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Date:   Tue, 17 May 2022 13:18:14 +0200
From:   Jacopo Mondi <jacopo@jmondi.org>
To:     Quentin Schulz <quentin.schulz@theobroma-systems.com>
Cc:     Quentin Schulz <foss+kernel@0leil.net>, shawnx.tu@intel.com,
        mchehab@kernel.org, robh+dt@kernel.org,
        krzysztof.kozlowski+dt@linaro.org, linux-media@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 4/4] media: i2c: ov5675: add .get_selection support
Message-ID: <20220517111814.gcu7zcog4atqzwdc@uno.localdomain>
References: <20220509143226.531117-1-foss+kernel@0leil.net>
 <20220509143226.531117-4-foss+kernel@0leil.net>
 <20220512090553.x7mzsj3ff3u5gqxq@uno.localdomain>
 <b77d43d5-4a50-3da2-67b4-65224a82a202@theobroma-systems.com>
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Hi Quentin

On Tue, May 17, 2022 at 11:25:17AM +0200, Quentin Schulz wrote:
> Hi Jacopo,
>
> On 5/12/22 11:05, Jacopo Mondi wrote:
> > Hi Quentin,
> >
> > On Mon, May 09, 2022 at 04:32:26PM +0200, Quentin Schulz wrote:
> > > From: Quentin Schulz <quentin.schulz@theobroma-systems.com>
> > >
> > > The sensor has 2592*1944 active pixels, surrounded by 16 active dummy
> > > pixels and there are an additional 24 black rows "at the bottom".
> > >
> > > As recommended in the SELECTION API documentation, let's put the first
> > > useful active pixel at the top/left corner (0,0).
> > >
> > > This window is the default and maximal crop allowed by the sensor.
> > >
> > > Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
> > > ---
> > >
> > > added in v3
> > >
> > >   drivers/media/i2c/ov5675.c | 25 +++++++++++++++++++++++++
> > >   1 file changed, 25 insertions(+)
> > >
> > > diff --git a/drivers/media/i2c/ov5675.c b/drivers/media/i2c/ov5675.c
> > > index 5544e1ae444e..8e3a5bc6c027 100644
> > > --- a/drivers/media/i2c/ov5675.c
> > > +++ b/drivers/media/i2c/ov5675.c
> > > @@ -78,6 +78,9 @@
> > >   #define OV5675_REG_FORMAT1		0x3820
> > >   #define OV5675_REG_FORMAT2		0x373d
> > >
> > > +#define OV5675_PIXEL_ARRAY_WIDTH	2592U
> > > +#define OV5675_PIXEL_ARRAY_HEIGHT	1944U
> > > +
> > >   #define to_ov5675(_sd)			container_of(_sd, struct ov5675, sd)
> > >
> > >   static const char * const ov5675_supply_names[] = {
> > > @@ -1115,6 +1118,27 @@ static int ov5675_get_format(struct v4l2_subdev *sd,
> > >   	return 0;
> > >   }
> > >
> > > +static int ov5675_get_selection(struct v4l2_subdev *sd,
> > > +				struct v4l2_subdev_state *state,
> > > +				struct v4l2_subdev_selection *sel)
> > > +{
> > > +	if (sel->which != V4L2_SUBDEV_FORMAT_ACTIVE)
> > > +		return -EINVAL;
> > > +
> > > +	switch (sel->target) {
> > > +	case V4L2_SEL_TGT_CROP:
> > > +	case V4L2_SEL_TGT_CROP_DEFAULT:
> > > +	case V4L2_SEL_TGT_CROP_BOUNDS:
> > > +		/* In HW, top/left corner is actually at (16,16) */
> > > +		sel->r.top = 0;
> > > +		sel->r.left = 0;
> > > +		sel->r.width = OV5675_PIXEL_ARRAY_WIDTH;
> > > +		sel->r.height = OV5675_PIXEL_ARRAY_HEIGHT;
> > > +		return 0;
> > > +	}
> >
> > CROP_NATIVE = the full pixel array size = 2592x1944
> >
> > CROP_BOUNDS = the rectangle that contains all possible crop
> >                rectangles, aka the readable portion of your pixel array.
> >                If in your case the sensor can read out dummy and non
> >                active lines this is == NATIVE
> >
> > CROP_DEFAULT = the active/valid pixel area. If there are any
> >                 dummy/invalid lines the DEFAULT rectangle should not
> >                 include them
> >
> > CROP = the portion of the active pixel area cropped to produce the
> >         final image. You should look into the modes definition and
> >         inspect what values are programmed in register 0x380x (I don't
> >         have a datasheet hence I don't know what corresponds to what)
> >
> > Does this make any sense to you ?
> >
>
> It's difficult to make sense of the datasheet to be honest.
>

Seems like it is made on purpose, isn't it :)


> There's a 3296x2480px "full-size" rectangle, then the sensor array output

Are there that many blank/invalid lines/cols ?

> area called CROP and configurable through registers 0x380x, then another
> output area called WIN (for window) also configurable through registers
> 0x380x. The WIN area seems to be just a mask applied on top of CROP area
> ("timing is not affected").
>
> On top of that, the schema shows 24 black lines - I assume - incorrectly,
> one after the start of the full-size rectangle, one after the end of the
> full-size rectangle.
>
> Then the sensor array area region in another section explicitly specifies
> the sensor to be 2624x2000px, active dummy pixels (2 16-pixel rows and
> columns) and black lines (1 24-pixel line) included. Which makes the actual
> useful area 2592x1944px.
>
> In the datasheet, the default size for the CROP area is
> 2624x1952px, offset (0,12) and for WIN it is 2592x1944px, offset (16,4)
> (assumed relative to CROP given the second coordinate).
>
> In the kernel driver though, the 2592x1944px mode configures the CROP area
> to be 2624x1968px offset (0,4) and the WIN area to be 2592x1944px, offset
> (16,13).
>
> The datasheet only ever mentions 2592x1944px as being the max resolution of
> the sensor though the sensor output area documentation and registers do not
> mention such a limitation.
>
> Since we're not modifying the crop area at the moment, CROP_DEFAULT and CROP
> would be the same, which would both be 2592x1944px.
>
> For the two others, I'm not sure. Any clue or hint with the info I just
> gave?

Is my intrpretation of the above correct ?

                     [2624]
        +------------------------------+
        |     |     16 dummy     |     |
        |------------------------------|
        |     |                  |     |
        |     |     [2592]       |     |
        |     |                  |     |
        |16   |      valid       | 16  |[2000]
        |dummy|                  |dummy|
        |     |            [1944]|     |
        |     |                  |     |
        |------------------------------|
        |     |     24 blacks    |     |
        |------------------------------|
        |     |     16 dummy     |     |
        -------------------------------|


Math looks right at least:

        2000 - 16 - 24 - 16 = 1944
        2624 - 16 - 16 = 2592

As V4L2 selection targets are defined in respect to they larger
sourrounding target, if the documentation about the full size array
(3296,2480) doesn't tell you what offset the readable part is, I think
it's fair to define
        - NATIVE == BOUNDS = (0, 0, 2624, 2000)
        - DEFAULT == CROP = (16, 16, 2592, 1944)

Or maybe better not define NATIVE at all.

The skipped rows/cols almost seems to match what the datasheet suggests by
combining the cropping and windowing rectangles top/left offsets (with
2 cols more skipped compared to my understanding)

        crop = (0, 12, 2624, 1952)
        win = (16, 6, 2592, 1944)

        CROP = (16, 18, 2592, 1944)

The driver does that a little differently with:

        crop = (0, 4, 2624, 1968)
        win = (16, 13, 2592, 1944)

        CROP = (16, 17, 2624, 1944)

Which seems a little off as the driver values for the VTS and HTS
register counts from 0 hence it seems to be skipping 17 rows and 18
cols (maybe the driver adjusts when writing the values to registers ?)

If you feel like it you can try to adjust the driver rectangles, but
in my experience subtle regressions might be introduced by moving
those values, hence I would not be too concerned and just report
whatever the driver does. I assume you're doing this in the context of
pleasing libcamera requirements, and as much as I don't like saying
this, if we're 1 or 2 columns off when reporting the CROP rectangle,
it's not a huge issue.

>
> Cheers,
> Quentin