Re: SCMI sensor reads unit scaling

From: Florian Fainelli <f.fainelli@gmail.com>
To: Guenter Roeck <linux@roeck-us.net>,
	Sudeep Holla <sudeep.holla@arm.com>,
	linux-hwmon@vger.kernel.org,
	"moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: SCMI sensor reads unit scaling
Date: Mon, 22 Apr 2019 11:31:18 -0700	[thread overview]
Message-ID: <2e6a6141-762f-7618-60b9-84fa1190813d@gmail.com> (raw)
In-Reply-To: <7d23066b-fd45-1784-3c2a-7e44eeae63bf@roeck-us.net>

On 4/20/19 10:31 AM, Guenter Roeck wrote:
> On 4/20/19 9:43 AM, Florian Fainelli wrote:
>> Hi Sudeep, Guenter,
>>
>> The current SCMI hwmon support does not seem to make use of the sensor
>> scale/unit as defined in the sensor replies (or the update scale for
>> that matter).
>>
>> I came up with the patch below which gets the job done, but I am worried
>> about possibly breaking people's SCMI deployments and sensors reading
>> because they may have intentionally or not already decided to return a
>> value which is scaled the way Linux's hwmon expect it, and may, or may
>> not have populated a valid unit number in the sensor reply.
>>
>> Ideally we should probably do two conversions:
>>
>> - from within scmi_sensor_reading_get(), scale the value as indicated by
>> the reply
>> - from within scmi_hwmon_read(), take that scaled value and apply the
>> necessary conversion expected by Linux's HWMON conventions (e.g.:
>> reporting voltage as mV values)
>>
>> What do you think?
>>
> 
> Reported values have to follow HWMON conventions. If not, this is a bug
> and needs to get fixed. The argument about not changing the userspace ABI
> does not apply to bugs.

Upon re-reading the SCMI specification, what is not clear to me is
whether you have the liberty to:

- target the Linux HWMON sensor units as specified by
Documentation/hwmon/sysfs-interface by e.g.: putting a millidegree
Celsius value in the SCMI reply and an power of 10 multiplier value of 0
(which would be treated as not doing any scaling)

- must provide the raw value as provided by the sensor as well as
indicate the scale, e.g.: millidegree celsius and specifying a scale of -3

Sudeep can you clarify how you would approach making use (or not) of the
power of 10 multiplier as defined by the SCMI specification?

Thanks!
-- 
Florian