Date:	Sat, 10 May 2014 08:32:40 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Andreas Werner <wernerandy@....de>
CC:	linux-kernel@...r.kernel.org, khali@...ux-fr.org,
	andreas.werner@....de
Subject: Re: Question: hwmon: Adding addtitional sysfs entries

On 05/10/2014 09:02 AM, Andreas Werner wrote:
> On Sat, May 10, 2014 at 05:32:13AM -0700, Guenter Roeck wrote:
>> On 05/10/2014 06:22 AM, Andreas Werner wrote:
>>> Hi,
>>> i am currently working on an implemenation of my Board Management Controller (BMC).
>>>
>>> This Controller is a MCR assembled on almost all of our Compact PCI or Compact PCI Serial
>>> Cards as well as on some other CPU Boards.
>>>
>>> The BMC includes LED´s, Watchdog, Voltage Monitoring and some other features.
>>> I have already implemented an MFD driver, LED and Watchdog driver.
>>>
>>> Currently i am working on the hwmon implemenation. This includes for the first step the
>>> voltage monitoring.
>>>
>>> The BMC has some other feature which are not supported by any of the subsystem.
>>>   - Errorcounter (Board status)
>>>   - Power Failure status
>>>   - Operating ours
>>
>> Guess you mean hours here.
>
> Yes sorry my mistake.
>
>>
>>>   - Do software reset, do cold reset
>>>   - Set power modes
>>>   - Backplate Geographical Address
>>>
>>> These are all informations related to the Board where the BMC is assembled to.
>>> I think all those features could be part of the hwmon implementation.
>>>
>> Why ?
>
> As all of the settings are CPU board related i thought it will be an good idea
> to put as much settings as possible to a group to one subsystem.
> This would be easier for the User Application.
>
Bad argument, even for the users. A user would not normally expect to find
the above information in unrelated subsystems such as hwmon. Keep in mind that
the Linux kernel is supposed to be useful for more than the application developers
in your company, and think about it - assume you would have to implement an
application which runs on more than one hardware, and attributes like the above
would be spread throughout the system. It would be all but impossible to write
generic applications, so you would want to avoid adding anything non-standard
as much as you can. And if you have to implement something non-standard, at least
implement it where people at least have a chance to find it.

>>
>>> Is it possible to add "custom" sysfs entries in my hwmon driver to support all these features?
>>> Does lm-sensor ingore those custom features or is it a problem for lm-sensors?
>>>
>>> What is the normal way to add those features?
>>>
>>
>> The hwmon subsystem isn't really the correct location for this functionality.
>> Power management should be done with a regulator driver (possibly power).
>> The other attributes and functionality would better fit into the mfd core
>> driver.
>>
>
> MFD Core sounds good. That is also perfect for the User App.
>
>> You might want to tie the reset functions into the system reset functionality.
>
> I do not understand this. Is there another subsystem or something like that
> were I can put the Reset stuff to?
>
Assuming you are talking about system reset, there is a function named machine_restart()
to accomplish that. This is architecture and, in many cases, machine specific functionality.
For example, many arm architectures have a different means to reset the system, and there
is a separate function pointer named arm_pm_restart to tie into that functionality.

Many restart functions are implemented in the drivers/power/reset directory, which may be
a good location for yours.

>
> I guess the hwmon subsys is the right place for the voltage monitoring (3,3V, 5V etc) isn´t it?
>
Yes, that is what it is for.

Guenter

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