linux-kernel - Re: [PATCH V2 2/3] soc: qcom: boot_stat: Add Driver Support for Boot Stats

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Date:   Wed, 12 Apr 2023 10:17:34 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Souradeep Chowdhury <quic_schowdhu@...cinc.com>,
        Bjorn Andersson <andersson@...nel.org>
Cc:     Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        Sibi Sankar <quic_sibis@...cinc.com>,
        Rajendra Nayak <quic_rjendra@...cinc.com>
Subject: Re: [PATCH V2 2/3] soc: qcom: boot_stat: Add Driver Support for Boot
 Stats

On 10/04/2023 08:48, Souradeep Chowdhury wrote:
> 
> 
> On 4/7/2023 9:11 PM, Bjorn Andersson wrote:
>> On Fri, Apr 07, 2023 at 07:34:36PM +0530, Souradeep Chowdhury wrote:
>>> All of Qualcomm's proprietary Android boot-loaders capture boot time
>>> stats, like the time when the bootloader started execution and at what
>>> point the bootloader handed over control to the kernel etc. in the IMEM
>>> region. This information is captured in a specific format by this driver
>>> by mapping a structure to the IMEM memory region and then accessing the
>>> members of the structure to print the information. This information is
>>> useful in verifying if the existing boot KPIs have regressed or not.
>>> A sample log in SM8450(waipio) device is as follows:-
>>>
>>> KPI: Pre ABL Time = 3s
>>> KPI: ABL Time = 14s
>>
>> Why are these in whole seconds?
> 
> This is to give a granular view of time.
> 
>>
>>> KPI: Kernel MPM timestamp = 890206
>>
>> And why is this presented in cycles?
> 
> This timestamp is used as an intermediate value for calculating one of 
> the KPIs. Can be changed to seconds as well for consistency.
> 
>>
>>>
>>> The Module Power Manager(MPM) sleep counter starts ticking at the PBL
>>> stage and the timestamp generated by the sleep counter is logged by
>>> the Qualcomm proprietary bootloader(ABL) at two points-> First when it
>>> starts execution which is logged here as "Pre ABL Time" and the second
>>> when it is about to load the kernel logged as "ABL Time". Both are
>>> logged in the unit of seconds.
>>
>> We have a policy to not taint the kernel log with "useless" information,
>> for kernel developers this seems to add no value and for end users
>> there's no benefit to this.
>>
>>> The current kernel timestamp is
>>> printed by the boot_stats driver as well.
>>>
>>
>> Why?
> 
> Same as stated above.

You did not answer. The question is "why do you think printing this
during boot is suitable for wide usage?". I don't find answer "give a
granular view of time" anyway related.

Please come with rationale why such printing should be used in Linux
kernel at all, given that as Bjorn said - we do not print
debugging/profiling information.

You should probably come with a debugfs interface for this.

Best regards,
Krzysztof