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

[Stefan Kerkmann <s.kerkmann@pengutronix.de>]

Hello Hugo,

On 3/11/26 21:02, Hugo Villeneuve wrote:
> From: Hugo Villeneuve <hvilleneuve@dimonoff.com>
> 
> 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:

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@git.hugovil.com: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.

> Thank you.
> 
> Link: https://lore.kernel.org/linux-rtc/20190910143945.9364-1-bruno.thomsen@gmail.com/ [1]
> Link: https://lore.kernel.org/linux-rtc/20191211163354.GC1463890@piout.net/ [2]
> Link: https://lore.kernel.org/linux-rtc/20230123170731.6064430c50f5fb7b484d8734@hugovil.com/ [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             |
Steuerwalder Str. 21                   | https://www.pengutronix.de/ |
31137 Hildesheim, Germany              | Phone: +49-5121-206917-128  |
Amtsgericht Hildesheim, HRA 2686       | Fax:   +49-5121-206917-9    |