Re: [lm-sensors] hwmon sysfs API: power attributes and PMBus devices

[Guenter Roeck <guenter.roeck@ericsson.com>]

Turns out there is another PMBus register which matches powerX_cap.
Updated list below.

Guenter

On Sun, Jun 27, 2010 at 03:53:29PM -0700, Guenter Roeck wrote:
> Hi all,
> 
> I am currently trying to match PMBus power related capabilities with the sysfs API.
> Unfortunately, the power attributes in the sysfs API don't follow the other attributes,
> so there is some degree of mismatch.
> 
> PMBus supports the following power limit registers:
> 
> PMBUS_POUT_OP_FAULT_LIMIT	Output power fault limit
> PMBUS_POUT_OP_WARN_LIMIT	Output power warning limit
> PMBUS_PIN_OP_WARN_LIMIT		Input power warning limit
> 
> the following status bits:
> 
> PB_POUT_OP_WARNING		Output power high warning
> PB_POUT_OP_FAULT		Output power fault
> PB_PIN_OP_WARNING		Input power high warning
> 
> and power usage registers for both input and output (PMBUS_READ_PIN and PMBUS_READ_POUT).
> 
> The sysfs API currently provides the following power attributes.
> 
> Attribute			Matching PMBus register
> 
> powerX_input			PMBUS_READ_PIN, PMBUS_READ_POUT
> powerX_cap			PMBUS_POUT_OP_WARN_LIMIT / PMBUS_PIN_OP_WARN_LIMIT ?
> 				Not an exact match, since the API specifically requires
> 				that "This file only appears if the cap is known
>                                 to be enforced by hardware".

	The PMBus POUT_MAX register is an exact match to the API.

> powerX_alarm			PB_POUT_OP_WARNING / PB_PIN_OP_WARNING ?
> 				Again not an exact match, since the API again requires
> 				that "This file only appears if the cap is known
>                                 to be enforced by hardware".

	There is also a POWER_LIMITING flag which indicates that the chip is in
	power limiting mode, enforcing POUT_MAX.

> 
> There is currently no support for powerX_crit (to reflect PMBUS_POUT_OP_FAULT_LIMIT)
> and powerX_fault (to reflect PB_POUT_OP_FAULT).

Given the above, there is also no match for the PMBUS_POUT_OP_WARN_LIMIT and
PMBUS_PIN_OP_WARN_LIMIT registers.

> 
> Questions: 
> - Can we remove the limitations of powerX_cap and powerX_alarm, ie no longer require
>   that the cap has to be enforced by hardware ?
	powerX_cap as defined should be retained and is supportable by PMBus devices.

> - If not, can we remove the requirement for powerX_alarm, and introduce powerX_max
>   to specify an alarm limit ?
	Introducing powerX_max would help to support the PMBUS_POUT_OP_WARN_LIMIT
   	and PMBUS_PIN_OP_WARN_LIMIT registers.
	powerX_alarm might then refer to "powerX_cap or powerX_max limit reached".

>   That would actually be more in line with other "_max" attributes, so it might be
>   a better solution.
> - Can we add new attributes powerX_crit and powerX_fault ?
> 
> Guenter

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