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

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Message-ID: <5639E9DD.1080309@linaro.org>
Date:	Wed, 4 Nov 2015 12:19:57 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
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 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 ?



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