linux-kernel - RE: [PATCH v6 2/2] clocksource: Add Intel Keem Bay timer support

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Date:   Fri, 24 Dec 2021 09:40:53 +0000
From:   "Sanil, Shruthi" <shruthi.sanil@...el.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
CC:     "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "kris.pan@...ux.intel.com" <kris.pan@...ux.intel.com>,
        "mgross@...ux.intel.com" <mgross@...ux.intel.com>,
        "Thokala, Srikanth" <srikanth.thokala@...el.com>,
        "Raja Subramanian, Lakshmi Bai" 
        <lakshmi.bai.raja.subramanian@...el.com>,
        "Sangannavar, Mallikarjunappa" 
        <mallikarjunappa.sangannavar@...el.com>
Subject: RE: [PATCH v6 2/2] clocksource: Add Intel Keem Bay timer support

> -----Original Message-----
> From: Daniel Lezcano <daniel.lezcano@...aro.org>
> Sent: Thursday, December 23, 2021 7:46 PM
> To: Sanil, Shruthi <shruthi.sanil@...el.com>; Thomas Gleixner
> <tglx@...utronix.de>; robh+dt@...nel.org; linux-kernel@...r.kernel.org;
> devicetree@...r.kernel.org
> Cc: andriy.shevchenko@...ux.intel.com; kris.pan@...ux.intel.com;
> mgross@...ux.intel.com; Thokala, Srikanth <srikanth.thokala@...el.com>;
> Raja Subramanian, Lakshmi Bai <lakshmi.bai.raja.subramanian@...el.com>;
> Sangannavar, Mallikarjunappa <mallikarjunappa.sangannavar@...el.com>
> Subject: Re: [PATCH v6 2/2] clocksource: Add Intel Keem Bay timer support
> 
> On 11/11/2021 11:42, Sanil, Shruthi wrote:
> >> -----Original Message-----
> >> From: Thomas Gleixner <tglx@...utronix.de>
> >> Sent: Monday, September 27, 2021 3:11 AM
> >> To: Sanil, Shruthi <shruthi.sanil@...el.com>;
> >> daniel.lezcano@...aro.org;
> >> robh+dt@...nel.org; linux-kernel@...r.kernel.org;
> >> devicetree@...r.kernel.org
> >> Cc: andriy.shevchenko@...ux.intel.com; kris.pan@...ux.intel.com;
> >> mgross@...ux.intel.com; Thokala, Srikanth
> >> <srikanth.thokala@...el.com>; Raja Subramanian, Lakshmi Bai
> >> <lakshmi.bai.raja.subramanian@...el.com>;
> >> Sangannavar, Mallikarjunappa
> <mallikarjunappa.sangannavar@...el.com>;
> >> Sanil, Shruthi <shruthi.sanil@...el.com>
> >> Subject: Re: [PATCH v6 2/2] clocksource: Add Intel Keem Bay timer
> >> support
> >>
> >> On Tue, Sep 07 2021 at 00:06, shruthi sanil wrote:
> >>> +
> >>> +/* Provides a unique ID for each timer */ static
> >>> +DEFINE_IDA(keembay_timer_ida);
> >>
> >>> +
> >>> +	timer_id = ida_alloc(&keembay_timer_ida, GFP_KERNEL);
> >>> +	if (timer_id < 0) {
> >>> +		ret = timer_id;
> >>> +		goto err_keembay_ce_to_free;
> >>> +	}
> >>
> >> May I ask what the purpose of the IDA, which is backed by a full
> >> blown xarray, is here?
> >>
> >> AFAICT all you want is a unique number for the timer name for up to 8
> >> timers.
> >>
> >>> +	timer_name = kasprintf(GFP_KERNEL, "keembay_timer%d",
> >> timer_id);
> >>
> >> So what's wrong about:
> >>
> >> static unsigned int keembay_timer_id;
> >>
> >> 	timer_name = kasprintf(GFP_KERNEL, "keembay_timer%d",
> >> keembay_timer_id++);
> >>
> >> Hmm?
> >
> > Yes, we had initially implemented it in the similar way, but in the
> > course of review it got changed to use IDA.
> >
> >>
> >>> +
> >>> +	clockevents_config_and_register(&keembay_ce_to->clkevt,
> >>> +					timer_of_rate(keembay_ce_to),
> >>> +					1,
> >>> +					U32_MAX);
> >>
> >> Aside of that what's the point of registering more than one of those
> >> timers as clock event? The core will only use one and the rest is
> >> just going to use memory for no value.
> >
> > Instead of
> > keembay_ce_to->clkevt.cpumask = cpumask_of(0); can I update it as
> > keembay_ce_to->clkevt.cpumask = cpu_possible_mask; so that each timer
> > would be associated with different cores?
> 
> Let me try to clarify:
> 
> The Intel Keem bay Soc is a 4 x Cortex-A53
> 
> The arch ARM timer is per CPU on this platform.
> 
> Case 1:
> -------
>  - the architected timer is not desired and this timer is wanted to be used
> instead (but rating tells the opposite) => rewrite per cpu code
> 
> Case 2:
> -------
>  - the architected timer are desired and this timer is used as a broadcast
> timer when a core is going done with cpuidle. One timer is needed.
> 
>  - In order to prevent useless wakeup, the timer uses the flag DYNIRQ.
> However, cpumask_of(0) is set and makes inoperative this flag. In order to
> make full use of it, clkevt.cpumask must be cpu_possible_mask
> 
> Hope that helps
> 
>   -- Daniel
> 

Thank You Daniel for the explanation.
In case of KMB, we are using the ARM architecture timer.
We would be using the timer for case2. So I need to register Just 1 timer.
I'll check and make the changes accordingly and submit the next patch.

Thank You!
Regards,
Shruthi

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