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

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Message-ID: <ni6gdeax2itvzagwbqkw6oj5xsbx6vqsidop6cbj2oqneovjib@mrwzqakbla35>
Date: Thu, 23 Oct 2025 09:39:47 -0400
From: Nick Bowler <nbowler@...conx.ca>
To: Esben Haabendal <esben@...nix.com>
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 Thu, Oct 23, 2025 at 07:21:21AM +0000, Esben Haabendal wrote:
> 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.

Yeah, about that.

> > 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?

It fails every time.

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

hwclock from util-linux 2.40.2

> Do you have CONFIG_RTC_INTF_DEV_UIE_EMUL enabled?

No, this option is not enabled.

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

6.18-rc2 (broken):

  # hwclock --verbose
  hwclock from util-linux 2.40.2
  System Time: 1761226454.799573
  Trying to open: /dev/rtc0
  Using the rtc interface to the clock.
  Last drift adjustment done at 1657523820 seconds after 1969
  Last calibration done at 1657523820 seconds after 1969
  Hardware clock is on UTC time
  Assuming hardware clock is kept in UTC time.
  Waiting for clock tick...
  hwclock: select() to /dev/rtc0 to wait for clock tick timed out
  ...synchronization failed

6.18-rc2 w/ revert (working):

  # hwclock --verbose
  hwclock from util-linux 2.40.2
  System Time: 1761226685.238753
  Trying to open: /dev/rtc0
  Using the rtc interface to the clock.
  Last drift adjustment done at 1657523820 seconds after 1969
  Last calibration done at 1657523820 seconds after 1969
  Hardware clock is on UTC time
  Assuming hardware clock is kept in UTC time.
  Waiting for clock tick...
  ioctl(3, RTC_UIE_ON, 0): Input/output error
  Waiting in loop for time from /dev/rtc0 to change
  ...got clock tick
  Time read from Hardware Clock: 2025/10/23 13:38:06
  Hw clock time : 2025/10/23 13:38:06 = 1761226686 seconds since 1969
  Time since last adjustment is 103702866 seconds
  Calculated Hardware Clock drift is 0.000000 seconds
  2025-10-23 09:38:05.239100-04:00

Thanks,
  Nick