Re: [PATCH v8 3/3] thermal/drivers/imx: Add calibration offset support

[CHENG Haoning (BCSC/ENG1)]

On 2026-07-17 06:23:44+00:00, CHENG Haoning (BCSC/ENG1) wrote:
> On Wed, Jul 15, 2026 at 11:06:07AM +0200, Frieder Schrempf wrote:
>
> > On 14.07.26 12:28, Haoning CHENG via B4 Relay wrote:
> >
> > Sorry to chime in so late. I just want to understand what this
> > calibration offset is about. Why would there be a need of a
> > board-specific offset? The sensor is in the SoC and if you add a
> > board-specific offset, you no longer measure the SoC core temperature,
> > right?
> >
> > How would you determine the offset in the first place? How would I know
> > what fsl,temp-calibration-offset-millicelsius should be set to? I could
> > put a sensor on the SoC case and use the delta as offset, but then I
> > would just account for the thermal resistance of the casing and not
> > measure the SoC core temperature anymore, right?
>
> Hi Frieder,
>
> Thanks for pointing this out. Your understanding is correct: after
> applying the offset, the reported value no longer represents the raw
> SoC die or junction temperature.
>
> For this board, the required "SoC temperature" is the package-surface
> temperature. The offset was derived by comparing the TEMPMON reading
> against a calibrated external sensor placed near the SoC package
> surface under steady-state thermal conditions, and is used to
> approximate the package-surface temperature from the internal TEMPMON
> reading.
>
> The Linux thermal framework does not require a thermal zone to use a
> specific temperature reference. It only requires the reported
> temperature and trip points to use the same temperature domain.
>
> In this driver, the offset is added in get_temp() and subtracted in
> set_alarm() and set_panic() when programming the hardware thresholds.
> This keeps the reported temperature and trip points in the same
> package-surface temperature domain. Doing this in the driver is
> necessary because the hardware alarm thresholds must also be offset-
> corrected; applying the offset only in userspace would cause the
> TEMPMON IRQ to fire at the wrong die temperature.
>
> I agree that "calibration offset" is misleading, since this does not
> calibrate TEMPMON to a more accurate junction temperature. In the next
> revision, the commit messages and DT binding have been updated to
> describe this as a board-specific conversion offset from the internal
> sensor reading to an estimated package-surface temperature.
>
> Thanks,
> Haoning
Hi Frieder,

I need to correct my previous reply. After further discussion with our hardware team, the offset does NOT convert die temperature to package-surface temperature.

Their thermal characterization shows that the raw TEMPMON sensor reading itself deviates from the theoretical junction temperature - this deviation exists even at the die level, before any board or package influence. The offset corrects the sensor reading toward the actual junction temperature, so it is a genuine sensor calibration.

Board-level effects may contribute additionally, but they are not the primary reason for this feature. I apologize for the confusion - my earlier description was inaccurate. The v10 series reflects this corrected understanding.

Thanks,
Haoning