Date:   Sat, 8 Feb 2020 16:03:27 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Olivier Moysan <olivier.moysan@...com>
Cc:     <robh+dt@...nel.org>, <mark.rutland@....com>, <knaack.h@....de>,
        <lars@...afoo.de>, <devicetree@...r.kernel.org>,
        <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pmeerw@...erw.net>, <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/4] iio: adc: sd modulator: add scale support

On Tue, 4 Feb 2020 11:10:06 +0100
Olivier Moysan <olivier.moysan@...com> wrote:

> Add scale support to sigma delta modulator.
> 
> Signed-off-by: Olivier Moysan <olivier.moysan@...com>

I note below that there are probably some complexities around
whether vref is used as you have it here or not.

A few other bits inline around a race condition introduced in probe / remove.

Thanks,

Jonathan

> ---
>  drivers/iio/adc/sd_adc_modulator.c | 108 ++++++++++++++++++++++++++---
>  1 file changed, 100 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/iio/adc/sd_adc_modulator.c b/drivers/iio/adc/sd_adc_modulator.c
> index 560d8c7d9d86..a83f35832050 100644
> --- a/drivers/iio/adc/sd_adc_modulator.c
> +++ b/drivers/iio/adc/sd_adc_modulator.c
> @@ -10,8 +10,7 @@
>  #include <linux/iio/triggered_buffer.h>
>  #include <linux/module.h>
>  #include <linux/of_device.h>
> -
> -static const struct iio_info iio_sd_mod_iio_info;
> +#include <linux/regulator/consumer.h>
>  
>  static const struct iio_chan_spec iio_sd_mod_ch = {
>  	.type = IIO_VOLTAGE,
> @@ -21,34 +20,126 @@ static const struct iio_chan_spec iio_sd_mod_ch = {
>  		.realbits = 1,
>  		.shift = 0,
>  	},
> +	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),

This relies on generic sigma delta modulators using an external vref.
They might have an internal always on regulator...
I would suggest we only support scale for devices with explicitly
defined compatibles where we can know what regulators are used etc.

For many devices, there will be a single powersupply called vdd-supply
or similar in DT.  It may also provide a reference voltage.

> +};
> +
> +static const struct iio_chan_spec iio_sd_mod_ch_ads = {
> +	.type = IIO_VOLTAGE,
> +	.indexed = 1,
> +	.scan_type = {
> +		.sign = 'u',
> +		.realbits = 1,
> +		.shift = 0,
> +	},
> +	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
> +	.differential = 1,
> +};
> +
> +struct iio_sd_mod_priv {
> +	struct regulator *vref;
> +	int vref_mv;
> +};
> +
> +static int iio_sd_mod_read_raw(struct iio_dev *indio_dev,
> +			       struct iio_chan_spec const *chan, int *val,
> +			       int *val2, long mask)
> +{
> +	struct iio_sd_mod_priv *priv = iio_priv(indio_dev);
> +
> +	switch (mask) {
> +	case IIO_CHAN_INFO_SCALE:
> +		*val = priv->vref_mv;
> +		*val2 = chan->scan_type.realbits;
> +		return IIO_VAL_INT;
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static const struct iio_info iio_sd_mod_iio_info = {
> +	.read_raw = iio_sd_mod_read_raw,
>  };
>  
>  static int iio_sd_mod_probe(struct platform_device *pdev)
>  {
>  	struct device *dev = &pdev->dev;
> +	struct iio_sd_mod_priv *priv;
>  	struct iio_dev *iio;
> +	int ret;
>  
> -	iio = devm_iio_device_alloc(dev, 0);
> +	iio = devm_iio_device_alloc(dev, sizeof(*priv));
>  	if (!iio)
>  		return -ENOMEM;
>  
> +	iio->channels = (const struct iio_chan_spec *)
> +					of_device_get_match_data(&pdev->dev);
> +
> +	priv = iio_priv(iio);
> +
>  	iio->dev.parent = dev;
>  	iio->dev.of_node = dev->of_node;
>  	iio->name = dev_name(dev);
>  	iio->info = &iio_sd_mod_iio_info;
>  	iio->modes = INDIO_BUFFER_HARDWARE;
> -
>  	iio->num_channels = 1;
> -	iio->channels = &iio_sd_mod_ch;
>  
>  	platform_set_drvdata(pdev, iio);
>  
> -	return devm_iio_device_register(&pdev->dev, iio);
> +	priv->vref = devm_regulator_get_optional(dev, "vref");
> +	if (IS_ERR(priv->vref)) {
> +		ret = PTR_ERR(priv->vref);
> +		if (ret != -ENODEV) {
> +			if (ret != -EPROBE_DEFER)
> +				dev_err(dev, "vref get failed, %d\n", ret);
> +			return ret;
> +		}
> +	}
> +
> +	if (!IS_ERR(priv->vref)) {
> +		ret = regulator_enable(priv->vref);
> +		if (ret < 0) {
> +			dev_err(dev, "vref enable failed %d\n", ret);
> +			return ret;
> +		}
> +
> +		ret = regulator_get_voltage(priv->vref);
> +		if (ret < 0) {
> +			dev_err(dev, "vref get failed, %d\n", ret);
> +			goto err_regulator_disable;
> +		}
> +
> +		priv->vref_mv = ret / 1000;
> +		dev_dbg(dev, "vref+=%dmV\n", priv->vref_mv);
> +	}
> +
> +	ret = devm_iio_device_register(&pdev->dev, iio);
This exposes the userspace and in kernel interfaces.  Those
are partly dependent on the regulator enable you have above.

Using devm_ version fo this interface leaves you with a race in remove.
The regulator is disabled before you have remove the interfaces that
will only work if we assume it is still on.

Hence, you should either use devm_add_action_or_reset magic
to ensure we still do everything in the right order, or do it
manually by using iio_device_register and iio_device_unregister.


> +	if (ret < 0) {
> +		dev_err(dev, "Failed to register sd-modulator, %d\n", ret);
> +		goto err_regulator_disable;
> +	}
> +
> +	return 0;
> +
> +err_regulator_disable:
> +	regulator_disable(priv->vref);
> +
> +	return ret;
> +}
> +
> +static int iio_sd_mod_remove(struct platform_device *pdev)
> +{
> +	struct iio_dev *indio_dev = platform_get_drvdata(pdev);
> +	struct iio_sd_mod_priv *priv = iio_priv(indio_dev);
> +
> +	if (priv->vref)
> +		return regulator_disable(priv->vref);
> +
> +	return 0;
>  }
>  
>  static const struct of_device_id sd_adc_of_match[] = {
> -	{ .compatible = "sd-modulator" },
> -	{ .compatible = "ads1201" },
> +	{ .compatible = "sd-modulator", .data = &iio_sd_mod_ch },
> +	{ .compatible = "ads1201", .data = &iio_sd_mod_ch_ads },
>  	{ }
>  };
>  MODULE_DEVICE_TABLE(of, sd_adc_of_match);
> @@ -59,6 +150,7 @@ static struct platform_driver iio_sd_mod_adc = {
>  		.of_match_table = of_match_ptr(sd_adc_of_match),
>  	},
>  	.probe = iio_sd_mod_probe,
> +	.remove = iio_sd_mod_remove,
>  };
>  
>  module_platform_driver(iio_sd_mod_adc);

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