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

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Message-ID: <krmiwpwogrvpehlqdrugb5glcmsu54qpw3mteonqeqymrvzz37@dzt7mes7qgxt>
Date: Thu, 23 Oct 2025 00:45:13 -0400
From: Nick Bowler <nbowler@...conx.ca>
To: linux-kernel@...r.kernel.org, regressions@...ts.linux.dev, 
	linux-rtc@...r.kernel.org
Cc: Esben Haabendal <esben@...nix.com>, stable@...r.kernel.org, 
	sparclinux@...r.kernel.org
Subject: PROBLEM: hwclock busted w/ M48T59 RTC (regression)

Hi,

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

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!

Thanks,
  Nick