linux-kernel - Re: [PATCH v2 01/03] clocksource: Add Kconfig entries for CMT, MTU2, TMU and STI

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Message-ID: <529D12DC.2000303@linaro.org>
Date:	Mon, 02 Dec 2013 15:08:12 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Magnus Damm <magnus.damm@...il.com>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	SH-Linux <linux-sh@...r.kernel.org>,
	"Simon Horman [Horms]" <horms@...ge.net.au>,
	Olof Johansson <olof@...om.net>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 01/03] clocksource: Add Kconfig entries for CMT, MTU2,
 TMU and STI

On 11/27/2013 09:42 PM, Magnus Damm wrote:
> Hi John, everyone,
>
> Let me get back to you all in a little while together with some code,
> but before that let me just clarify this:
>
> On Wed, Nov 13, 2013 at 5:47 AM, John Stultz <john.stultz@...aro.org> wrote:
>> On Tue, Nov 12, 2013 at 4:26 AM, Magnus Damm <magnus.damm@...il.com> wrote:
>>> On Sat, Nov 9, 2013 at 3:34 AM, John Stultz <john.stultz@...aro.org> wrote:
>>> Let me get back to the kernel configuration. Of course, it would be
>>> really nice if the kernel configuration was 100% fool proof, but what
>>> happens if the user doesn't compile-in certain parts? That hardware
>>> won't be used. What happens if wrong console device is passed on the
>>> kernel command line? The friendly answer is usually "don't do that".
>>>
>>> So in case of the serial console, no driver - no output. You can still
>>> use the network. If you have no timer then there won't be any timer
>>> ticks. You can still get to user space though, but don't try to rely
>>> on the timer. This CONFIG_TIMER=y/n case is pretty clear, but isn't
>>> there a grey zone too?
>> And on every new board, I have to fumble around with exactly those
>> sorts of no-serial output issues. Its never something I consider a
>> great use of my time :)
>>
>> And your example is a little flawed have no timer ticks, you're not
>> getting to userspace. The system won't boot.
> Correct me if I'm wrong here but I don't think so!
>
> On some platforms that may very well be the case, but on mach-shmobile
> we get to user space without any timers. If the timers are enabled
> then they are regular platform devices these days. From my experience
> the main blocker for going to user space without timer is on ARM
> usually the udelay() calculation and/or the TWD delay, but those can
> be handled with preset worst-case values for udelay() and getting the
> rate via CCF for the TWD.
>
> Also it of course depends on if some compiled-in driver needs to use
> the timer. If we just enable the serial console then all our platforms
> make it to initramfs-based user space without any clock source or
> clock event.
So something still triggers basic HZ timer functionality, and you get by
with the jiffies clocksource?

Ok, so thanks for the correction. But I still think booting in a
degraded mode isn't ideal when the user has already provided enough info
via config options that we could have sorted it out for them.

But yea, I look forward to your next patch so we can discuss things more
concretely, rather then in my more abstract handwaving rants ;)

thanks
-john


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