linux-kernel - Re: [PATCH 4/5] hwmon: pmbus: mpq8785: Implement voltage scale loop configuration

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Message-ID: <ec2dbc97-664a-4eee-80cc-929f13f80129@roeck-us.net>
Date: Mon, 28 Apr 2025 17:27:27 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Pawel Dembicki <paweldembicki@...il.com>, linux-hwmon@...r.kernel.org
Cc: Jean Delvare <jdelvare@...e.com>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Jonathan Corbet <corbet@....net>,
 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
 Shen Lichuan <shenlichuan@...o.com>, Greg KH <gregkh@...uxfoundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Charles Hsu <ythsu0511@...il.com>,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH 4/5] hwmon: pmbus: mpq8785: Implement voltage scale loop
 configuration

On 4/28/25 15:13, Pawel Dembicki wrote:
> Implement support for setting the VOUT_SCALE_LOOP PMBus register
> based on an optional device tree property "voltage-scale-loop".
> 
> This allows the driver to provide the correct VOUT value depending
> on the feedback voltage divider configuration for chips where the
> bootloader does not configure the voltage scale.
> 
> Signed-off-by: Pawel Dembicki <paweldembicki@...il.com>
> ---
>   drivers/hwmon/pmbus/mpq8785.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/hwmon/pmbus/mpq8785.c b/drivers/hwmon/pmbus/mpq8785.c
> index e6a643856f08..6e2325d7f37b 100644
> --- a/drivers/hwmon/pmbus/mpq8785.c
> +++ b/drivers/hwmon/pmbus/mpq8785.c
> @@ -78,6 +78,8 @@ static int mpq8785_probe(struct i2c_client *client)
>   	struct device *dev = &client->dev;
>   	struct pmbus_driver_info *info;
>   	enum chips chip_id;
> +	u32 voltage_scale;
> +	int ret;
>   
>   	info = devm_kmemdup(dev, &mpq8785_info, sizeof(*info), GFP_KERNEL);
>   	if (!info)
> @@ -105,6 +107,14 @@ static int mpq8785_probe(struct i2c_client *client)
>   		return -ENODEV;
>   	}
>   
> +	if (!of_property_read_u32(dev->of_node, "voltage-scale-loop",
> +				  &voltage_scale)) {
> +		ret = i2c_smbus_write_word_data(client, PMBUS_VOUT_SCALE_LOOP,
> +						voltage_scale);

Per PMBus specification this value can be in any supported scale.
Also, the chips do not support the full 16-bit value range. I see 10 bit for
mpq8785 and mpm3695, 7 bit for mp2853, and 12 bit for MPM82504. There will
have to be some device specific range check.

I don't understand the units. The devicetree document talks about
VOUT_SCALE_LOOP = VFB / VOUT * 1000, but I don't see how that translates
into chip values. For MPM82504, the datasheet says 1 LSB=2mV. For mpm3695
it is 0.001/LSB. I have no idea how that is supposed to translate to the
units suggested in the property patch.

Either case, I think the property needs to be something generic that is well
defined. Maybe "pmbus,voltage-scale-loop", but I really don't know what would
be acceptable for DT maintainers.

Guenter