Unlocking the Potential: A Comprehensive Guide to Using the Media Controller Tool

Modern System-on-Chip (SoC) devices support a wide range of functionality in the way of internal hardware blocks which has resulted in a variety of ways to interconnect functions within the SoC that alter the V4L device content.

The Media Controller kernel API has been designed to expose detailed information about media devices and capture devices to userspace in a way that allows them to be interconnected in dynamic and complex ways at runtime.

This media-ctl application from the v4l-utils package that uses the Linux Media Controller API to configure video pipeline entities.

Here taking reference of NXP imx8 & TI OMAP3 platform, see the top level camera pipeline flow

Ref: omap3isp

Once the device is booted successfully we can verify the above pipeline using media-ctl if device is bind via media controller driver.

How to view the device topology which help the user to see all required pad/linked created after successful boot

media-ctl -p 
OR
media-ctl --print-topology        

To create the links in the media control that actually connect the input pad with the output pad for the camera sensor node to the final camera frame output.

For example as per above omap3 platform Let say camera sensor is mt9p031 (RAW sensor)
Link will be:
mt9p031 --> OMAP3 ISP CCDC --> OMAP3 ISP Preview --> OMAP3 ISP Resizer
# media-ctl -r -l '"mt9p031 0-003c":0->"OMAP3 ISP CCDC":0[1], "OMAP3 ISP CCDC":2->"OMAP3 ISP preview":0[1], "OMAP3 ISP preview":1->"OMAP3 ISP resizer":0[1], "OMAP3 ISP resizer":1->"OMAP3 ISP resizer output":0[1]'        

In imx8 platform, this can be done by media dev driver after camera sensor and CSI driver loaded properly.

Ref: imx8 media dev

How to set the formats and resolutions to the source/sink pad using media-ctl

As per above linking the pads user need to configure formats and resolutions as well
# media-ctl -v -V '"mt9p031 0-003c":0 [SGRBG10 1280x1024], "OMAP3 ISP CCDC":2 [SGRBG10 752x480], "OMAP3 ISP preview":1 [UYVY 752x480], "OMAP3 ISP resizer":1 [UYVY 752x480]'        

User can also draw the the graph of linked pads which is created using media-ctl

media-ctl --print-topology
media-ctl --print-dot > graph.dot
dot -Tpng graph.dot > graph.png        

This will help the developer see if the links are properly created or if something is broken while creating the pipeline.

This is really a very powerful tool that can configure the camera sensor from user space if the camera is attached to the media sub device architecture. Hoping this info will help you in future to get some idea about the media-ctl tool.

Ref: https://trac.gateworks.com/wiki/linux/media

https://mediawiki.compulab.com/index.php/CM-T3730:_Linux:_Camera