Re: One of these things (CONFIG_HZ) is not like the others..

From: John Stultz <john.stultz@linaro.org>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>, Matt Sealey <matt@genesi-usa.com>,
	Linux ARM Kernel ML <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Ben Dooks <ben@simtec.co.uk>
Subject: Re: One of these things (CONFIG_HZ) is not like the others..
Date: Mon, 21 Jan 2013 14:18:20 -0800	[thread overview]
Message-ID: <50FDBEAC.2050606@linaro.org> (raw)
In-Reply-To: <20130121211218.GX23505@n2100.arm.linux.org.uk>

On 01/21/2013 01:12 PM, Russell King - ARM Linux wrote:
> On Mon, Jan 21, 2013 at 01:00:15PM -0800, John Stultz wrote:
>> So if you can not get actual timer ticks any faster then 200 HZ on that
>> hardware, setting HZ higher could cause some jiffies related timer
>> trouble
> Err, no John.  It's the other way around - especially on some platforms
> which are incapable of being converted to the clock source support.
>
> EBSA110 has _one_ counter.  It counts down at a certain rate, and when
> it rolls over from 0 to FFFF, it produces an interrupt and continues
> counting down from FFFF.
>
> To produce anything close to a reasonable regular tick rate from that,
> the only way to do it is - with interrupts disabled - read the current
> value to find out how far the timer has rolled over, and set it so that
> the next event will expire as close as possible to the desired HZ rate.
>
> So, none of the clcokevent stuff can be used; and we rely _purely_ on
> counting interrupts in jiffy based increments to provide any reference
> of time.
> Moreover, because the counter is only 16-bit, and it's clocked from
> something around 7MHz, well, maths will tell you why 200Hz had to be
> chosen rather than 100Hz.

Ah, so the counter can't do anything *lower* then ~107HZ, right? (7MHZ/2^16)

So we used to have the ACTHZ code to handle error from the HZ rate 
requested and the HZ rate possible given the underlying hardware. That's 
been moved to the register_refined_jiffies(), but do you have a sense if 
there a reason it couldn't be used? I don't quite recall the bounds at 
this second, so ~7% error might very well be too large.

So yes, I suspect these sorts of platforms, where there are no modern 
clocksource/clockevent driver, as well as further constraints (like 
specific HZ) are likely not good candidates for a multi-arch build.

thanks
-john