Re: [PATCH v2 2/5] media: v4l: ctrls: Add a control for temperature

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Hi Benjamin,

On Wed, Apr 20, 2022 at 03:01:18PM +0200, Benjamin Mugnier wrote:
> On 20/04/2022 00:04, Guenter Roeck wrote:
> > On 4/19/22 14:01, Laurent Pinchart wrote:
> >> On Tue, Apr 19, 2022 at 12:28:06PM -0700, Guenter Roeck wrote:
> >>> On 4/19/22 11:24, Nicolas Dufresne wrote:
> >>>> Hi,
> >>>>
> >>>> adding linux-hwmon in CC for a wider feedback.
> >>>>
> >>>> Le vendredi 15 avril 2022 à 13:18 +0200, Benjamin Mugnier a écrit :
> >>>>> Add V4L2_CID_TEMPERATURE control to get temperature from sensor in
> >>>>> celsius as a volatile and read-only control, and its documentation.
> >>>>> Useful to monitor thermals from v4l controls for sensors that support
> >>>>> this.
> >>>>
> >>>> Any justification to expose a temperature sensor outside of the dedicated kernel
> >>>> API hwmon ? I know if may makes it harder to use, as the sensor isn't bound to
> >>>> the camera driver, and also the sensor may not work if the camera is not
> >>>> streaming. But in the end, the API in hwmon does not look that complex, and is
> >>>> perhaps more precise ?
> 
> This sensor is able to read the temperature even if not streaming.
> 
> >>>>
> >>>> All in all, I think we need a strong justification to implement a custom
> >>>> thermometer interface, something described here and documented with care to
> >>>> prevent abuse. I would also see opinion from folks outside of the linux-media,
> >>>> hence why I have CCed hwmon mailing list.
> >>>
> >>> All I can say is that this seems to be odd and a bit outside the scope of
> >>> v4l2. I would have expected the vgxy61 driver to register a hwmon device
> >>> instead.
> >>
> >> I don't have a definitive opinion yet, but as Nicolas raised the issue
> >> by pushing towards hwmon, I'll offer counter-arguments for the sake of
> >> it :-)
> >>
> >> The temperature sensor in the imaging sensor is used to measure the die
> >> temperature, in order to tune the noise and spectral response model of
> >> the imaging sensor. It's thus not a generic-purpose temperature sensor.
> >> If the feature were to be exposed through hwmon, userspace would need to
> >> associate an hwmon device to the imaging sensor V4L2 subdev (we have a
> >> way to do so through the MC API, it doesn't support hwmon at this point,
> >> but I suppose it could be added). There are also various constraints
> >> that tie the temperature reading to the imaging side, it could be that
> >> the temperature would only be readable while capturing frames. That's
> >> probably possible to handle too but returning an error from the
> >> temperature read.
> >>
> >> Code-wise, both the driver and userspace would be more complex, for very
> >> little practical gain (I don't dispute a theorical gain).
> >>
> > 
> > All I can say is - not my subsystem, not my call to make. If you say this
> > is special and is better handled in V4L2, I'll take you by your word.
> > 
> > Guenter
> > 
> 
> I'll happily implement whatever conclusion we make here.
> 
> I could also drop this control for the first iteration of the driver,
> and come back later once a consensus is reached.

That would work too. By the way, what are your use cases for the
temperature sensor ? Have you added the control for the sake of
completeness, or do you have use cases ?

-- 
Regards,

Laurent Pinchart



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