linux-kernel - Re: [PATCH v2 3/3] hwmon: pmbus: mpq8785: force direct mode for VID VOUT on MPQ8785/MPQ8786

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Message-ID: <fa07bf22-d56f-46fc-8e39-a50dae2efc46@roeck-us.net>
Date: Thu, 5 Feb 2026 09:46:05 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: carl.lee@....com
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Charles Hsu <ythsu0511@...il.com>, linux-hwmon@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	peter.shen@....com, colin.huang2@....com
Subject: Re: [PATCH v2 3/3] hwmon: pmbus: mpq8785: force direct mode for VID
 VOUT on MPQ8785/MPQ8786

On Thu, Feb 05, 2026 at 06:01:39PM +0800, Carl Lee via B4 Relay wrote:
> From: Carl Lee <carl.lee@....com>
> 
> According to MPQ8785/MPQ8786 datasheet, VID mode configuration is
> the same as direct mode configuration. Therefore, when VOUT is
> reported in VID mode, it must be forced to use direct format.
> 
> Signed-off-by: Carl Lee <carl.lee@....com>
> ---
>  drivers/hwmon/pmbus/mpq8785.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/hwmon/pmbus/mpq8785.c b/drivers/hwmon/pmbus/mpq8785.c
> index f35534836cb8..d6624af076c3 100644
> --- a/drivers/hwmon/pmbus/mpq8785.c
> +++ b/drivers/hwmon/pmbus/mpq8785.c
> @@ -48,6 +48,25 @@ static int mpq8785_identify(struct i2c_client *client,
>  	return 0;
>  };
>  
> +static int mpq8785_read_byte_data(struct i2c_client *client, int page, int reg)
> +{
> +	int ret;
> +
> +	switch (reg) {
> +	case PMBUS_VOUT_MODE:
> +		ret = pmbus_read_byte_data(client, page, reg);
> +		if (ret < 0)
> +			return ret;
> +
> +		if ((ret >> 5) == 1)
> +			return PB_VOUT_MODE_DIRECT;
> +	default:
> +		return -ENODATA;
> +	}
> +
> +	return ret;
> +}

In addition to my earlier reply, here is AI code review feedback:

This switch statement appears to fall through to the default case when the
mode is not VID (when (ret >> 5) != 1). If it falls through, it returns
-ENODATA.  The core function _pmbus_read_byte_data() will then see -ENODATA
and call pmbus_read_byte_data() again, resulting in a second I2C transaction
for the same register.

Also, the `return ret;` at the end of the function is unreachable because
the default case returns.

Should the PMBUS_VOUT_MODE case return `ret` instead of falling through?

Guenter