Re: [PATCH] iio: adc: axp288: Fix the GPADC pin reading often wrongly returning 0

[Jonathan Cameron <jic23@kernel.org>]

On Sat,  8 Jul 2017 15:11:57 +0200
Hans de Goede <hdegoede@redhat.com> wrote:

> I noticed in its DSDT that one of my tablets actually is using the GPADC
> pin for temperature monitoring.
> 
> The whole axp288_adc_set_ts() function is a bit weird, in the past it was
> removed because it seems to make no sense, then this was reverted because
> of regressions.
> 
> So I decided to test the special GPADC pin handling on this tablet.
> Conclusion: not only is axp288_adc_set_ts() necessary, we need to sleep a
> bit after making the AXP288_ADC_TS_PIN_CTRL changes before sampling the
> GPADC, otherwise it will often (about 80% of the time) read 0 instead of
> its actual value.
> 
> It seems that there is only 1 bias current source and to be able to use it
> for the GPIO0 pin in GPADC mode it must be temporarily turned off for the
> TS pin, but the datasheet does not mention this.
> 
> This commit adds a sleep after disabling the TS pin bias current,
> fixing the GPADC more often then not wrongly returning 0.
> 
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Hans,

This is sufficiently weird that I want your opinion on what
to do with this.   Are we looking at a necessary fix for a hardware
platform that we should push out asap and mark for stable?

Or is this a bit of an odd corner case where a slower path makes
more sense?

I'm happy either way.

Jonathan
> ---
>  drivers/iio/adc/axp288_adc.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/axp288_adc.c b/drivers/iio/adc/axp288_adc.c
> index 7fd24949c0c1..462a99c13e7a 100644
> --- a/drivers/iio/adc/axp288_adc.c
> +++ b/drivers/iio/adc/axp288_adc.c
> @@ -126,11 +126,21 @@ static int axp288_adc_read_channel(int *val, unsigned long address,
>  static int axp288_adc_set_ts(struct regmap *regmap, unsigned int mode,
>  				unsigned long address)
>  {
> +	int ret;
> +
>  	/* channels other than GPADC do not need to switch TS pin */
>  	if (address != AXP288_GP_ADC_H)
>  		return 0;
>  
> -	return regmap_write(regmap, AXP288_ADC_TS_PIN_CTRL, mode);
> +	ret = regmap_write(regmap, AXP288_ADC_TS_PIN_CTRL, mode);
> +	if (ret)
> +		return ret;
> +
> +	/* When switching to the GPADC pin give things some time to settle */
> +	if (mode == AXP288_ADC_TS_PIN_GPADC)
> +		usleep_range(6000, 10000);
> +
> +	return 0;
>  }
>  
>  static int axp288_adc_read_raw(struct iio_dev *indio_dev,