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

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Date:	Wed, 04 Nov 2015 11:30:43 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Jisheng Zhang <jszhang@...vell.com>, 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

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.

	Arnd
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