Re: [PATCH v2] iio: imu: st_lsm6dsx: deselect shub page before reading whoami

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 2 Jul 2026 14:09:50 +0200
Lorenzo Bianconi <lorenzo@kernel.org> wrote:

> > As part of driver initialization, e.g. st_lsm6dsx_init_shub() selects
> > the shub register page using st_lsm6dsx_set_page(). Selecting the shub
> > register page shadows the regular register space so whoami, among other
> > registers, is no longer accessible.
> > 
> > In applications where the IMU is permanently powered separately from the
> > processor, there is a window where a reset of the CPU leaves the IMU in
> > the shub register page. Once this occurs, any subsequent probe attempt
> > fails because of the register shadowing.
> > 
> > Using the ism330dlc, the error typically looks like
> > 
> >     st_lsm6dsx_i2c 3-006a: unsupported whoami [10]
> > 
> > with the unknown whoami read from a reserved register in the shub page.
> > 
> > The reset register is also shadowed by the page select, preventing a
> > reset from recovering the chip.
> > 
> > Unconditionally clear the shub page before the whoami readout to ensure
> > normal register access and allow the initialization to proceed.
> > 
> > Place the fix in st_lsm6dsx_check_whoami() before the whoami check
> > because hw->settings, which st_lsm6dsx_set_page() relies on, is first
> > assigned in that function.
> > 
> > Placing the fix in a more logical place than the whoami check would
> > require a bigger restructuring of the code.
> > 
> > Signed-off-by: Andreas Kempe <andreas.kempe@actia.se>
> > ---
> > 
> >  Changes in v2:
> >    - Drop st_lsm6dsx_get_page() and unconditionally clear the shub page.
> >    - Make the code comment clearer.
> >    - Document the placement rationale in the commit message.
> >    - Link to v1: https://lore.kernel.org/linux-iio/20260604132646.1099072-1-andreas.kempe@actia.se/
> > 
> >  drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c | 21 +++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)  
> 
> Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
> 
Given I assume we want to backport this, please send an appropriate
Fixes tag in response to this email.

Thanks,

Jonathan

> > 
> > diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > index 630e2cae6f19..f4edcb73ec8c 100644
> > --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> > @@ -1712,6 +1712,26 @@ static int st_lsm6dsx_check_whoami(struct st_lsm6dsx_hw *hw, int id,
> >  		return -ENODEV;
> >  	}
> >  
> > +	hw->settings = &st_lsm6dsx_sensor_settings[i];
> > +
> > +	if (hw->settings->shub_settings.page_mux.addr) {
> > +		/*
> > +		 * If the IMU has the shub page selected on init, for example
> > +		 * after a CPU watchdog reset while the page is selected, the
> > +		 * regular register space is shadowed. While the regular
> > +		 * register space is shadowed, the registers needed for
> > +		 * initializing the IMU are not available.
> > +		 *
> > +		 * Unconditionally clear the shub page selection to ensure
> > +		 * normal register access.
> > +		 */
> > +		err = st_lsm6dsx_set_page(hw, false);
> > +		if (err < 0) {
> > +			dev_err(hw->dev, "failed to clear shub page\n");
> > +			return err;
> > +		}
> > +	}
> > +
> >  	err = regmap_read(hw->regmap, ST_LSM6DSX_REG_WHOAMI_ADDR, &data);
> >  	if (err < 0) {
> >  		dev_err(hw->dev, "failed to read whoami register\n");
> > @@ -1724,7 +1744,6 @@ static int st_lsm6dsx_check_whoami(struct st_lsm6dsx_hw *hw, int id,
> >  	}
> >  
> >  	*name = st_lsm6dsx_sensor_settings[i].id[j].name;
> > -	hw->settings = &st_lsm6dsx_sensor_settings[i];
> >  
> >  	return 0;
> >  }
> > -- 
> > 2.53.0