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SubjectRe: [PATCH v2] iio: bu27034: Ensure reset is written
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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! ~~

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