linux-kernel - Re: [PATCH v3 3/6] hwmon: (pmbus/core) add wp module param

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Message-ID: <1jfro783na.fsf@starbuckisacylon.baylibre.com>
Date: Mon, 04 Nov 2024 15:39:05 +0100
From: Jerome Brunet <jbrunet@...libre.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Jean Delvare <jdelvare@...e.com>,  Jonathan Corbet <corbet@....net>,
  Patrick Rudolph <patrick.rudolph@...ements.com>,  Naresh Solanki
 <naresh.solanki@...ements.com>,  Rob Herring <robh@...nel.org>,  Krzysztof
 Kozlowski <krzk+dt@...nel.org>,  Conor Dooley <conor+dt@...nel.org>,
  Delphine CC Chiu <Delphine_CC_Chiu@...ynn.com>,
  linux-hwmon@...r.kernel.org,  linux-kernel@...r.kernel.org,
  linux-doc@...r.kernel.org,  devicetree@...r.kernel.org,
  linux-i2c@...r.kernel.org
Subject: Re: [PATCH v3 3/6] hwmon: (pmbus/core) add wp module param

On Mon 04 Nov 2024 at 06:18, Guenter Roeck <linux@...ck-us.net> wrote:

> On 11/4/24 00:43, Jerome Brunet wrote:
>
>>>> +/*
>>>> + * PMBus write protect forced mode:
>>>> + * PMBus may come up with a variety of write protection configuration.
>>>> + * 'pmbus_wp' may be used if a particular write protection is necessary.
>>>> + * The ability to actually alter the protection may also depend on the chip
>>>> + * so the actual runtime write protection configuration may differ from
>>>> + * the requested one. pmbus_core currently support the following value:
>>>> + * - 0: write protection removed
>>>> + * - 1: write protection fully enabled, including OPERATION and VOUT_COMMAND
>>>> + *      registers. Chips essentially become read-only with this.
>>>
>>> Would it be desirable to also suppport the ability to set the output voltage
>>> but not limits (PB_WP_VOUT) ?
>> I was starting simple, it is doable sure.
>> It is not something I will be able to test on actual since does not
>> support that.
>> Do you want me to add "2: write protection enable execpt for
>> VOUT_COMMAND." ?
>> 
>
> Please add it. I have a number of PMBus test boards and will be able to test it.
>
> Thee are three options, though. Per specification:

Any preference for the value mapped to each mode ? Using the one from
the spec does not seem practical (32768, 16384, 8192).

The bit number, maybe (7, 6, 5) ?

or just by order strongest locking ?

>
> 1000 0000 Disable all writes except to the WRITE_PROTECT command

3

> 0100 0000 Disable all writes except to the WRITE_PROTECT, OPERATION and
>           PAGE commands

2

> 0010 0000 Disable all writes except to the WRITE_PROTECT, OPERATION,
>           PAGE, ON_OFF_CONFIG and VOUT_COMMAND commands

1 ?

>
> The driver uses OPERATION and VOUT_COMMAND, so we should have options
> to disable them separately. It may be desirable, for example, to be able
> to turn on a regulator but not to change the voltages. Also, since
> full write protection also disables writes to the page register,
> the impact of full write protection on multi-page chips needs to be
> documented.

Noted. I'll update the documentation.

>
> Thanks,
> Guenter

-- 
Jerome