a way to do
> that while keeping the driver self-contained.

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