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

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Message-ID: <20230507143049.4407c06e@jic23-huawei>
Date:   Sun, 7 May 2023 14:30:49 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: bu27034: Ensure reset is written

On Sun, 7 May 2023 12:58:52 +0300
Matti Vaittinen <mazziesaccount@...il.com> wrote:

> On 5/6/23 21:27, Jonathan Cameron wrote:
> > On Wed, 3 May 2023 13:01:32 +0300
> > Matti Vaittinen <mazziesaccount@...il.com> 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@...il.com>
> >> 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 :)

It's a reasonable change, but whether it's fixing a bug is more complex.
If we handle the cache correctly so it always says the bits need writing
then there will be no difference between update_bits() and write_bits().

Meh, better safe than sorry.

Jonathan

> 
> 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  
> >   
>