linux-kernel - Re: [RFC PATCH] time: introduce BOOT_TIME

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Message-ID: <87a53d2kju.ffs@tglx>
Date: Tue, 02 Sep 2025 12:52:37 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Kaiwan N Billimoria <kaiwan.billimoria@...il.com>
Cc: Llillian@...r-ark.net, agordeev@...ux.ibm.com,
 akpm@...ux-foundation.org, alexander.shishkin@...ux.intel.com,
 anna-maria@...utronix.de, bigeasy@...utronix.de, catalin.marinas@....com,
 chenhuacai@...nel.org, francesco@...la.it, frederic@...nel.org,
 guoweikang.kernel@...il.com, jstultz@...gle.com, kpsingh@...nel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 mark.rutland@....com, maz@...nel.org, mingo@...nel.org, pmladek@...e.com,
 rrangel@...omium.org, sboyd@...nel.org, urezki@...il.com, v-singh1@...com,
 will@...nel.org, Kaiwan N Billimoria <kaiwan.billimoria@...il.com>
Subject: Re: [RFC PATCH] time: introduce BOOT_TIME_TRACKER and minimal boot
 timestamp

On Tue, Sep 02 2025 at 12:32, Kaiwan N. Billimoria wrote:
> So, just confirming: here 1409443611 divided by 200 MHz gives us 7.047218055s
> since boot, and thus the actual timestamp here is that plus 0.000001s yes?
> (Over 7s here? yes, it's just that I haven't yet setup U-Boot properly for uSD
> card boot, thus am manually loading commands in U-Boot to boot up, that's all).

Looks about right.

> A question (perhaps very stupid): will the hwcnt - the output of the read() -
> be guaranteed to be (close to) the number of increments since processor
> power-up (or reset)? Meaning, it's simply a hardware feature and agnostic to
> what code the core was executing (ROM/BL/kernel), yes?

Under the assumption that nothing on the way resets the counter.

Thanks

        tglx