Hi Jacopo,
Thanks for the review.
On Wed, Jun 23, 2021 at 05:11:01PM +0200, Jacopo Mondi wrote:
> Hi Sakari,
> thanks for addressing comments on v1
>
> just a few minros, the rest looks very good!
>
> On Tue, Jun 22, 2021 at 02:21:55PM +0300, 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@xxxxxxxxxxxxxxx>
> > ---
> > .../driver-api/media/camera-sensor.rst | 45 +++----
> > Documentation/driver-api/media/index.rst | 2 +-
> > .../driver-api/media/{csi2.rst => tx-rx.rst} | 114 +++++++++++-------
> > .../media/v4l/ext-ctrls-image-process.rst | 2 +
> > 4 files changed, 90 insertions(+), 73 deletions(-)
> > rename Documentation/driver-api/media/{csi2.rst => tx-rx.rst} (39%)
> >
> > 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
> > +---------------------------------------------
> >
> > -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/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/csi2.rst b/Documentation/driver-api/media/tx-rx.rst
> > similarity index 39%
> > rename from Documentation/driver-api/media/csi2.rst
> > rename to Documentation/driver-api/media/tx-rx.rst
> > index 11c52b0be8b8..6331f93fb249 100644
> > --- a/Documentation/driver-api/media/csi2.rst
> > +++ b/Documentation/driver-api/media/tx-rx.rst
> > @@ -1,39 +1,71 @@
> > .. SPDX-License-Identifier: GPL-2.0
> >
> > -.. _MIPI_CSI_2:
> > +.. _transmitter-receiver:
> > +
> > +Pixel data transmitter and receiver drivers
> > +===========================================
> > +
> > +V4L2 supports various devices that transmit and receiver pixel data. Examples of
>
> s/and receiver/and receives/
"receive"; it's plural. :-)
>
> > +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`: http://www.mipi.org/
> > +.. _`MIPI alliance`: https://www.mipi.org/
> >
> > -Media bus formats
> > ------------------
> > +Parallel
> > +^^^^^^^^
> >
> > -See :ref:`v4l2-mbus-pixelcode` for details on which media bus formats should
> > -be used for CSI-2 interfaces.
> > +`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
> > -------------------
> >
> > -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.
> > +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
> > +^^^^^^^^^^^^^^^^^^^^
> > +
> > +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.
>
> I guess you intend to document that, in example,
> MEDIA_BUS_FMT_YUYV8_2X8 has to be used on parallel busses, as the bus
> width is meaningful, while MEDIA_BUS_FMT_YUYV8_1X16 should be used on
> serial busses as the bus width it's not. I guess this could be
> reported as an example ?
We actually have such an example in media bus pixel code documentation.
How about removing the paragraph and leaving just the reference below?
>
> > +
> > +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).
> > +
> > +``.s_stream()`` callback
> > +^^^^^^^^^^^^^^^^^^^^^^^^
> >
> > -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 struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
> > +receiver driver to control the transmitter driver's streaming state.
> >
> > -The value of the V4L2_CID_PIXEL_RATE is calculated as follows::
> > +
> > +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
> >
> > @@ -45,27 +77,38 @@ where
> > * - variable or constant
> > - description
> > * - link_freq
> > - - The value of the V4L2_CID_LINK_FREQ integer64 menu item.
> > + - 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.
> > + - 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
> >
> > -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*.
> > +.. note::
> > +
> > + The pixel rate calculated this way is **not** be the same thing than
> > + the pixel rate on the camera sensor's pixel array.
>
> .. as reported by the V4L2_CID_PIXEL_RATE control.
Good point. I'll use "... which is reported by ...", to
>
> > +
> > +LP-11 and LP-111 modes
> > +^^^^^^^^^^^^^^^^^^^^^^
> > +
> > +The transmitter drivers must, if possible, configure the CSI-2 transmitter to
>
> s/drivers/driver/
> Or if you want to keep it plural drop "The" ?
It could be singular, and indefinite. I think this is better since it
obviously applies to all such drivers.
>
> > +*LP-11 or LP-111 mode* whenever the transmitter is powered on but not active,
> > +and maintain *LP-11 or LP-111 mode* until stream on. Only at stream on should
> > +the transmitter activate the clock on the clock lane and transition to *HS
>
> "Only at stream on time the transmitter should"
>
> Sounds better to me, but not being a native speaker I might very well
> be wrong :)
Agreed, I added "time".
>
> All minors, thanks for clearing this up
> Reviewed-by: Jacopo Mondi <jacopo@xxxxxxxxxx>
Thank you!
--
Kind regards,
Sakari Ailus
