linux-kernel - Re: [PATCH 33/36] AArch64: Generic timers support

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Date:	Thu, 12 Jul 2012 12:56:31 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>,
	John Stultz <john.stultz@...aro.org>,
	Deepak Saxena <dsaxena@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	Linus Walleij <linus.ml.walleij@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Will Deacon <will.deacon@....com>,
	Mike Turquette <mike.turquette@...aro.org>,
	Marc Zyngier <maz@...terjones.org>
Subject: Re: [PATCH 33/36] AArch64: Generic timers support

On Thu, Jul 12, 2012 at 12:09 PM, Marc Zyngier <marc.zyngier@....com> wrote:

> On Thu, 12 Jul 2012 02:18:42 +0200, Linus Walleij
> <linus.ml.walleij@...il.com> wrote:
>
> Hi Linus,
>
>> I'm reviewing the only patch I really understand...
>>
>> 2012/7/6 Catalin Marinas <catalin.marinas@....com>:
>>
>>> +/* This isn't really used any more */
>>> +#define CLOCK_TICK_RATE 1000
>>
>> Is it still necessary to even have it there?
>
> It is used as part of the LATCH/TICK_* computation in
> include/linux/jiffies.h. It seems that any value could do, actually, and it
> only seem to be used in kernel/time/ntp.c. Any guidance on this much
> appreciated.
>
> By the way, there is a very interesting comment about this in
> arch/ia64/include/asm/timex.h.

Hmmmm paging John Stultz, TGLX, Deepak Saxena who have been in
this area...

>>> +       arch_timer_calibrate();
>>
>> Why is the ability to get this from a clk not contemplated?
>> I think this will be common. You could make it optional I think,
>> just like in the SMP TWD.
>
> _calibrate() is a misnomer here. It should really be _get_freq(). But your
> point still stand, and we could indeed use a clk to obtain the frequency.
>
> It should probably be selected in the following order: clock-frequency
> (DT), clk, CNTFRQ.

I agree. It can be bolted on later too, just trying to think ahead a bit.

I'm half-guessing that for mobile embedded with their complex clock trees
it will likely come from a clk whereas in servers etc there will be one
big clock for all and it will be fine to use the DT node.

Yours,
Linus Walleij
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