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 <linchuy...@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)
>
