Re: [PATCH] rtc: pcf85363: Fix phantom device registration on missing hardware

[Cosmo Chou]

On Thu, Jul 16, 2026 at 8:56 PM Alexandre Belloni
<alexandre.belloni@xxxxxxxxxxx> wrote:
>
> On 16/07/2026 20:51:42+0800, Cosmo Chou wrote:
> > During probe, pcf85363_load_capacitance() writes the oscillator load
> > capacitance configuration to the device. However, if the device is
> > not physically present on the bus (returning -ENXIO), the driver
> > only emits a warning and continues to execute the probe.
> >
> > This results in the successful registration of a phantom RTC device
> > via devm_rtc_register_device() and its associated nvmem regions.
> > Consequently, userspace may attempt to bind to a non-functional
> > /dev/rtc node.
> >
> > Propagate the I2C error back to the driver core using dev_err_probe()
> > to properly abort the probe and prevent phantom device registration.
> >
>
> No, there is no point in this, simply remove the RTC node from you
> device tree.
>

Hi Alexandre,

You are right, fixing the DT is the correct solution for missing
hardware. My commit message was misleading.

However, the current code ignores actual I2C transport errors (e.g.,
-ENXIO due to physical bus issues) from pcf85363_load_capacitance()
and continues probing.

Should I send a v2 with an updated commit message focusing solely on
properly handling these transport errors?

Thanks,
Cosmo

> > Fixes: fd9a6a13949a ("rtc: pcf85363: add support for the quartz-load-femtofarads property")
> > Signed-off-by: Cosmo Chou <chou.cosmo@xxxxxxxxx>
> > ---
> >  drivers/rtc/rtc-pcf85363.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/rtc/rtc-pcf85363.c b/drivers/rtc/rtc-pcf85363.c
> > index 540042b9eec8..5612330aff40 100644
> > --- a/drivers/rtc/rtc-pcf85363.c
> > +++ b/drivers/rtc/rtc-pcf85363.c
> > @@ -426,8 +426,8 @@ static int pcf85363_probe(struct i2c_client *client)
> >
> >       err = pcf85363_load_capacitance(pcf85363, client->dev.of_node);
> >       if (err < 0)
> > -             dev_warn(&client->dev, "failed to set xtal load capacitance: %d",
> > -                      err);
> > +             return dev_err_probe(&client->dev, err,
> > +                                  "failed to set xtal load capacitance\n");
> >
> >       pcf85363->rtc->ops = &rtc_ops;
> >       pcf85363->rtc->range_min = RTC_TIMESTAMP_BEGIN_2000;
> > --
> > 2.43.0
> >
>
> --
> Alexandre Belloni, co-owner and COO, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com