Re: [PATCH] media: Documentation: Fix frame interval calculation for raw camera sensors

[Jacopo Mondi <jacopo.mondi@ideasonboard.com>]

Hi Sakari

On Mon, Feb 23, 2026 at 12:06:01PM +0200, Sakari Ailus wrote:
> Hi Jacopo,
>
> On Mon, Feb 23, 2026 at 10:11:02AM +0100, Jacopo Mondi wrote:
> > Hi Jai
> >
> > On Thu, Feb 19, 2026 at 01:20:50PM +0530, Jai Luthra wrote:
> > > The previous frame interval formula used analogue crop dimensions. This
> > > breaks down for some sensors when binning.
> > >
> > > For example in imx219 the minimum FLL (frame length in lines) can be
> > > lower than the analogue crop height when binning, which would require a
> > > negative VBLANK to represent the actual timing. Similarly, imx283 allows
> > > a lower minimum HMAX (line length) when doing 2x2 or 3x3 binning than
> > > the analogue crop width of the full resolution mode.
> > >
> > > The CCS specification also describes under section "8.2.6 Line Length
> > > and Frame Length" how the horizontal and vertical readout minimums can
> > > be different when binning.
> > >
> > > Replace the formula with the underlying hardware concepts of LLP (line
> > > length in pixels) and FLL (frame length in lines). These terms were
> > > chosen to match the CCS specification on raw sensors, as it is a cleaner
> > > reference compared to a typical sensor vendor datasheet.
> > >
> > > Finally, define the blanking controls relative to the active pixel
> > > readout (post-binning) rather than the analogue crop size. This matches
> > > what most sensor drivers already do, and also what applications like
> > > libcamera expect. In "Figure 42" of CCS specification too, we see a
> > > similar definition:
> > >
> > >   frame interval = (output width + HBLANK) *
> > >                    (output height + VBLANK) / pixel rate
> > >
> > > Also add a note in the "Writing camera sensor drivers" guide, to ensure
> > > this formula is followed by new sensor drivers.
> >
> > I agree that using the analogue crop rectangle sizes is not correct,
> > however, with this new formulation the LLP and FLL values might be
> > smaller than the crop rectangles reported through the selection API,
> > which I'm not sure it's great from a consistency point of view ?
> >
> > Also the below suggested formulation:
> >
> >         LLP = active width + V4L2_CID_HBLANK
> >         FLL = active height + V4L2_CID_VBLANK
> >
> > Assumes the combined effect of [binning + subsampling] always directly
> > affect the readout time of pixels on the pixel array. The CCS specs
> > and driver seems to suggest that's not always the case ?
> >
> > Do we need to distinguish between binning modes that affect the
> > timings and binning modes that do not do that?
>
> For existing (pre-common raw sensor model) drivers, I'd say "no". CCS and

I wasn't thinking about existing drivers, they should continue to work
as they do nowadays not to break applications..


> common raw sensor model will use line length in pixels and frame length in
> lines for this, which reflects what the hardware does.
>

This won't support binning modes that do not change the pixel sampling
rate, when binning happens in the digital domain, after all lines and
pixels have been read out, if I understand correctly. I'm stepping
outside of my knowledge domain here as we're getting into the details
of a sensor implementation, but I guess configurations where binning
doesn't impact the frame timinings should be supported ?

> Existing non-CCS drivers have used the format for this purpose and adjusted
> the blanking values accordingly.

I don't think it's a problem for drivers, it's for userspace to have
a way to calculate the frame rate reliably in a way that supports all
sensors

>
> >
> > As we're going to introduce a control for binning to report the
> > binning factor in the image dimension domain, should we introduce
> > a control to specify the binning factor in the image timing domain ?
> >
> >         LLP = (analog_crop_width + HBLANK) / binning_timing_h
> >         FLL = (analog_crop_height + VBLANK) / binning_timing_v
> >         frame_interval = LLP * FLL / pixel_rate
> >
> > I'm not 100% sure this is correct however, as the blankings should be
> > expressed on a different clock domain that the pixel sampling rate,
> > but I guess this is a reasonable approximation ?
>
> If binning is done in analogue domain, the above would hold, but sensor do
> digital binning as well. The driver needs to adjust the blanking values

In which case, if a digital binning mode is used,
        binning_timing_h = binning_timing_v = 1;

> (and in the future frame length and line length values) to accommodate
> this.

Where should userspace get "frame length" and "line length" from "in
future" ?

If using the format, then the timings will always be affected by the
binning factor, wouldn't they ?

>
> >
> > >
> > > Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
> > > ---
> > >  Documentation/driver-api/media/camera-sensor.rst   | 11 ++++
> > >  .../userspace-api/media/drivers/camera-sensor.rst  | 59 +++++++++++++++-------
> > >  2 files changed, 53 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst
> > > index 94bd1dae82d5c570b2d11c7faee20dd45d2f4be6..8dcac7551f54ac4ffa71173281ae3bbea331c036 100644
> > > --- a/Documentation/driver-api/media/camera-sensor.rst
> > > +++ b/Documentation/driver-api/media/camera-sensor.rst
> > > @@ -120,6 +120,17 @@ The function returns a non-zero value if it succeeded getting the power count or
> > >  runtime PM was disabled, in either of which cases the driver may proceed to
> > >  access the device.
> > >
> > > +Frame interval
> > > +--------------
> > > +
> > > +If a sensor supports cropping or binning, it is the sensor driver's
> > > +responsibility to ensure that the frame interval formula (see
> > > +:ref:`media_using_camera_sensor_drivers`) remains valid regardless of the
> > > +pipeline configuration. The driver shall adjust the minimum and maximum allowed
> > > +values of ``V4L2_CID_HBLANK`` and ``V4L2_CID_VBLANK`` as needed when the mode
> > > +changes, so that application developers can always rely on the same formula to
> > > +calculate the frame interval.
> > > +
> > >  Rotation, orientation and flipping
> > >  ----------------------------------
> > >
> > > diff --git a/Documentation/userspace-api/media/drivers/camera-sensor.rst b/Documentation/userspace-api/media/drivers/camera-sensor.rst
> > > index 75fd9166383fdbb2dabdb6384ed0904c4e78a3c6..30dddea72a12da264fc9c30e37b561c762c09d29 100644
> > > --- a/Documentation/userspace-api/media/drivers/camera-sensor.rst
> > > +++ b/Documentation/userspace-api/media/drivers/camera-sensor.rst
> > > @@ -49,35 +49,60 @@ depends on the type of the device.
> > >  Raw camera sensors
> > >  ~~~~~~~~~~~~~~~~~~
> > >
> > > -Instead of a high level parameter such as frame interval, the frame interval is
> > > -a result of the configuration of a number of camera sensor implementation
> > > -specific parameters. Luckily, these parameters tend to be the same for more or
> > > -less all modern raw camera sensors.
> > > +Instead of a high level parameter such as frame interval, the frame interval on
> > > +a raw camera sensor is determined by a number of sensor-specific parameters.
> > > +These parameters tend to be common across most modern raw camera sensors.
> > >
> > > -The frame interval is calculated using the following equation::
> > > +The pixel array is the full grid of photosensitive elements on the sensor. A
> > > +subregion of it is selected by the analogue crop. The cropped image may then be
> > > +subject to binning (averaging of a NxN block) or subsampling which
> > > +further reduce the image dimensions. The resulting image is then read out by
> > > +the ADC (analogue-to-digital converter) line by line. After ADC readout,
> > > +optional digital crop or scaling may further reduce the image dimensions, see
> > > +:ref:`VIDIOC_SUBDEV_G_SELECTION <VIDIOC_SUBDEV_G_SELECTION>`.
> > >
> > > -	frame interval = (analogue crop width + horizontal blanking) *
> > > -			 (analogue crop height + vertical blanking) / pixel rate
> > > +The frame size is determined by two timing parameters: line length in pixels
> > > +(LLP) and frame length in lines (FLL). These are fundamental sensor timing
> > > +registers that control how fast the ADC reads out the image. They may go
> > > +by different names for a particular sensor, like HMAX and VMAX, or HTOTAL and
> > > +VTOTAL, or similar.
> > >
> > > -The formula is bus independent and is applicable for raw timing parameters on
> > > -large variety of devices beyond camera sensors. Devices that have no analogue
> > > -crop, use the full source image size, i.e. pixel array size.
> > > +LLP is the total number of pixel clock cycles per line, including both the
> > > +active readout width and horizontal blanking. FLL is the total number of lines
> > > +per frame, including both the active readout height and vertical blanking.
> > > +
> > > +The frame interval is::
> > > +
> > > +        frame interval = LLP * FLL / pixel rate
> > >
> > >  Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and
> > >  ``V4L2_CID_VBLANK``, respectively. The unit of the ``V4L2_CID_HBLANK`` control
> > >  is pixels and the unit of the ``V4L2_CID_VBLANK`` is lines. The pixel rate in
> > > -the sensor's **pixel array** is specified by ``V4L2_CID_PIXEL_RATE`` in the same
> > > -sub-device. The unit of that control is pixels per second.
> > > +the sensor's **pixel array** is specified by ``V4L2_CID_PIXEL_RATE`` in the
> > > +same sub-device. The unit of that control is pixels per second.
> > > +
> > > +The blanking is defined relative to the size of the image being sent out to the
> > > +host over the bus (like CSI-2)::
> > > +
> > > +        LLP = active width + V4L2_CID_HBLANK
> > > +        FLL = active height + V4L2_CID_VBLANK
> > > +
> > > +The driver shall set the minimum and maximum values of ``V4L2_CID_HBLANK`` and
> > > +``V4L2_CID_VBLANK`` such that the resulting LLP and FLL values correspond to the
> > > +range permitted by the sensor hardware for the current mode. Sensors that
> > > +support binning often define a lower minimum for LLP or FLL registers, which
> > > +can help achieve higher framerates when binning.
> > > +
> > > +Application developers can calculate the frame interval using the output
> > > +dimensions and the blanking controls::
> > > +
> > > +        frame interval = (output width + horizontal blanking) *
> > > +                         (output height + vertical blanking) / pixel rate
> > >
> > >  Register list-based drivers need to implement read-only sub-device nodes for the
> > >  purpose. Devices that are not register list based need these to configure the
> > >  device's internal processing pipeline.
> > >
> > > -The first entity in the linear pipeline is the pixel array. The pixel array may
> > > -be followed by other entities that are there to allow configuring binning,
> > > -skipping, scaling or digital crop, see :ref:`VIDIOC_SUBDEV_G_SELECTION
> > > -<VIDIOC_SUBDEV_G_SELECTION>`.
> > > -
> > >  USB cameras etc. devices
> > >  ~~~~~~~~~~~~~~~~~~~~~~~~
> > >
> > >
> > > ---
> > > base-commit: 956b9cbd7f156c8672dac94a00de3c6a0939c692
> > > change-id: 20260219-media-fps-docs-fd1da722cc38
> > >
> > > Best regards,
> > > --
> > > Jai Luthra <jai.luthra@ideasonboard.com>
> > >
>
> --
> Kind regards,
>
> Sakari Ailus