Re: [PATCH v2] iio: bu27034: Ensure reset is written

[Matti Vaittinen]

On 5/6/23 21:27, Jonathan Cameron wrote:
> On Wed, 3 May 2023 13:01:32 +0300
> Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
>
> > The reset bit must be always written to the hardware no matter what value
> > is in a cache or register. Ensure this by using regmap_write_bits()
> > instead of the regmap_update_bits(). Furthermore, the RESET bit may be
> > self-clearing, so mark the SYSTEM_CONTROL register volatile to guarantee
> > we do also read the right state - should we ever need to read it.
> >
> > Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
> > Fixes: e52afbd61039 ("iio: light: ROHM BU27034 Ambient Light Sensor")
>
> This obviously interacts with the regcache update.
>
> Fun question is whether a register is volatile if it results in all
> registers (including itself) resetting.  In my view, no it isn't volatile.
> So fixing the regcache stuff as in your other patch is more appropriate.

Hi Jonathan,

I think the key thing here is to ensure writing the reset-bit will 
always be performed no matter what value is found from cache/hardware. I 
guess marking the register as volatile is indeed unnecessary, although I 
don't think it is wrong though, as it underlines we have something 
special in this register. However, using the write_bits() instead of 
update_bits() is in my opinion very much "the right thing" to do :)

Yours¸
	-- Matti

> Jonathan
>
> > ---
> > Changelog:
> > v1 => v2:
> >    - Fix SoB tag
> >
> >
> > I haven't verified if the reset bit is self-clearin as I did temporarily
> > give away the HW.
> >
> > In worst case the bit is not self clearing - but we don't really
> > get performance penalty even if we set the register volatile because the
> > SYSTEM_CONTROL register only has the part-ID and the reset fields. The
> > part-id is only read once at probe.
> >
> > ---
> >   drivers/iio/light/rohm-bu27034.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iio/light/rohm-bu27034.c b/drivers/iio/light/rohm-bu27034.c
> > index 25c9b79574a5..740ebd86b6e5 100644
> > --- a/drivers/iio/light/rohm-bu27034.c
> > +++ b/drivers/iio/light/rohm-bu27034.c
> > @@ -231,6 +231,9 @@ struct bu27034_result {
> >   
> >   static const struct regmap_range bu27034_volatile_ranges[] = {
> >   	{
> > +		.range_min = BU27034_REG_SYSTEM_CONTROL,
> > +		.range_max = BU27034_REG_SYSTEM_CONTROL,
> > +	}, {
> >   		.range_min = BU27034_REG_MODE_CONTROL4,
> >   		.range_max = BU27034_REG_MODE_CONTROL4,
> >   	}, {
> > @@ -1272,7 +1275,7 @@ static int bu27034_chip_init(struct bu27034_data *data)
> >   	int ret, sel;
> >   
> >   	/* Reset */
> > -	ret = regmap_update_bits(data->regmap, BU27034_REG_SYSTEM_CONTROL,
> > +	ret = regmap_write_bits(data->regmap, BU27034_REG_SYSTEM_CONTROL,
> >   			   BU27034_MASK_SW_RESET, BU27034_MASK_SW_RESET);
> >   	if (ret)
> >   		return dev_err_probe(data->dev, ret, "Sensor reset failed\n");
> >
> > base-commit: 7fcbd72176076c44b47e8f68f0223c02c411f420

--
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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