Message-Id: <20251005180025.69602-1-asmirnou@pinefeat.co.uk>
Date: Sun,  5 Oct 2025 19:00:25 +0100
From: Aliaksandr Smirnou <asmirnou@...efeat.co.uk>
To: kieran.bingham@...asonboard.com
Cc: asmirnou@...efeat.co.uk,
	conor+dt@...nel.org,
	devicetree@...r.kernel.org,
	hverkuil@...all.nl,
	jacopo.mondi@...asonboard.com,
	krzk+dt@...nel.org,
	linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org,
	mchehab@...nel.org,
	robh@...nel.org
Subject: Re: [PATCH v5 2/2] media: i2c: Pinefeat cef168 lens control board driver

Hi Kieran,

On Sun, 05 Oct 2025 15:32:45 +0100, Kieran Bingham wrote:
> I'm really looking forward to trying this device out sometime. Very
> interesting piece of kit - Thank you for working directly to upstream
> the support! That's really awesome.

Thanks! I appreciate that. It's been interesting work, and it's great
to see the device getting some attention. I'm looking forward to hearing
your impressions once you get a chance to try it out.

> > +  rx_data->focus_distance_min = le16_to_cpup((__le16 *)&rx_data->focus_distance_min);
> > +  rx_data->focus_distance_max = le16_to_cpup((__le16 *)&rx_data->focus_distance_max);
>
> What is the focus distance in this case? Is it a measured distance that
> could be applied to focus?

The focus distance reported by the lens is the distance between the
camera's sensor and the subject currently in focus, measured in meters.
It correlates with the focus position - higher positions correspond to
greater distances - but the relationship is non-linear.

Libcamera's autofocus algorithm works in dioptres, which are the inverse
of distance. In the driver, reading the distance from the lens allows
the creation of a piecewise linear (PWL) function that maps inverse
distance to the hardware lens setting.

> > +       case CEF168_V4L2_CID_CUSTOM(calibrate):
> > +               return cef168_i2c_write(dev, INP_CALIBRATE, 0);
> 
> Is there any documentation on how to use this control?

The control performs a calibration action operated by the controller
to determine the total number of focus steps. The action is invoked
by the calibration utility, which moves the lens, reads the changing
focus distance, and finally generates a PWL function to be included
in the Libcamera tuning file.

End users are not expected to trigger this control manually through
the V4L2 API, as it is intended for use by the calibration tool during
system setup. For this reason, separate user-facing documentation for
the control has not been added.

In other use cases, calibration can be performed simply by toggling
the AF/MF switch on the lens three times within 15 seconds.

> > +       case CEF168_V4L2_CID_CUSTOM(focus_range):
> > +               ctrl->p_new.p_u32[0] = ((u32)data.focus_position_min << 16) |
> > +                                      (u32)data.focus_position_max;
> 
> Is this really a custom control ? Is there any way to convey this
> through the min/max of the FOCUS_ABSOLUTE control ?

We aimed to minimize the use of custom controls, but in this case one
is necessary.

When the driver is already loaded and initialized, the focus range of
the lens may vary depending on the lens state and configuration
options. Because the control's operational range is defined during
driver probe, the Linux V4L2 API does not allow the minimum and
maximum values to be changed while the driver is running.

The maximum focus position is not known until calibration is
performed, so initially the position is set to zero. The focus range
may vary slightly between calibration runs, as it is derived from
counting motor steps, and its precision depends on the lens's focusing
mechanics.

Additionally, some lenses provide a two-position switch that changes
the minimum focusing distance, significantly reducing the focus range
 - for example, from 2100 to 800.

> > +       case CEF168_V4L2_CID_CUSTOM(lens_id):
> > +               ctrl->p_new.p_u8[0] = data.lens_id;
> > +               return 0;
> 
> Is this a specific individual ID value for the connected lens (i.e.
> every lens has a custom id?) or is it a reference to the lens
> model/type?
> 
> Is this something we could use in libcamera for instance to select an
> appropriate tuning file per lens (type) ?

It represents the ID of the lens model/type. Correct, it's purpose to
assist in selecting the appropriate autofocus algorithm settings.

For calibrated lenses, the controller stores the focus range in EEPROM,
allowing lenses to be swapped without repeating calibration. The
Libcamera tuning file includes an autofocus algorithm section that is
linked to the lens's focus range, focus distance, and focusing speed.
Therefore, reading the lens ID when loading the corresponding tuning
file after a lens swap is both convenient and necessary.

It can be read as follows:

v4l2-ctl -d /dev/v4l-subdev3 -C lens_id

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