linux-kernel - Re: [PATCH v4 2/2] driver: clocksource: Add nxp system counter timer driver support

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Date:   Tue, 21 May 2019 14:20:15 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Jacky Bai <ping.bai@....com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Aisheng Dong <aisheng.dong@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH v4 2/2] driver: clocksource: Add nxp system counter timer
 driver support

On 21/05/2019 14:01, Jacky Bai wrote:
> 
>> -----Original Message-----
>> From: Daniel Lezcano [mailto:daniel.lezcano@...aro.org]
>> Sent: Tuesday, May 21, 2019 6:08 PM
>> To: Jacky Bai <ping.bai@....com>; tglx@...utronix.de; robh+dt@...nel.org;
>> shawnguo@...nel.org; mark.rutland@....com; Aisheng Dong
>> <aisheng.dong@....com>
>> Cc: linux-kernel@...r.kernel.org; devicetree@...r.kernel.org; dl-linux-imx
>> <linux-imx@....com>
>> Subject: Re: [PATCH v4 2/2] driver: clocksource: Add nxp system counter timer
>> driver support
>>
>> On 21/05/2019 09:18, Jacky Bai wrote:
>>> From: Bai Ping <ping.bai@....com>
>>>
>>> The system counter (sys_ctr) is a programmable system counter which
>>> provides a shared time base to the Cortex A15, A7, A53 etc cores.
>>> It is intended for use in applications where the counter is always
>>> powered on and supports multiple, unrelated clocks. The sys_ctr
>>> hardware
>>> supports:
>>>  - 56-bit counter width (roll-over time greater than 40 years)
>>
>> The benefit of using more than 32bits on a 32bits system is not proven.
>>
> 
> It is mainly used on 64bit ARMv8 system.

Oh, ok. Fair enough.

> 
>> The function to read and build the 56bits value can have a very significant
>> impact on the performance of your platform.
>>
>> Using a 32bits counter can be enough if it does not wrap too fast.
>>
>> Can you consider a 32 bits counter ?
> 
> this counter is ARMv8 arch timer's counter source. As it also has timer function, so I choose it
> to act as a broadcast timer for cpuidle. The timer interrupt can only be triggered when 'compare[55:0] <= counter[55:0]'.
> So you mean that only use the lower 32bit to implement this timer? If so, I can change to use only the lower 32bit.

IMO it is preferable but you decide (probably compare with how long it
takes to wrap when 32bits).



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