rtc: pcf85363: Fix phantom device registration on missing hardware

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

Posted by Cosmo Chou 1 week, 2 days ago

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.

Fixes: fd9a6a13949a ("rtc: pcf85363: add support for the quartz-load-femtofarads property")
Signed-off-by: Cosmo Chou <chou.cosmo@gmail.com>
---
 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

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

Posted by Alexandre Belloni 1 week, 2 days ago

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.

> Fixes: fd9a6a13949a ("rtc: pcf85363: add support for the quartz-load-femtofarads property")
> Signed-off-by: Cosmo Chou <chou.cosmo@gmail.com>
> ---
>  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

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

Posted by Cosmo Chou 1 week, 2 days ago

On Thu, Jul 16, 2026 at 8:56 PM Alexandre Belloni
<alexandre.belloni@bootlin.com> 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@gmail.com>
> > ---
> >  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

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

Posted by Alexandre Belloni 1 week, 2 days ago

On 16/07/2026 22:56:04+0800, Cosmo Chou wrote:
> On Thu, Jul 16, 2026 at 8:56 PM Alexandre Belloni
> <alexandre.belloni@bootlin.com> 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?

Then you should add error checking to all the regmap calls in probe()

> 
> Thanks,
> Cosmo
> 
> > > Fixes: fd9a6a13949a ("rtc: pcf85363: add support for the quartz-load-femtofarads property")
> > > Signed-off-by: Cosmo Chou <chou.cosmo@gmail.com>
> > > ---
> > >  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

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com