Re: [PATCH v3 1/6] Documentation: media: Improve camera sensor documentation

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

Hi Kieran,

Thanks for the review.

On Tue, Jul 27, 2021 at 11:54:14AM +0100, Kieran Bingham wrote:
> Hi Sakari
> 
> On 24/06/2021 09:40, Sakari Ailus wrote:
> > Modernise the documentation to make it more precise and update the use of
> > pixel rate control and various other changes. In particular:
> > 
> > - Use non-proportional font for file names, properties as well as
> >   controls.
> > 
> > - The unit of the HBLANK control is pixels, not lines.
> > 
> > - The unit of PIXEL_RATE control is pixels per second, not Hz.
> > 
> > - Merge common requirements for CSI-2 and parallel busses.>
> > - Include all DT properties needed for assigned clocks.
> > 
> > - Fix referencing the link rate control.
> > 
> > - SMIA driver's new name is CCS driver.
> > 
> > - The PIXEL_RATE control denotes pixel rate on the pixel array on camera
> >   sensors. Do not suggest it is used to tell the maximum pixel rate on the
> >   bus anymore.
> > 
> > - Improve ReST syntax (plain struct and function names).
> > 
> > - Remove the suggestion to use s_power() in receiver drivers.
> > 
> > - Make MIPI website URL use HTTPS, add Wikipedia links to BT.601 and
> >   BT.656.
> > 
> > Fixes: e4cf8c58af75 ("media: Documentation: media: Document how to write camera sensor drivers")
> > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
> > Reviewed-by: Andrey Konovalov <andrey.konovalov@linaro.org>
> > ---
> >  .../driver-api/media/camera-sensor.rst        |  45 +++----
> >  Documentation/driver-api/media/csi2.rst       |  94 --------------
> >  Documentation/driver-api/media/index.rst      |   2 +-
> >  Documentation/driver-api/media/tx-rx.rst      | 117 ++++++++++++++++++
> >  .../media/v4l/ext-ctrls-image-process.rst     |   2 +
> >  5 files changed, 137 insertions(+), 123 deletions(-)
> >  delete mode 100644 Documentation/driver-api/media/csi2.rst
> >  create mode 100644 Documentation/driver-api/media/tx-rx.rst
> > 
> > diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst
> > index 7160336aa475..c7d4891bd24e 100644
> > --- a/Documentation/driver-api/media/camera-sensor.rst
> > +++ b/Documentation/driver-api/media/camera-sensor.rst
> > @@ -3,10 +3,10 @@
> >  Writing camera sensor drivers
> >  =============================
> >  
> > -CSI-2
> > ------
> > +CSI-2 and parallel (BT.601 and BT.656) busses
> 
> Busses looks odd to me, so I've got to look it up:
>   https://www.grammarly.com/blog/busses-buses/
> 
> Though I found this one more of an entertaining read:
>   https://www.merriam-webster.com/words-at-play/plural-of-bus
> 
> Technically, some dictionaries still reference busses it seems, so this
> isn't specifically an error, just that buses is more commonly used (and
> it doesn't appear to be a US/UK thing?)

It's interesting you say that.

The Cambridge dictionary notes "busses" is American and then contradicts
itself by saying it's the other way around. I thought so, too.

Foldoc exclusively uses "busses" as plural of "bus" in its description of
the word "bus".

Then I also read in Wiktionary "buss" is also entirely valid, but has other
meanings, too. I thought it was only in Swedish. "Bus" means a "hobo" in
Swedish.

I like busses. I don't think there's much risk of mistaking it with e.g. a
herring buss in this case.

> 
> 
> > +---------------------------------------------
> >  
> > -Please see what is written on :ref:`MIPI_CSI_2`.
> > +Please see :ref:`transmitter-receiver`.
> >  
> >  Handling clocks
> >  ---------------
> > @@ -26,15 +26,16 @@ user.
> >  ACPI
> >  ~~~~
> >  
> > -Read the "clock-frequency" _DSD property to denote the frequency. The driver can
> > -rely on this frequency being used.
> > +Read the ``clock-frequency`` _DSD property to denote the frequency. The driver
> > +can rely on this frequency being used.
> >  
> >  Devicetree
> >  ~~~~~~~~~~
> >  
> > -The currently preferred way to achieve this is using "assigned-clock-rates"
> > -property. See Documentation/devicetree/bindings/clock/clock-bindings.txt for
> > -more information. The driver then gets the frequency using clk_get_rate().
> > +The currently preferred way to achieve this is using ``assigned-clocks``,
> > +``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See
> > +``Documentation/devicetree/bindings/clock/clock-bindings.txt`` for more
> > +information. The driver then gets the frequency using ``clk_get_rate()``.
> >  
> >  This approach has the drawback that there's no guarantee that the frequency
> >  hasn't been modified directly or indirectly by another driver, or supported by
> > @@ -55,7 +56,7 @@ processing pipeline as one or more sub-devices with different cropping and
> >  scaling configurations. The output size of the device is the result of a series
> >  of cropping and scaling operations from the device's pixel array's size.
> >  
> > -An example of such a driver is the smiapp driver (see drivers/media/i2c/smiapp).
> > +An example of such a driver is the CCS driver (see ``drivers/media/i2c/ccs``).
> >  
> >  Register list based drivers
> >  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > @@ -67,7 +68,7 @@ level are independent. How a driver picks such configuration is based on the
> >  format set on a source pad at the end of the device's internal pipeline.
> >  
> >  Most sensor drivers are implemented this way, see e.g.
> > -drivers/media/i2c/imx319.c for an example.
> > +``drivers/media/i2c/imx319.c`` for an example.
> >  
> >  Frame interval configuration
> >  ----------------------------
> > @@ -94,9 +95,10 @@ large variety of devices beyond camera sensors. Devices that have no analogue
> >  crop, use the full source image size, i.e. pixel array size.
> >  
> >  Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and
> > -``V4L2_CID_VBLANK``, respectively. The unit of these controls are lines. The
> > -pixel rate is specified by ``V4L2_CID_PIXEL_RATE`` in the same sub-device. The
> > -unit of that control is Hz.
> > +``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.
> >  
> >  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
> > @@ -125,14 +127,14 @@ general, the device must be powered on at least when its registers are being
> >  accessed and when it is streaming.
> >  
> >  Existing camera sensor drivers may rely on the old
> > -:c:type:`v4l2_subdev_core_ops`->s_power() callback for bridge or ISP drivers to
> > +struct v4l2_subdev_core_ops->s_power() callback for bridge or ISP drivers to
> >  manage their power state. This is however **deprecated**. If you feel you need
> >  to begin calling an s_power from an ISP or a bridge driver, instead please add
> >  runtime PM support to the sensor driver you are using. Likewise, new drivers
> >  should not use s_power.
> >  
> >  Please see examples in e.g. ``drivers/media/i2c/ov8856.c`` and
> > -``drivers/media/i2c/smiapp/smiapp-core.c``. The two drivers work in both ACPI
> > +``drivers/media/i2c/ccs/ccs-core.c``. The two drivers work in both ACPI
> >  and DT based systems.
> >  
> >  Control framework
> > @@ -149,16 +151,3 @@ used to obtain device's power state after the power state transition:
> >  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.
> > -
> > -Controls
> > ---------
> > -
> > -For camera sensors that are connected to a bus where transmitter and receiver
> > -require common configuration set by drivers, such as CSI-2 or parallel (BT.601
> > -or BT.656) bus, the ``V4L2_CID_LINK_FREQ`` control is mandatory on transmitter
> > -drivers. Receiver drivers can use the ``V4L2_CID_LINK_FREQ`` to query the
> > -frequency used on the bus.
> > -
> > -The transmitter drivers should also implement ``V4L2_CID_PIXEL_RATE`` control in
> > -order to tell the maximum pixel rate to the receiver. This is required on raw
> > -camera sensors.
> > diff --git a/Documentation/driver-api/media/csi2.rst b/Documentation/driver-api/media/csi2.rst
> > deleted file mode 100644
> > index 11c52b0be8b8..000000000000
> > --- a/Documentation/driver-api/media/csi2.rst
> > +++ /dev/null
> > @@ -1,94 +0,0 @@
> > -.. SPDX-License-Identifier: GPL-2.0
> > -
> > -.. _MIPI_CSI_2:
> > -
> > -MIPI CSI-2
> > -==========
> > -
> > -CSI-2 is a data bus intended for transferring images from cameras to
> > -the host SoC. It is defined by the `MIPI alliance`_.
> > -
> > -.. _`MIPI alliance`: http://www.mipi.org/
> > -
> > -Media bus formats
> > ------------------
> > -
> > -See :ref:`v4l2-mbus-pixelcode` for details on which media bus formats should
> > -be used for CSI-2 interfaces.
> > -
> > -Transmitter drivers
> > --------------------
> > -
> > -CSI-2 transmitter, such as a sensor or a TV tuner, drivers need to
> > -provide the CSI-2 receiver with information on the CSI-2 bus
> > -configuration. These include the V4L2_CID_LINK_FREQ and
> > -V4L2_CID_PIXEL_RATE controls and
> > -(:c:type:`v4l2_subdev_video_ops`->s_stream() callback). These
> > -interface elements must be present on the sub-device represents the
> > -CSI-2 transmitter.
> > -
> > -The V4L2_CID_LINK_FREQ control is used to tell the receiver driver the
> > -frequency (and not the symbol rate) of the link. The V4L2_CID_PIXEL_RATE
> > -control may be used by the receiver to obtain the pixel rate the transmitter
> > -uses. The :c:type:`v4l2_subdev_video_ops`->s_stream() callback provides an
> > -ability to start and stop the stream.
> > -
> > -The value of the V4L2_CID_PIXEL_RATE is calculated as follows::
> > -
> > -	pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample
> > -
> > -where
> > -
> > -.. list-table:: variables in pixel rate calculation
> > -   :header-rows: 1
> > -
> > -   * - variable or constant
> > -     - description
> > -   * - link_freq
> > -     - The value of the V4L2_CID_LINK_FREQ integer64 menu item.
> > -   * - nr_of_lanes
> > -     - Number of data lanes used on the CSI-2 link. This can
> > -       be obtained from the OF endpoint configuration.
> > -   * - 2
> > -     - Two bits are transferred per clock cycle per lane.
> > -   * - bits_per_sample
> > -     - Number of bits per sample.
> > -   * - k
> > -     - 16 for D-PHY and 7 for C-PHY
> > -
> > -The transmitter drivers must, if possible, configure the CSI-2
> > -transmitter to *LP-11 mode* whenever the transmitter is powered on but
> > -not active, and maintain *LP-11 mode* until stream on. Only at stream
> > -on should the transmitter activate the clock on the clock lane and
> > -transition to *HS mode*.
> > -
> > -Some transmitters do this automatically but some have to be explicitly
> > -programmed to do so, and some are unable to do so altogether due to
> > -hardware constraints.
> > -
> > -Stopping the transmitter
> > -^^^^^^^^^^^^^^^^^^^^^^^^
> > -
> > -A transmitter stops sending the stream of images as a result of
> > -calling the ``.s_stream()`` callback. Some transmitters may stop the
> > -stream at a frame boundary whereas others stop immediately,
> > -effectively leaving the current frame unfinished. The receiver driver
> > -should not make assumptions either way, but function properly in both
> > -cases.
> > -
> > -Receiver drivers
> > -----------------
> > -
> > -Before the receiver driver may enable the CSI-2 transmitter by using
> > -the :c:type:`v4l2_subdev_video_ops`->s_stream(), it must have powered
> > -the transmitter up by using the
> > -:c:type:`v4l2_subdev_core_ops`->s_power() callback. This may take
> > -place either indirectly by using :c:func:`v4l2_pipeline_pm_get` or
> > -directly.
> > -
> > -Formats
> > --------
> > -
> > -The media bus pixel codes document parallel formats. Should the pixel data be
> > -transported over a serial bus, the media bus pixel code that describes a
> > -parallel format that transfers a sample on a single clock cycle is used.
> > diff --git a/Documentation/driver-api/media/index.rst b/Documentation/driver-api/media/index.rst
> > index 813d7db59da7..08e206567408 100644
> > --- a/Documentation/driver-api/media/index.rst
> > +++ b/Documentation/driver-api/media/index.rst
> > @@ -37,7 +37,7 @@ Documentation/userspace-api/media/index.rst
> >      rc-core
> >      mc-core
> >      cec-core
> > -    csi2
> > +    tx-rx
> >      camera-sensor
> >  
> >      drivers/index
> > diff --git a/Documentation/driver-api/media/tx-rx.rst b/Documentation/driver-api/media/tx-rx.rst
> > new file mode 100644
> > index 000000000000..4c8584e7b6f2
> > --- /dev/null
> > +++ b/Documentation/driver-api/media/tx-rx.rst
> > @@ -0,0 +1,117 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +.. _transmitter-receiver:
> > +
> > +Pixel data transmitter and receiver drivers
> > +===========================================
> > +
> > +V4L2 supports various devices that transmit and receiver pixel data. Examples of
> > +these devices include a camera sensor, a TV tuner and a parallel or a CSI-2
> > +receiver in an SoC.
> > +
> > +Bus types
> > +---------
> > +
> > +The following busses are the most common. This section discusses these two only.
> > +
> > +MIPI CSI-2
> > +^^^^^^^^^^
> > +
> > +CSI-2 is a data bus intended for transferring images from cameras to
> > +the host SoC. It is defined by the `MIPI alliance`_.
> > +
> > +.. _`MIPI alliance`: https://www.mipi.org/
> > +
> > +Parallel
> > +^^^^^^^^
> > +
> > +`BT.601`_ and `BT.656`_ are the most common parallel busses.
> > +
> > +.. _`BT.601`: https://en.wikipedia.org/wiki/Rec._601
> > +.. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656
> > +
> > +Transmitter drivers
> > +-------------------
> > +
> > +Transmitter drivers generally need to provide the receiver drivers with the
> > +configuration of the transmitter. What is required depends on the type of the
> > +bus. These are common for both busses.
> > +
> > +Media bus pixel code
> > +^^^^^^^^^^^^^^^^^^^^
> > +
> > +See :ref:`v4l2-mbus-pixelcode`.
> > +
> > +Link frequency
> > +^^^^^^^^^^^^^^
> > +
> > +The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the
> > +receiver the frequency of the bus (i.e. it is not the same as the symbol rate).
> > +
> 
> Would the symbol rate be the same as the pixel rate, or is the 'bit'
> rate? ( I believe it's the bit rate, but I wonder if it needs to be
> defined here to make it clear?)

Neither. :-) Actually we could probably just drop this note as the symbol
rate isn't what the user space works with in general anyway.

> 
> I guess this is the distinction that the bus may send two bits per clock
> cycle or such.
> 
> 
> > +``.s_stream()`` callback
> > +^^^^^^^^^^^^^^^^^^^^^^^^
> > +
> > +The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
> > +receiver driver to control the transmitter driver's streaming state.
> > +
> > +
> > +CSI-2 transmitter drivers
> > +-------------------------
> > +
> > +Pixel rate
> > +^^^^^^^^^^
> > +
> > +The pixel rate on the bus is calculated as follows::
> > +
> > +	pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample
> > +
> > +where
> > +
> > +.. list-table:: variables in pixel rate calculation
> > +   :header-rows: 1
> > +
> > +   * - variable or constant
> > +     - description
> > +   * - link_freq
> > +     - The value of the ``V4L2_CID_LINK_FREQ`` integer64 menu item.
> > +   * - nr_of_lanes
> > +     - Number of data lanes used on the CSI-2 link. This can
> > +       be obtained from the OF endpoint configuration.
> > +   * - 2
> > +     - Data is transferred on both rising and falling edge of the signal.
> > +   * - bits_per_sample
> > +     - Number of bits per sample.
> > +   * - k
> > +     - 16 for D-PHY and 7 for C-PHY
> 
> Is 'k' a defined term here? It makes me assume kilo ... which clearly
> isn't its usage with values of 16 and 7?

It's a name of a constant used in relevant MIPI specs. So I thought using
the same should be fine.

> 
> 
> Fairly optional comments though so :
> 
> Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

Thank you. I already sent a pull request so if something goes wrong I'll
put this there, too.

-- 
Kind regards,

Sakari Ailus