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

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Message-ID: <fa79de78-aed9-4cd3-bff9-310f2b4a32c9@roeck-us.net>
Date: Mon, 4 Nov 2024 06:18:27 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Jerome Brunet <jbrunet@...libre.com>
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 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:

1000 0000 Disable all writes except to the WRITE_PROTECT command
0100 0000 Disable all writes except to the WRITE_PROTECT, OPERATION and
           PAGE commands
0010 0000 Disable all writes except to the WRITE_PROTECT, OPERATION,
           PAGE, ON_OFF_CONFIG and VOUT_COMMAND commands

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.

Thanks,
Guenter