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

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Message-ID: <5639D409.1030302@linaro.org>
Date:	Wed, 4 Nov 2015 10:46:49 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Jisheng Zhang <jszhang@...vell.com>, tglx@...utronix.de,
	linux@....linux.org.uk, arnd@...db.de
Cc:	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 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.


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