[v8,2/2] Added Digiteq Automotive MGB4 driver documentation

  From: Martin Tůma <martin.tuma@digiteqautomotive.com>

The "admin-guide" documentation for the Digiteq Automotive MGB4 driver.

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
---
 Documentation/admin-guide/media/mgb4.rst      | 369 ++++++++++++++++++
 .../admin-guide/media/pci-cardlist.rst        |   1 +
 .../admin-guide/media/v4l-drivers.rst         |   1 +
 3 files changed, 371 insertions(+)
 create mode 100644 Documentation/admin-guide/media/mgb4.rst
  

On 04/07/2023 15:13, tumic@gpxsee.org wrote:
> From: Martin Tůma <martin.tuma@digiteqautomotive.com>
> 
> The "admin-guide" documentation for the Digiteq Automotive MGB4 driver.
> 
> Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
> ---
>  Documentation/admin-guide/media/mgb4.rst      | 369 ++++++++++++++++++
>  .../admin-guide/media/pci-cardlist.rst        |   1 +
>  .../admin-guide/media/v4l-drivers.rst         |   1 +
>  3 files changed, 371 insertions(+)
>  create mode 100644 Documentation/admin-guide/media/mgb4.rst
> 
> diff --git a/Documentation/admin-guide/media/mgb4.rst b/Documentation/admin-guide/media/mgb4.rst
> new file mode 100644
> index 000000000000..e1bb708a2265
> --- /dev/null
> +++ b/Documentation/admin-guide/media/mgb4.rst
> @@ -0,0 +1,369 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +====================
> +mgb4 sysfs interface
> +====================
> +
> +The mgb4 driver provides a sysfs interface, that is used to configure video
> +stream related parameters (some of them must be set properly before the v4l2
> +device can be opened) and obtain the video device/stream status.
> +
> +There are two types of parameters - global / PCI card related, found under
> +``/sys/class/video4linux/videoX/device`` and module specific found under
> +``/sys/class/video4linux/videoX``.
> +
> +
> +Global (PCI card) parameters
> +============================
> +
> +**module_type** (R):
> +    Module type.
> +
> +    | 0 - No module present
> +    | 1 - FPDL3
> +    | 2 - GMSL
> +
> +**module_version** (R):
> +    Module version number. Zero in case of a missing module.
> +
> +**fw_type** (R):
> +    Firmware type.
> +
> +    | 1 - FPDL3
> +    | 2 - GMSL
> +
> +**fw_version** (R):
> +    Firmware version number.
> +
> +**serial_number** (R):
> +    Card serial number. The format is::
> +
> +        PRODUCT-REVISION-SERIES-SERIAL
> +
> +    where each component is a 8b number.
> +
> +
> +Common FPDL3/GMSL input parameters
> +==================================
> +
> +**input_id** (R):
> +    Input number ID, zero based.
> +
> +**oldi_lane_width** (RW):
> +    Number of deserializer output lanes.
> +
> +    | 0 - single
> +    | 1 - dual
> +
> +**color_mapping** (RW):
> +    Mapping of the incoming bits in the signal to the colour bits of the pixels.
> +
> +    | 0 - OLDI/JEIDA
> +    | 1 - SPWG/VESA
> +
> +**link_status** (R):
> +    Video link status. If the link is locked, chips are properly connected and
> +    communicating at the same speed and protocol. The link can be locked without
> +    an active video stream.
> +
> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SYNC flag of the V4L2
> +    VIDIOC_ENUMINPUT status bits.
> +
> +    | 0 - unlocked
> +    | 1 - locked
> +
> +**stream_status** (R):
> +    Video stream status. A stream is detected if the link is locked, the input
> +    pixel clock is running and the DE signal is moving.
> +
> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SIGNAL flag of the V4L2
> +    VIDIOC_ENUMINPUT status bits.
> +
> +    | 0 - not detected
> +    | 1 - detected
> +
> +**video_width** (R):
> +    Video stream width. This is the actual width as detected by the HW.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the width
> +    field of the v4l2_bt_timings struct.
> +
> +**video_height** (R):
> +    Video stream height. This is the actual height as detected by the HW.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the height
> +    field of the v4l2_bt_timings struct.
> +
> +**vsync_status** (R):
> +    The type of VSYNC pulses as detected by the video format detector.
> +
> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
> +    the polarities field of the v4l2_bt_timings struct.
> +
> +    | 0 - active low
> +    | 1 - active high
> +    | 2 - not available
> +
> +**hsync_status** (R):
> +    The type of HSYNC pulses as detected by the video format detector.
> +
> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
> +    the polarities field of the v4l2_bt_timings struct.
> +
> +    | 0 - active low
> +    | 1 - active high
> +    | 2 - not available
> +
> +**vsync_gap_length** (RW):
> +    If the incoming video signal does not contain synchronization VSYNC and
> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
> +    the correct frame ordering. This value indicates, how many "empty" pixels

Pixels or lines? This is vsync, so lines would be more logical.

Even if the hardware wants pixels, perhaps the driver should use lines and
translate it to pixels. It's much easier for userspace to work with lines.

> +    (pixels with deasserted Data Enable signal) are necessary to generate the
> +    internal VSYNC pulse.
> +
> +**hsync_gap_length** (RW):
> +    If the incoming video signal does not contain synchronization VSYNC and
> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
> +    the correct frame ordering. This value indicates, how many "empty" pixels
> +    (pixels with deasserted Data Enable signal) are necessary to generate the
> +    internal HSYNC pulse. The value must be greater than 1 and smaller than
> +    vsync_gap_length.

Does this make sense? vsync_gap_length can be many video lines, which makes
not sense for hsync_gap_length.

I wonder if it isn't easier to just change this to v/hsync_blanking_length
(lines for vsync, pixels for hsync) to indicate the length of the blanking
periods, and then let the driver pick a suitable hsync/vsync position.

> +
> +**pclk_frequency** (R):
> +    Input pixel clock frequency in kHz.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the pixelclock field of the v4l2_bt_timings struct.
> +
> +    *Note: The frequency_range parameter must be set properly first to get
> +    a valid frequency here.*
> +
> +**hsync_width** (R):
> +    Width of the HSYNC signal in PCLK clock ticks.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the hsync field of the v4l2_bt_timings struct.
> +
> +**vsync_width** (R):
> +    Width of the VSYNC signal in PCLK clock ticks.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the vsync field of the v4l2_bt_timings struct.
> +
> +**hback_porch** (R):
> +    Number of PCLK pulses between deassertion of the HSYNC signal and the first
> +    valid pixel in the video line (marked by DE=1).
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the hbackporch field of the v4l2_bt_timings struct.
> +
> +**hfront_porch** (R):
> +    Number of PCLK pulses between the end of the last valid pixel in the video
> +    line (marked by DE=1) and assertion of the HSYNC signal.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the hfrontporch field of the v4l2_bt_timings struct.
> +
> +**vback_porch** (R):
> +    Number of video lines between deassertion of the VSYNC signal and the video
> +    line with the first valid pixel (marked by DE=1).
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the vbackporch field of the v4l2_bt_timings struct.
> +
> +**vfront_porch** (R):
> +    Number of video lines between the end of the last valid pixel line (marked
> +    by DE=1) and assertion of the VSYNC signal.
> +
> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
> +    the vfrontporch field of the v4l2_bt_timings struct.

Does setting v/hsync_gap_length also update these fields accordingly?

> +
> +**frequency_range** (RW)
> +    PLL frequency range of the OLDI input clock generator. The PLL frequency is
> +    derived from the Pixel Clock Frequency (PCLK) and is equal to PCLK if
> +    oldi_lane_width is set to "single" and PCLK/2 if oldi_lane_width is set to
> +    "dual".
> +
> +    | 0 - PLL < 50MHz
> +    | 1 - PLL >= 50MHz
> +
> +    *Note: This parameter can not be changed while the input v4l2 device is
> +    open.*
> +
> +
> +Common FPDL3/GMSL output parameters
> +===================================
> +
> +**output_id** (R):
> +    Output number ID, zero based.
> +
> +**video_source** (RW):
> +    Output video source. If set to 0 or 1, the source is the corresponding card
> +    input and the v4l2 output devices are disabled. If set to 2 or 3, the source
> +    is the corresponding v4l2 video output device.
> +
> +    | 0 - input 0
> +    | 1 - input 1
> +    | 2 - v4l2 output 0
> +    | 3 - v4l2 output 1
> +
> +    *Note: This parameter can not be changed while ANY of the input/output v4l2
> +    devices is open.*
> +
> +**display_width** (RW):
> +    Display width. There is no autodetection of the connected display, so the
> +    proper value must be set before the start of streaming.
> +
> +    *Note: This parameter can not be changed while the output v4l2 device is
> +    open.*
> +
> +**display_height** (RW):
> +    Display height. There is no autodetection of the connected display, so the
> +    proper value must be set before the start of streaming.
> +
> +    *Note: This parameter can not be changed while the output v4l2 device is
> +    open.*
> +
> +**frame_rate** (RW):
> +    Output video frame rate in frames per second.
> +
> +**hsync_polarity** (RW):
> +    HSYNC signal polarity.
> +
> +    | 0 - active low
> +    | 1 - active high
> +
> +**vsync_polarity** (RW):
> +    VSYNC signal polarity.
> +
> +    | 0 - active low
> +    | 1 - active high
> +
> +**de_polarity** (RW):
> +    DE signal polarity.
> +
> +    | 0 - active low
> +    | 1 - active high
> +
> +**pclk_frequency** (RW):
> +    Output pixel clock frequency. Allowed values are between 25000-190000(kHz)
> +    and there is a non-linear stepping between two consecutive allowed
> +    frequencies. The driver finds the nearest allowed frequency to the given
> +    value and sets it. When reading this property, you get the exact
> +    frequency set by the driver.
> +
> +    *Note: This parameter can not be changed while the output v4l2 device is
> +    open.*
> +
> +**hsync_width** (RW):
> +    Width of the HSYNC signal in PCLK clock ticks.

Isn't "PCLK clock ticks" the equivalent of "pixels"? That's a much more natural
way of describing this.

> +
> +**vsync_width** (RW):
> +    Width of the VSYNC signal in PCLK clock ticks.

VSYNC is usually described in lines and 'height'.

> +
> +**hback_porch** (RW):
> +    Number of PCLK pulses between deassertion of the HSYNC signal and the first
> +    valid pixel in the video line (marked by DE=1).
> +
> +**hfront_porch** (RW):
> +    Number of PCLK pulses between the end of the last valid pixel in the video
> +    line (marked by DE=1) and assertion of the HSYNC signal.
> +
> +**vback_porch** (RW):
> +    Number of video lines between deassertion of the VSYNC signal and the video
> +    line with the first valid pixel (marked by DE=1).

As is done here.

> +
> +**vfront_porch** (RW):
> +    Number of video lines between the end of the last valid pixel line (marked
> +    by DE=1) and assertion of the VSYNC signal.
> +
> +
> +FPDL3 specific input parameters
> +===============================
> +
> +**fpdl3_input_width** (RW):
> +    Number of deserializer input lines.
> +
> +    | 0 - auto
> +    | 1 - single
> +    | 2 - dual
> +
> +FPDL3 specific output parameters
> +================================
> +
> +**fpdl3_output_width** (RW):
> +    Number of serializer output lines.
> +
> +    | 0 - auto
> +    | 1 - single
> +    | 2 - dual
> +
> +GMSL specific input parameters
> +==============================
> +
> +**gmsl_mode** (RW):
> +    GMSL speed mode.
> +
> +    | 0 - 12Gb/s
> +    | 1 - 6Gb/s
> +    | 2 - 3Gb/s
> +    | 3 - 1.5Gb/s
> +
> +**gmsl_stream_id** (RW):
> +    The GMSL multi-stream contains up to four video streams. This parameter
> +    selects which stream is captured by the video input. The value is the
> +    zero-based index of the stream.
> +
> +    *Note: This parameter can not be changed while the input v4l2 device is
> +    open.*
> +
> +**gmsl_fec** (RW):
> +    GMSL Forward Error Correction (FEC).
> +
> +    | 0 - disabled
> +    | 1 - enabled
> +
> +
> +====================
> +mgb4 mtd partitions
> +====================
> +
> +The mgb4 driver creates a MTD device with two partitions:
> + - mgb4-fw.X - FPGA firmware.
> + - mgb4-data.X - Factory settings, e.g. card serial number.
> +
> +The *mgb4-fw* partition is writable and is used for FW updates, *mgb4-data* is
> +read-only. The *X* attached to the partition name represents the card number.
> +Depending on the CONFIG_MTD_PARTITIONED_MASTER kernel configuration, you may
> +also have a third partition named *mgb4-flash* available in the system. This
> +partition represents the whole, unpartitioned, card's FLASH memory and one should
> +not fiddle with it...
> +
> +====================
> +mgb4 iio (triggers)
> +====================
> +
> +The mgb4 driver creates an Industrial I/O (IIO) device that provides trigger and
> +signal level status capability. The following scan elements are available:
> +
> +**activity**:
> +	The trigger levels and pending status.
> +
> +	| bit 1 - trigger 1 pending
> +	| bit 2 - trigger 2 pending
> +	| bit 5 - trigger 1 level
> +	| bit 6 - trigger 2 level
> +
> +**timestamp**:
> +	The trigger event timestamp.
> +
> +The iio device can operate either in "raw" mode where you can fetch the signal
> +levels (activity bits 5 and 6) using sysfs access or in triggered buffer mode.
> +In the triggered buffer mode you can follow the signal level changes (activity
> +bits 1 and 2) using the iio device in /dev. If you enable the timestamps, you
> +will also get the exact trigger event time that can be matched to a video frame
> +(every mgb4 video frame has a timestamp with the same clock source).
> +
> +*Note: although the activity sample always contains all the status bits, it makes
> +no sense to get the pending bits in raw mode or the level bits in the triggered
> +buffer mode - the values do not represent valid data in such case.*
> diff --git a/Documentation/admin-guide/media/pci-cardlist.rst b/Documentation/admin-guide/media/pci-cardlist.rst
> index 42528795d4da..7d8e3c8987db 100644
> --- a/Documentation/admin-guide/media/pci-cardlist.rst
> +++ b/Documentation/admin-guide/media/pci-cardlist.rst
> @@ -77,6 +77,7 @@ ipu3-cio2         Intel ipu3-cio2 driver
>  ivtv              Conexant cx23416/cx23415 MPEG encoder/decoder
>  ivtvfb            Conexant cx23415 framebuffer
>  mantis            MANTIS based cards
> +mgb4              Digiteq Automotive MGB4 frame grabber
>  mxb               Siemens-Nixdorf 'Multimedia eXtension Board'
>  netup-unidvb      NetUP Universal DVB card
>  ngene             Micronas nGene
> diff --git a/Documentation/admin-guide/media/v4l-drivers.rst b/Documentation/admin-guide/media/v4l-drivers.rst
> index 1c41f87c3917..61283d67ceef 100644
> --- a/Documentation/admin-guide/media/v4l-drivers.rst
> +++ b/Documentation/admin-guide/media/v4l-drivers.rst
> @@ -17,6 +17,7 @@ Video4Linux (V4L) driver-specific documentation
>  	imx7
>  	ipu3
>  	ivtv
> +	mgb4
>  	omap3isp
>  	omap4_camera
>  	philips

General note regarding the RW attributes: are there default values set by the driver?
Should those be documented? What happens if you just start streaming without setting
any of the RW attrs?

The standard V4L2 model is that there is no such thing as an uninitialized value, i.e.
there always are reasonable defaults created at probe time. Streaming might not actually
work if the defaults do not match the incoming signal, but you won't have to deal with
e.g. zero values or anything like that.

The same should be done here: there do have to be sane documented initial values.

Regards,

	Hans
  

On 26. 07. 23 12:40, Hans Verkuil wrote:
> On 04/07/2023 15:13, tumic@gpxsee.org wrote:
>> From: Martin Tůma <martin.tuma@digiteqautomotive.com>
>>
>> The "admin-guide" documentation for the Digiteq Automotive MGB4 driver.
>>
>> Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
>> ---
>>   Documentation/admin-guide/media/mgb4.rst      | 369 ++++++++++++++++++
>>   .../admin-guide/media/pci-cardlist.rst        |   1 +
>>   .../admin-guide/media/v4l-drivers.rst         |   1 +
>>   3 files changed, 371 insertions(+)
>>   create mode 100644 Documentation/admin-guide/media/mgb4.rst
>>
>> diff --git a/Documentation/admin-guide/media/mgb4.rst b/Documentation/admin-guide/media/mgb4.rst
>> new file mode 100644
>> index 000000000000..e1bb708a2265
>> --- /dev/null
>> +++ b/Documentation/admin-guide/media/mgb4.rst
>> @@ -0,0 +1,369 @@
>> +.. SPDX-License-Identifier: GPL-2.0
>> +
>> +====================
>> +mgb4 sysfs interface
>> +====================
>> +
>> +The mgb4 driver provides a sysfs interface, that is used to configure video
>> +stream related parameters (some of them must be set properly before the v4l2
>> +device can be opened) and obtain the video device/stream status.
>> +
>> +There are two types of parameters - global / PCI card related, found under
>> +``/sys/class/video4linux/videoX/device`` and module specific found under
>> +``/sys/class/video4linux/videoX``.
>> +
>> +
>> +Global (PCI card) parameters
>> +============================
>> +
>> +**module_type** (R):
>> +    Module type.
>> +
>> +    | 0 - No module present
>> +    | 1 - FPDL3
>> +    | 2 - GMSL
>> +
>> +**module_version** (R):
>> +    Module version number. Zero in case of a missing module.
>> +
>> +**fw_type** (R):
>> +    Firmware type.
>> +
>> +    | 1 - FPDL3
>> +    | 2 - GMSL
>> +
>> +**fw_version** (R):
>> +    Firmware version number.
>> +
>> +**serial_number** (R):
>> +    Card serial number. The format is::
>> +
>> +        PRODUCT-REVISION-SERIES-SERIAL
>> +
>> +    where each component is a 8b number.
>> +
>> +
>> +Common FPDL3/GMSL input parameters
>> +==================================
>> +
>> +**input_id** (R):
>> +    Input number ID, zero based.
>> +
>> +**oldi_lane_width** (RW):
>> +    Number of deserializer output lanes.
>> +
>> +    | 0 - single
>> +    | 1 - dual
>> +
>> +**color_mapping** (RW):
>> +    Mapping of the incoming bits in the signal to the colour bits of the pixels.
>> +
>> +    | 0 - OLDI/JEIDA
>> +    | 1 - SPWG/VESA
>> +
>> +**link_status** (R):
>> +    Video link status. If the link is locked, chips are properly connected and
>> +    communicating at the same speed and protocol. The link can be locked without
>> +    an active video stream.
>> +
>> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SYNC flag of the V4L2
>> +    VIDIOC_ENUMINPUT status bits.
>> +
>> +    | 0 - unlocked
>> +    | 1 - locked
>> +
>> +**stream_status** (R):
>> +    Video stream status. A stream is detected if the link is locked, the input
>> +    pixel clock is running and the DE signal is moving.
>> +
>> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SIGNAL flag of the V4L2
>> +    VIDIOC_ENUMINPUT status bits.
>> +
>> +    | 0 - not detected
>> +    | 1 - detected
>> +
>> +**video_width** (R):
>> +    Video stream width. This is the actual width as detected by the HW.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the width
>> +    field of the v4l2_bt_timings struct.
>> +
>> +**video_height** (R):
>> +    Video stream height. This is the actual height as detected by the HW.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the height
>> +    field of the v4l2_bt_timings struct.
>> +
>> +**vsync_status** (R):
>> +    The type of VSYNC pulses as detected by the video format detector.
>> +
>> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
>> +    the polarities field of the v4l2_bt_timings struct.
>> +
>> +    | 0 - active low
>> +    | 1 - active high
>> +    | 2 - not available
>> +
>> +**hsync_status** (R):
>> +    The type of HSYNC pulses as detected by the video format detector.
>> +
>> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
>> +    the polarities field of the v4l2_bt_timings struct.
>> +
>> +    | 0 - active low
>> +    | 1 - active high
>> +    | 2 - not available
>> +
>> +**vsync_gap_length** (RW):
>> +    If the incoming video signal does not contain synchronization VSYNC and
>> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
>> +    the correct frame ordering. This value indicates, how many "empty" pixels
> 
> Pixels or lines? This is vsync, so lines would be more logical.
> 
> Even if the hardware wants pixels, perhaps the driver should use lines and
> translate it to pixels. It's much easier for userspace to work with lines.
>

According to our HW engineers, this is properly documented. I do not 
have the full insight to the "signal parameters" topic, so my answers 
here will be just some kind of "free" translation of what I got. The 
justification here was, that the vsync gap length (in our case/HW) 
represents something slightly different than you may think.

>> +    (pixels with deasserted Data Enable signal) are necessary to generate the
>> +    internal VSYNC pulse.
>> +
>> +**hsync_gap_length** (RW):
>> +    If the incoming video signal does not contain synchronization VSYNC and
>> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
>> +    the correct frame ordering. This value indicates, how many "empty" pixels
>> +    (pixels with deasserted Data Enable signal) are necessary to generate the
>> +    internal HSYNC pulse. The value must be greater than 1 and smaller than
>> +    vsync_gap_length.
> 
> Does this make sense? vsync_gap_length can be many video lines, which makes
> not sense for hsync_gap_length.
> 
> I wonder if it isn't easier to just change this to v/hsync_blanking_length
> (lines for vsync, pixels for hsync) to indicate the length of the blanking
> periods, and then let the driver pick a suitable hsync/vsync position.
> 

Dtto.

>> +
>> +**pclk_frequency** (R):
>> +    Input pixel clock frequency in kHz.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the pixelclock field of the v4l2_bt_timings struct.
>> +
>> +    *Note: The frequency_range parameter must be set properly first to get
>> +    a valid frequency here.*
>> +
>> +**hsync_width** (R):
>> +    Width of the HSYNC signal in PCLK clock ticks.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the hsync field of the v4l2_bt_timings struct.
>> +
>> +**vsync_width** (R):
>> +    Width of the VSYNC signal in PCLK clock ticks.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the vsync field of the v4l2_bt_timings struct.
>> +
>> +**hback_porch** (R):
>> +    Number of PCLK pulses between deassertion of the HSYNC signal and the first
>> +    valid pixel in the video line (marked by DE=1).
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the hbackporch field of the v4l2_bt_timings struct.
>> +
>> +**hfront_porch** (R):
>> +    Number of PCLK pulses between the end of the last valid pixel in the video
>> +    line (marked by DE=1) and assertion of the HSYNC signal.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the hfrontporch field of the v4l2_bt_timings struct.
>> +
>> +**vback_porch** (R):
>> +    Number of video lines between deassertion of the VSYNC signal and the video
>> +    line with the first valid pixel (marked by DE=1).
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the vbackporch field of the v4l2_bt_timings struct.
>> +
>> +**vfront_porch** (R):
>> +    Number of video lines between the end of the last valid pixel line (marked
>> +    by DE=1) and assertion of the VSYNC signal.
>> +
>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in
>> +    the vfrontporch field of the v4l2_bt_timings struct.
> 
> Does setting v/hsync_gap_length also update these fields accordingly?
> 

According to our HW engineers, those values are independent.

>> +
>> +**frequency_range** (RW)
>> +    PLL frequency range of the OLDI input clock generator. The PLL frequency is
>> +    derived from the Pixel Clock Frequency (PCLK) and is equal to PCLK if
>> +    oldi_lane_width is set to "single" and PCLK/2 if oldi_lane_width is set to
>> +    "dual".
>> +
>> +    | 0 - PLL < 50MHz
>> +    | 1 - PLL >= 50MHz
>> +
>> +    *Note: This parameter can not be changed while the input v4l2 device is
>> +    open.*
>> +
>> +
>> +Common FPDL3/GMSL output parameters
>> +===================================
>> +
>> +**output_id** (R):
>> +    Output number ID, zero based.
>> +
>> +**video_source** (RW):
>> +    Output video source. If set to 0 or 1, the source is the corresponding card
>> +    input and the v4l2 output devices are disabled. If set to 2 or 3, the source
>> +    is the corresponding v4l2 video output device.
>> +
>> +    | 0 - input 0
>> +    | 1 - input 1
>> +    | 2 - v4l2 output 0
>> +    | 3 - v4l2 output 1
>> +
>> +    *Note: This parameter can not be changed while ANY of the input/output v4l2
>> +    devices is open.*
>> +
>> +**display_width** (RW):
>> +    Display width. There is no autodetection of the connected display, so the
>> +    proper value must be set before the start of streaming.
>> +
>> +    *Note: This parameter can not be changed while the output v4l2 device is
>> +    open.*
>> +
>> +**display_height** (RW):
>> +    Display height. There is no autodetection of the connected display, so the
>> +    proper value must be set before the start of streaming.
>> +
>> +    *Note: This parameter can not be changed while the output v4l2 device is
>> +    open.*
>> +
>> +**frame_rate** (RW):
>> +    Output video frame rate in frames per second.
>> +
>> +**hsync_polarity** (RW):
>> +    HSYNC signal polarity.
>> +
>> +    | 0 - active low
>> +    | 1 - active high
>> +
>> +**vsync_polarity** (RW):
>> +    VSYNC signal polarity.
>> +
>> +    | 0 - active low
>> +    | 1 - active high
>> +
>> +**de_polarity** (RW):
>> +    DE signal polarity.
>> +
>> +    | 0 - active low
>> +    | 1 - active high
>> +
>> +**pclk_frequency** (RW):
>> +    Output pixel clock frequency. Allowed values are between 25000-190000(kHz)
>> +    and there is a non-linear stepping between two consecutive allowed
>> +    frequencies. The driver finds the nearest allowed frequency to the given
>> +    value and sets it. When reading this property, you get the exact
>> +    frequency set by the driver.
>> +
>> +    *Note: This parameter can not be changed while the output v4l2 device is
>> +    open.*
>> +
>> +**hsync_width** (RW):
>> +    Width of the HSYNC signal in PCLK clock ticks.
> 
> Isn't "PCLK clock ticks" the equivalent of "pixels"? That's a much more natural
> way of describing this.
> 

Should really have been pixels, fixed in v9.

>> +
>> +**vsync_width** (RW):
>> +    Width of the VSYNC signal in PCLK clock ticks.
> 
> VSYNC is usually described in lines and 'height'.
> 

That was a bug too. Fixed to "lines".

>> +
>> +**hback_porch** (RW):
>> +    Number of PCLK pulses between deassertion of the HSYNC signal and the first
>> +    valid pixel in the video line (marked by DE=1).
>> +
>> +**hfront_porch** (RW):
>> +    Number of PCLK pulses between the end of the last valid pixel in the video
>> +    line (marked by DE=1) and assertion of the HSYNC signal.
>> +
>> +**vback_porch** (RW):
>> +    Number of video lines between deassertion of the VSYNC signal and the video
>> +    line with the first valid pixel (marked by DE=1).
> 
> As is done here.

According to our HW engineers, this is properly documented.

> 
>> +
>> +**vfront_porch** (RW):
>> +    Number of video lines between the end of the last valid pixel line (marked
>> +    by DE=1) and assertion of the VSYNC signal.
>> +
>> +
>> +FPDL3 specific input parameters
>> +===============================
>> +
>> +**fpdl3_input_width** (RW):
>> +    Number of deserializer input lines.
>> +
>> +    | 0 - auto
>> +    | 1 - single
>> +    | 2 - dual
>> +
>> +FPDL3 specific output parameters
>> +================================
>> +
>> +**fpdl3_output_width** (RW):
>> +    Number of serializer output lines.
>> +
>> +    | 0 - auto
>> +    | 1 - single
>> +    | 2 - dual
>> +
>> +GMSL specific input parameters
>> +==============================
>> +
>> +**gmsl_mode** (RW):
>> +    GMSL speed mode.
>> +
>> +    | 0 - 12Gb/s
>> +    | 1 - 6Gb/s
>> +    | 2 - 3Gb/s
>> +    | 3 - 1.5Gb/s
>> +
>> +**gmsl_stream_id** (RW):
>> +    The GMSL multi-stream contains up to four video streams. This parameter
>> +    selects which stream is captured by the video input. The value is the
>> +    zero-based index of the stream.
>> +
>> +    *Note: This parameter can not be changed while the input v4l2 device is
>> +    open.*
>> +
>> +**gmsl_fec** (RW):
>> +    GMSL Forward Error Correction (FEC).
>> +
>> +    | 0 - disabled
>> +    | 1 - enabled
>> +
>> +
>> +====================
>> +mgb4 mtd partitions
>> +====================
>> +
>> +The mgb4 driver creates a MTD device with two partitions:
>> + - mgb4-fw.X - FPGA firmware.
>> + - mgb4-data.X - Factory settings, e.g. card serial number.
>> +
>> +The *mgb4-fw* partition is writable and is used for FW updates, *mgb4-data* is
>> +read-only. The *X* attached to the partition name represents the card number.
>> +Depending on the CONFIG_MTD_PARTITIONED_MASTER kernel configuration, you may
>> +also have a third partition named *mgb4-flash* available in the system. This
>> +partition represents the whole, unpartitioned, card's FLASH memory and one should
>> +not fiddle with it...
>> +
>> +====================
>> +mgb4 iio (triggers)
>> +====================
>> +
>> +The mgb4 driver creates an Industrial I/O (IIO) device that provides trigger and
>> +signal level status capability. The following scan elements are available:
>> +
>> +**activity**:
>> +	The trigger levels and pending status.
>> +
>> +	| bit 1 - trigger 1 pending
>> +	| bit 2 - trigger 2 pending
>> +	| bit 5 - trigger 1 level
>> +	| bit 6 - trigger 2 level
>> +
>> +**timestamp**:
>> +	The trigger event timestamp.
>> +
>> +The iio device can operate either in "raw" mode where you can fetch the signal
>> +levels (activity bits 5 and 6) using sysfs access or in triggered buffer mode.
>> +In the triggered buffer mode you can follow the signal level changes (activity
>> +bits 1 and 2) using the iio device in /dev. If you enable the timestamps, you
>> +will also get the exact trigger event time that can be matched to a video frame
>> +(every mgb4 video frame has a timestamp with the same clock source).
>> +
>> +*Note: although the activity sample always contains all the status bits, it makes
>> +no sense to get the pending bits in raw mode or the level bits in the triggered
>> +buffer mode - the values do not represent valid data in such case.*
>> diff --git a/Documentation/admin-guide/media/pci-cardlist.rst b/Documentation/admin-guide/media/pci-cardlist.rst
>> index 42528795d4da..7d8e3c8987db 100644
>> --- a/Documentation/admin-guide/media/pci-cardlist.rst
>> +++ b/Documentation/admin-guide/media/pci-cardlist.rst
>> @@ -77,6 +77,7 @@ ipu3-cio2         Intel ipu3-cio2 driver
>>   ivtv              Conexant cx23416/cx23415 MPEG encoder/decoder
>>   ivtvfb            Conexant cx23415 framebuffer
>>   mantis            MANTIS based cards
>> +mgb4              Digiteq Automotive MGB4 frame grabber
>>   mxb               Siemens-Nixdorf 'Multimedia eXtension Board'
>>   netup-unidvb      NetUP Universal DVB card
>>   ngene             Micronas nGene
>> diff --git a/Documentation/admin-guide/media/v4l-drivers.rst b/Documentation/admin-guide/media/v4l-drivers.rst
>> index 1c41f87c3917..61283d67ceef 100644
>> --- a/Documentation/admin-guide/media/v4l-drivers.rst
>> +++ b/Documentation/admin-guide/media/v4l-drivers.rst
>> @@ -17,6 +17,7 @@ Video4Linux (V4L) driver-specific documentation
>>   	imx7
>>   	ipu3
>>   	ivtv
>> +	mgb4
>>   	omap3isp
>>   	omap4_camera
>>   	philips
> 
> General note regarding the RW attributes: are there default values set by the driver?
> Should those be documented? What happens if you just start streaming without setting
> any of the RW attrs?
> 
> The standard V4L2 model is that there is no such thing as an uninitialized value, i.e.
> there always are reasonable defaults created at probe time. Streaming might not actually
> work if the defaults do not match the incoming signal, but you won't have to deal with
> e.g. zero values or anything like that.
> 
> The same should be done here: there do have to be sane documented initial values.

There are everywhere default values, i.e. no uninitialized values exist 
in the mgb4 driver like in v4l2. I have extended the documentation and 
added the defaults where applicable in v9.

M.
  

On 8/10/23 13:54, Martin Tůma wrote:
> On 26. 07. 23 12:40, Hans Verkuil wrote:
>> On 04/07/2023 15:13, tumic@gpxsee.org wrote:
>>> From: Martin Tůma <martin.tuma@digiteqautomotive.com>
>>>
>>> The "admin-guide" documentation for the Digiteq Automotive MGB4 driver.
>>>
>>> Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
>>> ---
>>>   Documentation/admin-guide/media/mgb4.rst      | 369 ++++++++++++++++++
>>>   .../admin-guide/media/pci-cardlist.rst        |   1 +
>>>   .../admin-guide/media/v4l-drivers.rst         |   1 +
>>>   3 files changed, 371 insertions(+)
>>>   create mode 100644 Documentation/admin-guide/media/mgb4.rst
>>>
>>> diff --git a/Documentation/admin-guide/media/mgb4.rst b/Documentation/admin-guide/media/mgb4.rst
>>> new file mode 100644
>>> index 000000000000..e1bb708a2265
>>> --- /dev/null
>>> +++ b/Documentation/admin-guide/media/mgb4.rst
>>> @@ -0,0 +1,369 @@
>>> +.. SPDX-License-Identifier: GPL-2.0
>>> +
>>> +====================
>>> +mgb4 sysfs interface
>>> +====================
>>> +
>>> +The mgb4 driver provides a sysfs interface, that is used to configure video
>>> +stream related parameters (some of them must be set properly before the v4l2
>>> +device can be opened) and obtain the video device/stream status.
>>> +
>>> +There are two types of parameters - global / PCI card related, found under
>>> +``/sys/class/video4linux/videoX/device`` and module specific found under
>>> +``/sys/class/video4linux/videoX``.
>>> +
>>> +
>>> +Global (PCI card) parameters
>>> +============================
>>> +
>>> +**module_type** (R):
>>> +    Module type.
>>> +
>>> +    | 0 - No module present
>>> +    | 1 - FPDL3
>>> +    | 2 - GMSL
>>> +
>>> +**module_version** (R):
>>> +    Module version number. Zero in case of a missing module.
>>> +
>>> +**fw_type** (R):
>>> +    Firmware type.
>>> +
>>> +    | 1 - FPDL3
>>> +    | 2 - GMSL
>>> +
>>> +**fw_version** (R):
>>> +    Firmware version number.
>>> +
>>> +**serial_number** (R):
>>> +    Card serial number. The format is::
>>> +
>>> +        PRODUCT-REVISION-SERIES-SERIAL
>>> +
>>> +    where each component is a 8b number.
>>> +
>>> +
>>> +Common FPDL3/GMSL input parameters
>>> +==================================
>>> +
>>> +**input_id** (R):
>>> +    Input number ID, zero based.
>>> +
>>> +**oldi_lane_width** (RW):
>>> +    Number of deserializer output lanes.
>>> +
>>> +    | 0 - single
>>> +    | 1 - dual
>>> +
>>> +**color_mapping** (RW):
>>> +    Mapping of the incoming bits in the signal to the colour bits of the pixels.
>>> +
>>> +    | 0 - OLDI/JEIDA
>>> +    | 1 - SPWG/VESA
>>> +
>>> +**link_status** (R):
>>> +    Video link status. If the link is locked, chips are properly connected and
>>> +    communicating at the same speed and protocol. The link can be locked without
>>> +    an active video stream.
>>> +
>>> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SYNC flag of the V4L2
>>> +    VIDIOC_ENUMINPUT status bits.
>>> +
>>> +    | 0 - unlocked
>>> +    | 1 - locked
>>> +
>>> +**stream_status** (R):
>>> +    Video stream status. A stream is detected if the link is locked, the input
>>> +    pixel clock is running and the DE signal is moving.
>>> +
>>> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SIGNAL flag of the V4L2
>>> +    VIDIOC_ENUMINPUT status bits.
>>> +
>>> +    | 0 - not detected
>>> +    | 1 - detected
>>> +
>>> +**video_width** (R):
>>> +    Video stream width. This is the actual width as detected by the HW.
>>> +
>>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the width
>>> +    field of the v4l2_bt_timings struct.
>>> +
>>> +**video_height** (R):
>>> +    Video stream height. This is the actual height as detected by the HW.
>>> +
>>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the height
>>> +    field of the v4l2_bt_timings struct.
>>> +
>>> +**vsync_status** (R):
>>> +    The type of VSYNC pulses as detected by the video format detector.
>>> +
>>> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
>>> +    the polarities field of the v4l2_bt_timings struct.
>>> +
>>> +    | 0 - active low
>>> +    | 1 - active high
>>> +    | 2 - not available
>>> +
>>> +**hsync_status** (R):
>>> +    The type of HSYNC pulses as detected by the video format detector.
>>> +
>>> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
>>> +    the polarities field of the v4l2_bt_timings struct.
>>> +
>>> +    | 0 - active low
>>> +    | 1 - active high
>>> +    | 2 - not available
>>> +
>>> +**vsync_gap_length** (RW):
>>> +    If the incoming video signal does not contain synchronization VSYNC and
>>> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
>>> +    the correct frame ordering. This value indicates, how many "empty" pixels
>>
>> Pixels or lines? This is vsync, so lines would be more logical.
>>
>> Even if the hardware wants pixels, perhaps the driver should use lines and
>> translate it to pixels. It's much easier for userspace to work with lines.
>>
> 
> According to our HW engineers, this is properly documented. I do not 
> have the full insight to the "signal parameters" topic, so my answers 
> here will be just some kind of "free" translation of what I got. The 
> justification here was, that the vsync gap length (in our case/HW) 
> represents something slightly different than you may think.
> 
>>> +    (pixels with deasserted Data Enable signal) are necessary to generate the
>>> +    internal VSYNC pulse.
>>> +
>>> +**hsync_gap_length** (RW):
>>> +    If the incoming video signal does not contain synchronization VSYNC and
>>> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
>>> +    the correct frame ordering. This value indicates, how many "empty" pixels
>>> +    (pixels with deasserted Data Enable signal) are necessary to generate the
>>> +    internal HSYNC pulse. The value must be greater than 1 and smaller than
>>> +    vsync_gap_length.
>>
>> Does this make sense? vsync_gap_length can be many video lines, which makes
>> not sense for hsync_gap_length.
>>
>> I wonder if it isn't easier to just change this to v/hsync_blanking_length
>> (lines for vsync, pixels for hsync) to indicate the length of the blanking
>> periods, and then let the driver pick a suitable hsync/vsync position.
>>
> 
> Dtto.

So the problem here is that if I don't understand what is meant here, how will
a user of this driver be able to understand it?

I think it would be better to give one or two examples of devices and their
configuration. Or refer to freely available documentation, if that is available.

Also note that v9 doesn't mentioned default values for these two properties.

Regards,

	Hans
  

On 10. 08. 23 14:43, Hans Verkuil wrote:
> On 8/10/23 13:54, Martin Tůma wrote:
>> On 26. 07. 23 12:40, Hans Verkuil wrote:
>>> On 04/07/2023 15:13, tumic@gpxsee.org wrote:
>>>> From: Martin Tůma <martin.tuma@digiteqautomotive.com>
>>>>
>>>> The "admin-guide" documentation for the Digiteq Automotive MGB4 driver.
>>>>
>>>> Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
>>>> ---
>>>>    Documentation/admin-guide/media/mgb4.rst      | 369 ++++++++++++++++++
>>>>    .../admin-guide/media/pci-cardlist.rst        |   1 +
>>>>    .../admin-guide/media/v4l-drivers.rst         |   1 +
>>>>    3 files changed, 371 insertions(+)
>>>>    create mode 100644 Documentation/admin-guide/media/mgb4.rst
>>>>
>>>> diff --git a/Documentation/admin-guide/media/mgb4.rst b/Documentation/admin-guide/media/mgb4.rst
>>>> new file mode 100644
>>>> index 000000000000..e1bb708a2265
>>>> --- /dev/null
>>>> +++ b/Documentation/admin-guide/media/mgb4.rst
>>>> @@ -0,0 +1,369 @@
>>>> +.. SPDX-License-Identifier: GPL-2.0
>>>> +
>>>> +====================
>>>> +mgb4 sysfs interface
>>>> +====================
>>>> +
>>>> +The mgb4 driver provides a sysfs interface, that is used to configure video
>>>> +stream related parameters (some of them must be set properly before the v4l2
>>>> +device can be opened) and obtain the video device/stream status.
>>>> +
>>>> +There are two types of parameters - global / PCI card related, found under
>>>> +``/sys/class/video4linux/videoX/device`` and module specific found under
>>>> +``/sys/class/video4linux/videoX``.
>>>> +
>>>> +
>>>> +Global (PCI card) parameters
>>>> +============================
>>>> +
>>>> +**module_type** (R):
>>>> +    Module type.
>>>> +
>>>> +    | 0 - No module present
>>>> +    | 1 - FPDL3
>>>> +    | 2 - GMSL
>>>> +
>>>> +**module_version** (R):
>>>> +    Module version number. Zero in case of a missing module.
>>>> +
>>>> +**fw_type** (R):
>>>> +    Firmware type.
>>>> +
>>>> +    | 1 - FPDL3
>>>> +    | 2 - GMSL
>>>> +
>>>> +**fw_version** (R):
>>>> +    Firmware version number.
>>>> +
>>>> +**serial_number** (R):
>>>> +    Card serial number. The format is::
>>>> +
>>>> +        PRODUCT-REVISION-SERIES-SERIAL
>>>> +
>>>> +    where each component is a 8b number.
>>>> +
>>>> +
>>>> +Common FPDL3/GMSL input parameters
>>>> +==================================
>>>> +
>>>> +**input_id** (R):
>>>> +    Input number ID, zero based.
>>>> +
>>>> +**oldi_lane_width** (RW):
>>>> +    Number of deserializer output lanes.
>>>> +
>>>> +    | 0 - single
>>>> +    | 1 - dual
>>>> +
>>>> +**color_mapping** (RW):
>>>> +    Mapping of the incoming bits in the signal to the colour bits of the pixels.
>>>> +
>>>> +    | 0 - OLDI/JEIDA
>>>> +    | 1 - SPWG/VESA
>>>> +
>>>> +**link_status** (R):
>>>> +    Video link status. If the link is locked, chips are properly connected and
>>>> +    communicating at the same speed and protocol. The link can be locked without
>>>> +    an active video stream.
>>>> +
>>>> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SYNC flag of the V4L2
>>>> +    VIDIOC_ENUMINPUT status bits.
>>>> +
>>>> +    | 0 - unlocked
>>>> +    | 1 - locked
>>>> +
>>>> +**stream_status** (R):
>>>> +    Video stream status. A stream is detected if the link is locked, the input
>>>> +    pixel clock is running and the DE signal is moving.
>>>> +
>>>> +    A value of 0 is equivalent to the V4L2_IN_ST_NO_SIGNAL flag of the V4L2
>>>> +    VIDIOC_ENUMINPUT status bits.
>>>> +
>>>> +    | 0 - not detected
>>>> +    | 1 - detected
>>>> +
>>>> +**video_width** (R):
>>>> +    Video stream width. This is the actual width as detected by the HW.
>>>> +
>>>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the width
>>>> +    field of the v4l2_bt_timings struct.
>>>> +
>>>> +**video_height** (R):
>>>> +    Video stream height. This is the actual height as detected by the HW.
>>>> +
>>>> +    The value is identical to what VIDIOC_QUERY_DV_TIMINGS returns in the height
>>>> +    field of the v4l2_bt_timings struct.
>>>> +
>>>> +**vsync_status** (R):
>>>> +    The type of VSYNC pulses as detected by the video format detector.
>>>> +
>>>> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
>>>> +    the polarities field of the v4l2_bt_timings struct.
>>>> +
>>>> +    | 0 - active low
>>>> +    | 1 - active high
>>>> +    | 2 - not available
>>>> +
>>>> +**hsync_status** (R):
>>>> +    The type of HSYNC pulses as detected by the video format detector.
>>>> +
>>>> +    The value is equivalent to the flags returned by VIDIOC_QUERY_DV_TIMINGS in
>>>> +    the polarities field of the v4l2_bt_timings struct.
>>>> +
>>>> +    | 0 - active low
>>>> +    | 1 - active high
>>>> +    | 2 - not available
>>>> +
>>>> +**vsync_gap_length** (RW):
>>>> +    If the incoming video signal does not contain synchronization VSYNC and
>>>> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
>>>> +    the correct frame ordering. This value indicates, how many "empty" pixels
>>>
>>> Pixels or lines? This is vsync, so lines would be more logical.
>>>
>>> Even if the hardware wants pixels, perhaps the driver should use lines and
>>> translate it to pixels. It's much easier for userspace to work with lines.
>>>
>>
>> According to our HW engineers, this is properly documented. I do not
>> have the full insight to the "signal parameters" topic, so my answers
>> here will be just some kind of "free" translation of what I got. The
>> justification here was, that the vsync gap length (in our case/HW)
>> represents something slightly different than you may think.
>>
>>>> +    (pixels with deasserted Data Enable signal) are necessary to generate the
>>>> +    internal VSYNC pulse.
>>>> +
>>>> +**hsync_gap_length** (RW):
>>>> +    If the incoming video signal does not contain synchronization VSYNC and
>>>> +    HSYNC pulses, these must be generated internally in the FPGA to achieve
>>>> +    the correct frame ordering. This value indicates, how many "empty" pixels
>>>> +    (pixels with deasserted Data Enable signal) are necessary to generate the
>>>> +    internal HSYNC pulse. The value must be greater than 1 and smaller than
>>>> +    vsync_gap_length.
>>>
>>> Does this make sense? vsync_gap_length can be many video lines, which makes
>>> not sense for hsync_gap_length.
>>>
>>> I wonder if it isn't easier to just change this to v/hsync_blanking_length
>>> (lines for vsync, pixels for hsync) to indicate the length of the blanking
>>> periods, and then let the driver pick a suitable hsync/vsync position.
>>>
>>
>> Dtto.
> 
> So the problem here is that if I don't understand what is meant here, how will
> a user of this driver be able to understand it?
> 
> I think it would be better to give one or two examples of devices and their
> configuration. Or refer to freely available documentation, if that is available.
>

Those two parameters are very rare to be used and it is expected that 
users with the corresponding HW (signal source/displays) will either be 
experts on that topic or they will simply get the parameters as "magic 
numbers" from us after we test the HW and obtain the correct values. As 
far as I can decipher the info from the HW engineers, this is a quiet 
non-standard parameter that is hard to find in any documentation, but 
for some "exotic" systems it is required for the mgb4 to work with the 
corresponding HW.

> Also note that v9 doesn't mentioned default values for these two properties.
> 

I have not put the defaults here because in most cases the "default" is 
that this value is not used (the signal has VSYNC/HSYNC). And when it is 
needed, the value has to be changed to fit the HW anyway as chances to 
match any default value are almost zero. All other values are the 
defaults of a particular display/entertainment system (Škoda Octavia 
III)* but here I myself (unlike the HW people) have not even access to 
any HW to test it...

One can however always read-out the value if he desperately needs the 
default value.

M.

* Officially this is the reference system that shall be quiet accessible 
for the customers as this infotainment system is quite widespread in the 
VW concern, but the real reason is, it is the system laying on my desk ;-)