linux-kernel - Re: [PATCH v2] hwmon: pmbus: max20730: adjust the vout reading given voltage divider

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Message-ID: <196e30ce-634c-a543-814d-d17c6a926cf8@roeck-us.net>
Date:   Tue, 15 Sep 2020 20:02:47 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Chu Lin <linchuyuan@...gle.com>, jdelvare@...e.com,
        linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
        qiongwang@...gle.com, zhongqil@...gle.com, jasonling@...gle.com,
        belgaied@...gle.com
Subject: Re: [PATCH v2] hwmon: pmbus: max20730: adjust the vout reading given
 voltage divider

On 9/10/20 5:00 PM, 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@...gle.com>

For my reference:

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

We'll have to wait for DT review before I can apply the patch.

Thanks,
Guenter

> ---
> ChangeLog v1 -> v2
>   - Don't do anything to the ret if an error is returned from
>     pmbus_read_word
>   - avoid overflow when doing multiplication
> 
>  drivers/hwmon/pmbus/max20730.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/hwmon/pmbus/max20730.c b/drivers/hwmon/pmbus/max20730.c
> index c0bb05487e0e..83affdad4060 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 +112,14 @@ 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 && data->vout_voltage_divider[0] && data->vout_voltage_divider[1]) {
> +			u64 temp = DIV_ROUND_CLOSEST_ULL((u64)ret * data->vout_voltage_divider[1],
> +							 data->vout_voltage_divider[0]);
> +			ret = clamp_val(temp, 0, 0xffff);
> +		}
> +		break;
>  	default:
>  		ret = -ENODATA;
>  		break;
> @@ -329,6 +338,15 @@ 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)
>