linux-kernel - Re: PROBLEM: hwclock busted w/ M48T59 RTC (regression)

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Message-ID: <DmLaDrfp-izPBqLjB9SAGPy3WVKOPNgg9FInsykhNO3WPEWgltKF5GoDknld3l5xoJxovduV8xn8ygSupvyIFOCCZl0Q0aTXwKT2XhPM1n8=@geanix.com>
Date: Thu, 23 Oct 2025 07:21:21 +0000
From: Esben Haabendal <esben@...nix.com>
To: Nick Bowler <nbowler@...conx.ca>
Cc: linux-kernel@...r.kernel.org, regressions@...ts.linux.dev, linux-rtc@...r.kernel.org, stable@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: PROBLEM: hwclock busted w/ M48T59 RTC (regression)

On Thursday, 23 October 2025 at 06:45, Nick Bowler <nbowler@...conx.ca> wrote:

> After a stable kernel update, the hwclock command seems no longer
> functional on my SPARC system with an ST M48T59Y-70PC1 RTC:
> 
> # hwclock
> [...long delay...]

I assume this is 10 seconds long.

> hwclock: select() to /dev/rtc0 to wait for clock tick timed out

And this is 100% reproducible, or does it sometimes work and sometimes fail?

> On prior kernels, there is no problem:
> 
> # hwclock
> 2025-10-22 22:21:04.806992-04:00
> 
> I reproduced the same failure on 6.18-rc2 and bisected to this commit:
> 
> commit 795cda8338eab036013314dbc0b04aae728880ab
> Author: Esben Haabendal esben@...nix.com
> 
> Date: Fri May 16 09:23:35 2025 +0200
> 
> rtc: interface: Fix long-standing race when setting alarm
> 
> This commit was backported to all current 6.x stable branches,
> as well as 5.15.x, so they all have the same regression.
> 
> Reverting this commit on top of 6.18-rc2 corrects the problem.
> 
> Let me know if you need any more info!

Are you using the util-linux hwclock command? Which version?

Do you have CONFIG_RTC_INTF_DEV_UIE_EMUL enabled?

Can you run `hwclock --verbose`, both with and without the reverted commit,
and send the output from that?

/Esben