Re: [PATCH 0/4] rtc: pcf2127: add support for battery low voltage detection

[Hugo Villeneuve]

Hi Stefan,

On Wed, 29 Jul 2026 17:18:59 +0200
Stefan Kerkmann <s.kerkmann@xxxxxxxxxxxxxx> wrote:

> Hello Hugo,
> 
> On 3/11/26 21:02, Hugo Villeneuve wrote:
> > From: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>
> > 
> > Hello,
> > this patch series adds support for battery low voltage detection configuration
> > for RTC devicesm, with specifc changes targeted at the pcf2127/2131 devices.
> > 
> > The origin of this patch goes back to the initial commit to add support for the
> > PCF2131 device. On the PCF2131, the battery low voltage detection is disabled by
> > default at the hardware level, contrary to the PCF2127 where it is enabled by
> > default. Because of that, a lot of people are stuck with a PCF2131 RTC device
> > using a battery backup, but unable to use it!
> > 
> > A lot of people are writing to me in private emails to report this as a bug
> > in the PCF2131 driver. Others (and me) have proposed to implement device tree
> > properties to enable these functions, but this was rejected [1][2][3].
> > 
> > It is important to note that some projects do not have the luxury to modify
> > the bootloder to enable that function at boot, and for these having a DT
> > property that could be put in a DT overlay would simplify a lot their life.
> > Also having to rely on a userspace application to configure the RTC is also
> > not ideal, as some projects use stock Debian distros (for example), and
> > adding a new application to their repository is not trivial or easy.
> > 
> > So as the next best thing, this patch aims to add what is missing in the driver,
> > the ability to enable/disable the battery low voltage detection with ioctls,
> > similarly to what is done with the BSM.
> > 
> 
> Thank you for the series. We have customer that has this exact use case:

Ok, a lot of people affected by this...

> 
> Enabling the backup battery switch-over and low battery detection on a pcf2131 
> rtc. With util-linux's hwclock the former can already be enabled e.g. hwclock 
> --param-set bsm <xyz> and for the later this patch series would enable it as well.
> 
> > This patch has been tested on a custom board with a PCF2131 and using my
> > userspace application:
> > 
> >      git clone -b batlow_param git@xxxxxxxxxxxxxxx:repos/hvrtc.git
> > 
> 
> If this series gets merged I can sent a PR to util-linux to enable the low 
> battery detection via hwclock as well.

Good.

Unfortunately, I haven't received any feedback from the RTC
maintainer(s) about this series, nor have I received any answers
to my previous inquiries about this problem.

I send patches to other kernel subsystem, and eventually I always
receive feedback, but RTC seems to be in a
different category. I sometimes wonder if there is still an
active RTC maintainer...

Hugo.


> > Thank you.
> > 
> > Link: https://lore.kernel.org/linux-rtc/20190910143945.9364-1-bruno.thomsen@xxxxxxxxx/ [1]
> > Link: https://lore.kernel.org/linux-rtc/20191211163354.GC1463890@xxxxxxxxx/ [2]
> > Link: https://lore.kernel.org/linux-rtc/20230123170731.6064430c50f5fb7b484d8734@xxxxxxxxxxx/ [3]
> > 
> > Hugo Villeneuve (4):
> >    rtc: pcf2127: remove redundant break statement in switch-case
> >    rtc: pcf2127: add pcf2127_pwrmng_get/set
> >    rtc: add battery low voltage detection feature
> >    rtc: pcf2127: support battery low voltage detection function
> > 
> >   drivers/rtc/rtc-pcf2127.c | 94 ++++++++++++++++++++++++++++++---------
> >   include/uapi/linux/rtc.h  |  7 ++-
> >   2 files changed, 80 insertions(+), 21 deletions(-)
> > 
> > 
> > base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
> 
> Best regards,
> Stefan
> 
> -- 
> Pengutronix e.K.                       | Stefan Kerkmann             |
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-- 
Hugo Villeneuve