Re: [PATCH 1/4] media: docs: Describe pixel array properties

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Laurent,

On Wed, Aug 19, 2020 at 04:06:23AM +0300, Laurent Pinchart wrote:
> Hi Sakari,
> 
> On Tue, Aug 18, 2020 at 11:17:43AM +0300, Sakari Ailus wrote:
> > On Mon, Aug 10, 2020 at 10:17:57AM +0200, Jacopo Mondi wrote:
> > > On Sun, Aug 09, 2020 at 08:58:21PM +0300, Laurent Pinchart wrote:
> > > > On Wed, Aug 05, 2020 at 12:57:18PM +0200, Jacopo Mondi wrote:
> > > > > The V4L2 selection API are also used to access the pixel array
> > > > > properties of an image sensor, such as the size and position of active
> > > > > pixels and the cropped area of the pixel matrix used to produce images.
> > > > >
> > > > > Currently no clear definition of the different areas that compose an
> > > > > image sensor pixel array matrix is provided in the specification, and
> > > > > the actual meaning of each selection target when applied to an image
> > > > > sensor was not provided.
> > > > >
> > > > > Provide in the sub-device documentation the definition of the pixel
> > > > > matrix properties and the selection target associated to each of them.
> > > > >
> > > > > Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
> > > > > ---
> > > > >  .../userspace-api/media/v4l/dev-subdev.rst    | 81 +++++++++++++++++++
> > > > >  1 file changed, 81 insertions(+)
> > > > >
> > > > > diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
> > > > > index 134d2fb909fa4..c47861dff9b9b 100644
> > > > > --- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
> > > > > +++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
> > > > > @@ -386,6 +386,87 @@ requests on all selection targets, unless specifically told otherwise.
> > > > >  ``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
> > > > >  the image size either up or down. :ref:`v4l2-selection-flags`
> > > > >
> > > > > +.. _v4l2-subdev-pixel-array-properties:
> > > > > +
> > > > > +Selection targets for image sensors properties
> > > > > +----------------------------------------------
> > > > > +
> > > > > +The V4L2 selection API can be used on sub-devices that represent an image
> > > > > +sensor to retrieve the sensor's pixel array matrix properties by using the
> > > > > +:ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` ioctls.
> > > > > +
> > > > > +Sub-device drivers for image sensor usually register a single source pad, but in
> > > > > +the case they expose more, the pixel array properties can be accessed from
> > > > > +any of them.
> > > > > +
> > > > > +The ``V4L2_SEL_TGT_NATIVE``, ``V4L2_SEL_TGT_CROP_BOUNDS``,
> > > > > +``V4L2_SEL_TGT_CROP_DEFAULT`` and ``V4L2_TGT_CROP`` targets are used to retrieve
> > > > > +the immutable properties of the several different areas that compose the sensor
> > > > > +pixel array matrix. Each area describes a rectangle of logically adjacent pixel
> > > > > +units. The logical disposition of pixels is defined by the sensor read-out
> > > > > +starting point and direction, and may differ from the physical disposition of
> > > > > +the pixel units in the pixel array matrix.
> > > > > +
> > > > > +Each pixel matrix portion is contained in a larger rectangle, with the most
> > > > > +largest being the one that describes the pixel matrix physical size. This
> > > > > +defines a hierarchical positional system, where each rectangle is defined
> > > > > +relatively to the largest available one among the ones exposed by the
> > > > > +sub-device driver. Each selection target and the associated pixel array portion
> > > > > +it represents are below presented in order from the largest to the smallest one.
> > > > > +
> > > > > +Pixel array physical size
> > > > > +^^^^^^^^^^^^^^^^^^^^^^^^^
> > > > > +
> > > > > +The image sensor chip is composed by a number of physical pixels, not all of
> > > > > +them readable by the application processor. Invalid or unreadable lines might
> > > > > +not be transmitted on the data bus at all, or in case on CSI-2 capable sensors
> > > > > +they might be tagged with an invalid data type (DT) so that the receiver
> > > > > +automatically discard them. The size of the whole pixel matrix area is
> > > > > +retrieved using the V4L2_SEL_TGT_NATIVE target, which has its top-left corner
> > > > > +defined as position (0, 0). All the other selection targets are defined
> > > > > +relatively to this, larger, rectangle. The rectangle returned by
> > > > > +V4L2_SEL_TGT_NATIVE describes an immutable property of the image sensor, it
> > > > > +does not change at run-time and cannot be modified from userspace.
> > > >
> > > > As I think I've mentioned previously (not sure if it was by e-mail or on
> > > > IRC), we could also decide to set V4L2_SEL_TGT_NATIVE_SIZE ==
> > > > V4L2_SEL_TGT_CROP_BOUNDS by ignoring the non-readable pixels completely.
> > > > What's the advantage of exposing them in the API, when the sensors
> > > > doesn't provide them to the rest of the pipeline ?
> > > 
> > > I don't know :) I'm also  bit confused on what's the purpose of
> > > NATIVE, this commit seems to suggest it was meant to replace
> > > CROP_BOUNDS, but I'm not sure about that.
> > > 
> > > commit b518d86609cc066b626120fe6ec6fe3a4ccfcd54
> > > Author: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > Date:   Thu Nov 6 16:54:33 2014 -0300
> > > 
> > >     [media] smiapp: Support V4L2_SEL_TGT_NATIVE_SIZE
> > > 
> > >     Add support for selection target V4L2_SEL_TGT_NATIVE_SIZE. It is equivalent
> > >     of what V4L2_SEL_TGT_CROP_BOUNDS used to be. Support for
> > >     V4L2_SEL_TGT_CROP_BOUNDS is still supported by the driver as a compatibility
> > >     interface.
> > > 
> > > Sakari, do you recall if that's was the original plan ?
> > 
> > That was to denote the size of the pixel array indeed. We didn't discuss
> > dark or invalid pixels at the time.
> > 
> > So this was just there to tell that it's the pixel array you're cropping
> > from.
> > 
> > But as long as it's API-wise compatible, I don't think anything prevents
> > re-purposing this to include other areas. The documentation (AFAIR) does
> > not say this has to be the same as the crop bounds rectangle.
> 
> What do you think would be best ? Should we include the non-readable
> pixels in V4L2_SEL_TGT_NATIVE_SIZE, with V4L2_SEL_TGT_CROP_BOUNDS then
> being strictly smaller, or drop them completely from the API, with
> V4L2_SEL_TGT_CROP_BOUNDS being equal to V4L2_SEL_TGT_NATIVE_SIZE ? It
> may be that we have to allow both to support existing drivers, but we
> should pick one of the two options and make it mandatory for new
> drivers.

That's a very good question.

What would be the purpose of adding pixels that cannot be read? I assume
they would not affect sensor timing either in that case, so there would be
no difference whether they are there or not. The crop bounds should contain
everything whereas for the default crop should reflect the area of the
visible pixels.

I guess in theory the visible pixels could not be cropped by the sensor in
analogue cropping step, so it might be worth having a separate rectangle
for those, too.

There could also be sensors that read the dark pixels internally and use
them somehow without sending their data out. don't think we should try to
model that though as it's likely entirely internal to the sensor in that
case.

-- 
Sakari Ailus