Re: [PATCH v3 2/3] iio: adc: tsc2046: add vref support

[Oleksij Rempel <o.rempel@pengutronix.de>]

Hi Marco,

On Thu, Sep 01, 2022 at 10:24:25AM +0200, Marco Felsch wrote:
> Hi Oleksij,
> 
> sorry for jumping in, please see below.
> 
> On 22-09-01, Oleksij Rempel wrote:
> > If VREF pin is attached, we should use external VREF source instead of
> > the internal. Otherwise we will get wrong measurements on some of channel
> > types.
> > 
> > Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > ---
> >  drivers/iio/adc/ti-tsc2046.c | 64 +++++++++++++++++++++++++++++++-----
> >  1 file changed, 55 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ti-tsc2046.c b/drivers/iio/adc/ti-tsc2046.c
> > index 0d9436a69cbfb..bbc8b4137b0b1 100644
> > --- a/drivers/iio/adc/ti-tsc2046.c
> > +++ b/drivers/iio/adc/ti-tsc2046.c
> > @@ -8,6 +8,7 @@
> >  #include <linux/bitfield.h>
> >  #include <linux/delay.h>
> >  #include <linux/module.h>
> > +#include <linux/regulator/consumer.h>
> >  #include <linux/spi/spi.h>
> >  
> >  #include <asm/unaligned.h>
> > @@ -175,6 +176,9 @@ struct tsc2046_adc_priv {
> >  	u32 time_per_bit_ns;
> >  
> >  	struct tsc2046_adc_ch_cfg ch_cfg[TI_TSC2046_MAX_CHAN];
> > +	bool use_internal_vref;
> > +	unsigned int vref_mv;
> 
> We wouldn't need those members if we just enable/disable the regulator
> on demand. This would also be more power efficient.

Not really.
- it is reference voltage. There is no load.
- usually no one care to add extra switch to disable reference voltage
  on a controller which should be enabled all the time any way.
- we add more CPU cycles on every access to enable/disable things, which are
  usually never have dedicated regulator.

> > +	struct regulator *vref_reg;
> >  };
> >  
> >  #define TI_TSC2046_V_CHAN(index, bits, name)			\
> > @@ -252,7 +256,9 @@ static u8 tsc2046_adc_get_cmd(struct tsc2046_adc_priv *priv, int ch_idx,
> >  	case TI_TSC2046_ADDR_AUX:
> >  	case TI_TSC2046_ADDR_VBAT:
> >  	case TI_TSC2046_ADDR_TEMP0:
> > -		pd |= TI_TSC2046_SER | TI_TSC2046_PD1_VREF_ON;
> > +		pd |= TI_TSC2046_SER;
> > +		if (priv->use_internal_vref)
> > +			pd |= TI_TSC2046_PD1_VREF_ON;
> 
> Then this line would become:

ack. I agree here.

> +		if (!priv->vref_reg)
> +			pd |= TI_TSC2046_PD1_VREF_ON;
> 
> 
> >  	}
> >  
> >  	return TI_TSC2046_START | FIELD_PREP(TI_TSC2046_ADDR, ch_idx) | pd;
> > @@ -468,7 +474,7 @@ static int tsc2046_adc_read_raw(struct iio_dev *indio_dev,
> 
> This function needs to enable the vref_reg before the switch-case and
> disable it afterwards.

it will not enable power supply of this controller. There is no need to
do this.

> >  		 * So, it is better to use external voltage-divider driver
> >  		 * instead, which is calculating complete chain.
> >  		 */
> > -		*val = TI_TSC2046_INT_VREF;
> > +		*val = priv->vref_mv;
> >  		*val2 = chan->scan_type.realbits;
> >  		return IIO_VAL_FRACTIONAL_LOG2;
> >  	}
> > @@ -781,22 +787,42 @@ static int tsc2046_adc_probe(struct spi_device *spi)
> >  	indio_dev->num_channels = dcfg->num_channels;
> >  	indio_dev->info = &tsc2046_adc_info;
> >  
> > +	priv->vref_reg = devm_regulator_get_optional(&spi->dev, "vref");
> 
> This line would be enough and we could drop
> 
> > +	if (!IS_ERR(priv->vref_reg)) {
> > +		ret = regulator_enable(priv->vref_reg);
> > +		if (ret)
> > +			return ret;
> > +
> > +		ret = regulator_get_voltage(priv->vref_reg);
> > +		if (ret < 0)
> > +			goto err_regulator_disable;
> > +
> > +		priv->vref_mv = ret / 1000;
> > +		priv->use_internal_vref = false;
> > +	} else {
> > +		/* Use internal reference */
> > +		priv->vref_mv = TI_TSC2046_INT_VREF;
> > +		priv->use_internal_vref = true;
> > +	}
> > +
> 
> this part.

In this case we will not get any power saving, but instead we will need
to run enable/disable + regulator_get_voltage + extra calculation
without any advantage.

> 
> >  	tsc2046_adc_parse_fwnode(priv);
> >  
> >  	ret = tsc2046_adc_setup_spi_msg(priv);
> >  	if (ret)
> > -		return ret;
> > +		goto err_regulator_disable;
> >  
> >  	mutex_init(&priv->slock);
> >  
> >  	ret = devm_request_irq(dev, spi->irq, &tsc2046_adc_irq,
> >  			       IRQF_NO_AUTOEN, indio_dev->name, indio_dev);
> >  	if (ret)
> > -		return ret;
> > +		goto err_regulator_disable;
> >  
> >  	trig = devm_iio_trigger_alloc(dev, "touchscreen-%s", indio_dev->name);
> > -	if (!trig)
> > -		return -ENOMEM;
> > +	if (!trig) {
> > +		ret = -ENOMEM;
> > +		goto err_regulator_disable;
> > +	}
> >  
> >  	priv->trig = trig;
> >  	iio_trigger_set_drvdata(trig, indio_dev);
> > @@ -811,20 +837,39 @@ static int tsc2046_adc_probe(struct spi_device *spi)
> >  	ret = devm_iio_trigger_register(dev, trig);
> >  	if (ret) {
> >  		dev_err(dev, "failed to register trigger\n");
> > -		return ret;
> > +		goto err_regulator_disable;
> >  	}
> >  
> >  	ret = devm_iio_triggered_buffer_setup(dev, indio_dev, NULL,
> >  					      &tsc2046_adc_trigger_handler, NULL);
> >  	if (ret) {
> >  		dev_err(dev, "Failed to setup triggered buffer\n");
> > -		return ret;
> > +		goto err_regulator_disable;
> >  	}
> >  
> >  	/* set default trigger */
> >  	indio_dev->trig = iio_trigger_get(priv->trig);
> >  
> > -	return devm_iio_device_register(dev, indio_dev);
> > +	ret = devm_iio_device_register(dev, indio_dev);
> > +	if (ret)
> > +		goto err_regulator_disable;
> > +
> > +	return 0;
> > +
> > +err_regulator_disable:
> > +	if (!IS_ERR(priv->vref_reg))
> > +		regulator_disable(priv->vref_reg);
> > +
> > +	return ret;
> 
> As well as the whole new error handling and
> 
> > +}
> > +
> > +static void tsc2046_adc_remove(struct spi_device *spi)
> > +{
> > +	struct iio_dev *indio_dev = spi_get_drvdata(spi);
> > +	struct tsc2046_adc_priv *priv = iio_priv(indio_dev);
> > +
> > +	if (!IS_ERR(priv->vref_reg))
> > +		regulator_disable(priv->vref_reg);
> >  }
> 
> the remove callback.

no.

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