linux-kernel - Re: [PATCH] iio: light: vcnl4000: Don't power on/off chip in config

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Date:   Mon, 11 Sep 2023 08:47:30 +0200
From:   Mårten Lindahl <martenli@...s.com>
To:     Jonathan Cameron <jic23@...nel.org>
CC:     Lars-Peter Clausen <lars@...afoo.de>, <linux-iio@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kernel@...s.com>
Subject: Re: [PATCH] iio: light: vcnl4000: Don't power on/off chip in config

Hi Jonathan!

Thanks! I'll rephrase it and add the fixes tags.

Kind regards

Mårten

On 9/10/23 15:32, Jonathan Cameron wrote:
> On Thu, 7 Sep 2023 12:53:14 +0200
> Mårten Lindahl <marten.lindahl@...s.com> wrote:
>
> Hi Mårten,
>
> Agree with your reasoning etc (and I guess you've triggered this for real)
> so just a few patch description etc comments.
>
>> After enabling/disabling interrupts on the vcnl4040 chip the als and/or
>> ps sensor is powered on or off depending on the interrupt enable bits.
>> This is made as a last step in write_event_config.
>>
>> But there is no reason to do this as the runtime PM handles the power
>> state of the sensors. Interfering with this may impact sensor readings.
>>
> I think the following example could be clearer. I haven't checked
> the naming of states in runtime pm, but a few things seem backwards to me.
>
>> Consider the following:
>>   1. Userspace makes sensor data reading which triggers 2000ms RPM resume
>>      (sensor powered on) timeout
> It triggers a timeout to do runtime suspend if no access in 2000ms, not resume.
>
>>   2. Userspace disables interrupts => powers sensor off
>>   3. Userspace reads sensor data = 0 because sensor is off and RPM didn't
>>      power on the sensor as resume timeout is still active
> suspend timeout hasn't yet run out, so the runtime pm subsystem thinks the
> device is still powered up and doesn't need resuming.
>
>>   4. RPM resume timeout passed
> suspend timeout.
>
>> Powering sensor off in (2) risks a time window of close to 2000ms where
>> sensor data readings are disabled as in (3).
>>
>> Skip setting power state when writing new event config.
>>
>> Signed-off-by: Mårten Lindahl <marten.lindahl@...s.com>
> Fixes tag - probably 2 of them. One for the recent change that added
> the || als_int part and one for wherever the bug originally came from
> with comments to say why there are two fixes tags.
>
>> ---
>>   drivers/iio/light/vcnl4000.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/drivers/iio/light/vcnl4000.c b/drivers/iio/light/vcnl4000.c
>> index 3a52b09c2823..fdf763a04b0b 100644
>> --- a/drivers/iio/light/vcnl4000.c
>> +++ b/drivers/iio/light/vcnl4000.c
>> @@ -1513,7 +1513,6 @@ static int vcnl4040_write_event_config(struct iio_dev *indio_dev,
>>   
>>   out:
>>   	mutex_unlock(&data->vcnl4000_lock);
>> -	data->chip_spec->set_power_state(data, data->ps_int || data->als_int);
> This will need manual backporting as this line changed recently and I'm fairly
> sure the argument is equally valid for the older code.
>
>>   
>>   	return ret;
>>   }
>>
>> ---
>> base-commit: 7ba2090ca64ea1aa435744884124387db1fac70f
>> change-id: 20230907-vcnl4000-pm-fix-b58dc0dffb5c
>>
>> Best regards,