linux-kernel - Re: [PATCH v2 2/2] hwmon: pmbus: Add ltc4286 driver

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Message-ID: <4b73a239-c90e-4515-b89d-65de15a1e9a9@roeck-us.net>
Date:   Mon, 6 Nov 2023 19:29:56 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Delphine_CC_Chiu/WYHQ/Wiwynn <Delphine_CC_Chiu@...ynn.com>,
        "patrick@...cx.xyz" <patrick@...cx.xyz>,
        Jean Delvare <jdelvare@...e.com>,
        Jonathan Corbet <corbet@....net>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] hwmon: pmbus: Add ltc4286 driver

On 11/6/23 19:08, Delphine_CC_Chiu/WYHQ/Wiwynn wrote:
>> -----Original Message-----
>> From: Guenter Roeck <groeck7@...il.com> On Behalf Of Guenter Roeck
>> Sent: Tuesday, October 31, 2023 9:47 PM
>> To: Delphine_CC_Chiu/WYHQ/Wiwynn <Delphine_CC_Chiu@...ynn.com>;
>> patrick@...cx.xyz; Jean Delvare <jdelvare@...e.com>; Jonathan Corbet
>> <corbet@....net>
>> Cc: Rob Herring <robh+dt@...nel.org>; Krzysztof Kozlowski
>> <krzysztof.kozlowski+dt@...aro.org>; Conor Dooley <conor+dt@...nel.org>;
>> linux-i2c@...r.kernel.org; linux-hwmon@...r.kernel.org;
>> devicetree@...r.kernel.org; linux-kernel@...r.kernel.org;
>> linux-doc@...r.kernel.org
>> Subject: Re: [PATCH v2 2/2] hwmon: pmbus: Add ltc4286 driver
>>
>>    Security Reminder: Please be aware that this email is sent by an external
>> sender.
>>
>> On 10/30/23 23:46, Delphine_CC_Chiu/WYHQ/Wiwynn wrote:
>> [ ... ]
>>>>
>>>>> +
>>>>> +     ret = of_property_read_u32(client->dev.of_node,
>>>>> +                                "shunt-resistor-micro-ohms",
>>>> &rsense);
>>>>> +     if (ret < 0)
>>>>> +             return ret;
>>>>> +
>>>>> +     if (rsense == 0)
>>>>> +             return -EINVAL;
>>>>> +
>>>>> +     info = &ltc4286_info;
>>>>> +
>>>>> +     /* Default of VRANGE_SELECT = 1, 102.4V */
>>>>> +     if (device_property_read_bool(&client->dev,
>>>> "adi,vrange-select-25p6")) {
>>>>
>>>> What if the adi,vrange-select-25p6 property is not provided, but the
>>>> chip is programmed for this range ?
>>> The binding document tells programmers how to fill the dts.
>>> Thus, programmers must fill this property if their system is 25.6 volts voltage
>> range.
>>>
>>
>> Sure, but there is no else case, meaning VRANGE_SELECT is unmodified in that
>> case. There is no guarantee that the chip is in its power-on state.
> 
> The else case is in v2 ltc4286.c line 133
> It means that the voltage range for programmer is 102.4 volts which is default value,
> so driver doesn't need to do any change for VRANGE_SELECT bit.

There is no guarantee that the value wasn't changed before the driver was loaded.

Guenter

> Additionally, we have checked the behavior of VRANGE_SELECT bit with vendor.
> Below is the reply from vendor:
> [Our question]
> If we change the VRANGE_SELECT bit value to 0, the value would return to default after we restart this device?
> Or VRANGE_SELECT bit value would never change util someone change its value?
> [ADI reply]
> chip reset will go back to default. Thanks.
> 
> Thus, we must overwrite this bit if the user fill adi,vrange-select-25p6 here whenever driver probes
> 
>>
>> Guenter
>