[libcamera-devel] [PATCH 1/1] libipa: Add CameraSensorHelper for IMX258

[Dave Stevenson]

Hi Umang and Laurent

On Tue, 20 Jul 2021 at 01:02, Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
>
> Hi Umang,
>
> Thank you for the patch.
>
> On Mon, Jul 19, 2021 at 03:44:36PM +0530, Umang Jain wrote:
> > Extend the CameraSensorHelper factory with support for an
> > IMX258 sensor as found in the Nautilus Chromebook.
> >
> > The values are read by hacking the IMX258 kernel driver.
> > The values for analog gain constants are obtained by reading the
> > register indexes, corresponding to the analog gain constants, as
> > mentioned in MIPI CCS v1.1 specification.
>
> I'd expand this to explain why.
>
> The imx258 kernel driver hints that the IMX258 sensor may be compatible
> with the MIPI CCS specification, as the register set matches. The gain
> parameter values have been retrieved from the sensor by reading the
> analog gain read-only registers as defined by CCS.
>
> This being said, given that the maximum gain value doesn't match the
> driver, I think we need to run additional tests to figure out if this is
> actually correct. Setting the gain to 0, 256, 384 and 448 should result
> in gain values of x1, x2, x4 and x8 according to the coefficients, and
> it should be possible to validate that by capturing images with a fixed
> exposure time and calculate the average luminance for the different gain
> values. The exposure time should be picked to have a luminance value of
> about 10% when the gain is x1, as lower valus would result in measuring
> noise rather than actual data, and higher values would saturate at
> higher gains.
>
> If the formula holds with those values, you could try setting the gain
> to values higher than 448, up to the maximum value of 0x1fff accepted by
> the driver, to see what happens.
>
> If 448 is indeed the maximum value, then we should fix the driver
> accordingly.

Do these comments mean you're working without a datasheet?
We do have a datasheet for IMX258 (sorry, can't share it), and David
has IMX258 largely working on the Pi with some extra patches due to
only having 2 data lanes available or missing controls [1].

The gain formula is stated in the datasheet as:
Analog gain = 512 / (512 – X)
(m0 = 0, m1 = -1, c0 = 512, c1 = 512, as you've read back).

Range 0 to 480.
0 = x1
256 = x2
384 = x4
448 = x8
480 = x16.

Tested empirically, an exposure of 200 lines with analogue gain 480
(x16) is giving the same intensity image as exposure of 400 lines with
analogue gain 448 (x8).

  Dave

[1] Slightly hacky patches and need cleaning up, but look on our
shared kernel tree and branch 6by9/imx258.

> > Signed-off-by: Umang Jain <umang.jain at ideasonboard.com>
> > ---
> >  src/ipa/libipa/camera_sensor_helper.cpp | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/src/ipa/libipa/camera_sensor_helper.cpp b/src/ipa/libipa/camera_sensor_helper.cpp
> > index 709835a8..c43368df 100644
> > --- a/src/ipa/libipa/camera_sensor_helper.cpp
> > +++ b/src/ipa/libipa/camera_sensor_helper.cpp
> > @@ -295,6 +295,16 @@ public:
> >  };
> >  REGISTER_CAMERA_SENSOR_HELPER("imx219", CameraSensorHelperImx219)
> >
> > +class CameraSensorHelperImx258 : public CameraSensorHelper
> > +{
> > +public:
> > +        CameraSensorHelperImx258()
> > +        {
> > +                analogueGainConstants_ = { AnalogueGainLinear, 0, 512, -1, 512 };
> > +        }
> > +};
> > +REGISTER_CAMERA_SENSOR_HELPER("imx258", CameraSensorHelperImx258)
> > +
> >  class CameraSensorHelperOv5670 : public CameraSensorHelper
> >  {
> >  public:
>
> --
> Regards,
>
> Laurent Pinchart