linux-kernel - Re: [PATCH v2 0/3] let Marvell Berlin SoCs make use of the best delay timer

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Message-ID: <20151105103628.10d6f1f9@xhacker>
Date:	Thu, 5 Nov 2015 10:36:28 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>, <tglx@...utronix.de>,
	<linux@....linux.org.uk>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] let Marvell Berlin SoCs make use of the best
 delay timer

Dear Arnd and Daniel,

On Wed, 4 Nov 2015 13:19:53 +0100
Arnd Bergmann wrote:

> On Wednesday 04 November 2015 12:19:57 Daniel Lezcano wrote:
> > On 11/04/2015 11:30 AM, Arnd Bergmann wrote:  
> > > On Wednesday 04 November 2015 10:46:49 Daniel Lezcano wrote:  
> > >> On 11/03/2015 03:28 PM, Jisheng Zhang wrote:  
> > >>> In case there are several possible delay timers, we purely base the
> > >>> selection on the frequency, which is suboptimal in some cases. Take
> > >>> one Marvell Berlin platform for example: we have arch timer and dw-apb
> > >>> timer. The arch timer freq is 25MHZ while the dw-apb timer freq is
> > >>> 100MHZ, current selection would choose the dw-apb timer. But the dw
> > >>> apb timer is on the APB bus while arch timer sits in CPU, the cost
> > >>> of accessing the apb timer is higher than the arch timer.
> > >>>
> > >>> This series firstly modifies register_current_timer_delay() to choose
> > >>> the highest rating delay timer: use the rating as a primary indication
> > >>> and fall back to comparing the frequency if the rating is not set or
> > >>> the same. Then we set the arch_delay_timer rating as 400, finally
> > >>> Implement ARM delay timer for the dw_apb_timer and set its rating as 300.  
> > >>
> > >> Hi Jisheng, Arnd,
> > >>
> > >> I don't feel comfortable with the rating / freq think. I am afraid this
> > >> approach based on heuristic will bring a lot of complexity and
> > >> workarounds in the code for a small benefit.
> > >>
> > >> Why don't we define a DT entry for the delay timer ? So we delegate the
> > >> choice to the platform DT definition.  
> > >
> > > That would be wrong, because the fact that Linux uses a timer to
> > > optimize its udelay() function is not a feature of the hardware.  
> > 
> > True.
> > 
> > Any ideas / suggestions for an alternative ?  
> 
> How about simply hardcoding the fact that we prefer the arch timer
> over any other one for delay as I suggested earlier?
> 
> Another idea I just had is to do nothing: According to Jisheng's
> description for this series, the reason for preferring the arch
> timer is that it is faster to access. However, we could argue
> that this actually doesn't matter at all, because the entire
> point of the ndelay()/udelay()/mdelay() functions is to waste
> CPU cycles doing not much at all, so we can just as well waste
> them reading the timer register than spinning on the CPU reading
> the arch timer more often.
> 

I like this "Another idea", indeed, the delay timer speed doesn't matter
at all. So I just cooked v3 to simply register dw apb based delay timer.

Thanks a lot for the inspiration,
Jisheng
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