Re: Maximum rate value of input power for pmbus device

[Guenter Roeck <linux@roeck-us.net>]

On 7/8/20 3:50 AM, zlukwins wrote:
> 
> On 6/29/20 6:59 PM, Guenter Roeck wrote:
>> +linux-hwmon@vger.kernel.org
>>
>> On Mon, Jun 29, 2020 at 08:31:11AM +0200, zlukwins wrote:
>>> Hi,
>>>
>>>
>>> I am OpenBMC FW developer working currently on some power measurement stuff.
>>>
>>> I would like to have maximum rated input power for pmubus device available
>>> in hwmon sysfs. This value is read by MFR_PIN_MAX command:
>>>
>>> /MFR_PIN_MAX//
>>> //The MFR_PIN_MIN command sets or retrieves the maximum rated value, in
>>> watts, of//
>>> //the input power./
>> Interesting typo in the PMBus specification. Yes, it really does associate
>> MFR_PIN_MIN - which doesn't seem to exist - with the maximum rated output
>> power.
>>> And I wondering which attribute shell be used to expose that value in sysfs.
>>> I went through documentation
>>> (https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface) and found
>>> the following:
>>>
>>> power[1-*]_max            Maximum power.
>>>                 Unit: microWatt
>>>                 RW
>>>
>>> But it looks like it is already occupied by PIN_OP_WARN_LIMIT.
>>>
>>> Maybe new attribute shall be used? If so how to call that?
>>>
>> None of the standard attributes reports (or is supposed to report) rated
>> values, so we can not just use any of those.
>>
>> Also, we can not just add a single attribute to handle this situation,
>> for the simple reason that there are many more similar attributes.
>> PMBus specifies (this is from version 1.3.1):
>>
>> MFR_VIN_MIN
>> MFR_VIN_MAX
>> MFR_IIN_MAX
>> MFR_PIN_MAX
>> MFR_VOUT_MIN
>> MFR_VOUT_MAX
>> MFR_IOUT_MAX
>> MFR_POUT_MAX
>> MFR_TAMBIENT_MAX
>> MFR_TAMBIENT_MIN
>> MFR_MAX_TEMP_{1,2.3}
>>
>> All those report rated values. I do see the need/desire for reporting such
>> information. The only real solution I can see is to add a new set of
>> attributes to the hwmon ABI. Something like:
>>
>>     currentX_rated_min    # for consistency
>>     currentX_rated_max
>>     inX_rated_min
>>     inX_rated_max
>>     powerX_rated_min    # for consistency
>>     powerX_rated_max
>>     tempX_rated_min
>>     tempX_rated_max
>> plus maybe, for consistency:
>>     humidityX_rated_min
>>     humidityX_rated_max
>>
>> Those would be read-only attributes.
>>
>> Thoughts, comments, feedback anyone ?
>>
>> Thanks,
>> Guenter
> 
> 
> I really like your proposition but I guess we need to wait few more days for the feedback.
> 
> But have some questions here.
> 
> What about potential next steps when we all agreed to follow that approach. Should documentation modification reach upstream repository first and then e.g. pmbus hwmon module implementation?
> 
We'll need a series of patches. One to amend the documentation, one to add
the necessary code to the hwmon core (so that the core supports it with
the _with_info API), one to add support to the PMBus core, and one each
to add support to affected drivers.

Once this is all complete, the lm-sensors package should be updated
as well.

Guenter