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

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

[Matti Vaittinen]

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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@gmail.com>
Fixes: e52afbd61039 ("iio: light: ROHM BU27034 Ambient Light Sensor")

---
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
-- 
2.40.0


-- 
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =] 

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

[Jonathan Cameron]

On Wed, 3 May 2023 13:01:32 +0300
Matti Vaittinen <mazziesaccount@gmail.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@gmail.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.

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

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@gmail.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@gmail.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 :)

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

~~ When things go utterly wrong vim users can always type :help! ~~

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

[Jonathan Cameron]

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

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