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 12:45:36 +0000
From:   Jacky Bai <ping.bai@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        "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



> -----Original Message-----
> From: Daniel Lezcano [mailto:daniel.lezcano@...aro.org]
> Sent: Tuesday, May 21, 2019 8:20 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 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).
> 

Normally, it is driven by fixed 8MHz clock. 32bit has risk, when the lower 32bit wrapped,
then the 'counter value >= compare value' is true if only use 32bit for timer, then IRQ will
Pending all the time. I will keep use the whole 56bit.

> 
>