Re: [PATCH] hwmon: pmbus: max20730: adjust the vout reading given voltage divider

[Guenter Roeck <linux@roeck-us.net>]

On 9/10/20 10:08 AM, Chu Lin wrote:
> Problem:
> We use voltage dividers so that the voltage presented at the voltage
> sense pins is confusing. We might need to convert these readings to more
> meaningful readings given the voltage divider.
> 
> Solution:
> Read the voltage divider resistance from dts and convert the voltage
> reading to a more meaningful reading.
> 
> Testing:
> max20730 with voltage divider
> 
> Signed-off-by: Chu Lin <linchuyuan@google.com>
> ---
>  drivers/hwmon/pmbus/max20730.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/hwmon/pmbus/max20730.c b/drivers/hwmon/pmbus/max20730.c
> index c0bb05487e0e..4b56810fa894 100644
> --- a/drivers/hwmon/pmbus/max20730.c
> +++ b/drivers/hwmon/pmbus/max20730.c
> @@ -29,6 +29,7 @@ struct max20730_data {
>  	struct pmbus_driver_info info;
>  	struct mutex lock;	/* Used to protect against parallel writes */
>  	u16 mfr_devset1;
> +	u32 vout_voltage_divider[2];
>  };
>  
>  #define to_max20730_data(x)  container_of(x, struct max20730_data, info)
> @@ -111,6 +112,12 @@ static int max20730_read_word_data(struct i2c_client *client, int page,
>  		max_c = max_current[data->id][(data->mfr_devset1 >> 5) & 0x3];
>  		ret = val_to_direct(max_c, PSC_CURRENT_OUT, info);
>  		break;
> +	case PMBUS_READ_VOUT:
> +		ret = pmbus_read_word_data(client, page, phase, reg);

		if (ret < 0)
			return ret;

> +		if (data->vout_voltage_divider[0] && data->vout_voltage_divider[1])
> +			ret = DIV_ROUND_CLOSEST(ret * data->vout_voltage_divider[1],
> +						data->vout_voltage_divider[0]);

With no range checks, ret * data->vout_voltage_divider[1] can
easily overflow if vout_voltage_divider[] is larger than 0xffff.
Also, the pmbus core does not expect register values larger than 0xffff
(it typecasts the returned value explicitly to u16).

[ Grumble. The PMBus specification has VOUT_SCALE commands to handle
  this situation. It is quite annoying that this chip does not support it. ]

Guenter

> +		break;
>  	default:
>  		ret = -ENODATA;
>  		break;
> @@ -329,6 +336,15 @@ static int max20730_probe(struct i2c_client *client,
>  	data->id = chip_id;
>  	mutex_init(&data->lock);
>  	memcpy(&data->info, &max20730_info[chip_id], sizeof(data->info));
> +	if (of_property_read_u32_array(client->dev.of_node, "vout-voltage-divider",
> +				       data->vout_voltage_divider,
> +				       ARRAY_SIZE(data->vout_voltage_divider)) != 0)
> +		memset(data->vout_voltage_divider, 0, sizeof(data->vout_voltage_divider));
> +	if (data->vout_voltage_divider[1] < data->vout_voltage_divider[0]) {
> +		dev_err(dev,
> +			"The total resistance of voltage divider is less than output resistance\n");
> +		return -ENODEV;
> +	}
>  
>  	ret = i2c_smbus_read_word_data(client, MAX20730_MFR_DEVSET1);
>  	if (ret < 0)
>