3.6.4. Display

TI SoCs are equipped with Display SubSystem (DSS) hardware to provide hardware acceleration for alpha blending of overlays and color conversion. The DSS hardware is exposed to the software drm API available through libdrm module. Through this drm interface, a user space program can perform mode setting of the display.

The drm module models the display hardware as a series of abstract hardware blocks and manages them through the API. The blocks are:

CRTC1: represents a scanout engine that generates video timing signal from the data pointed to by the scanout buffer

Connector: represents where the video timing signal is sent across to the display

Encoder: transforms the video timing signal from CRTC to a format that is suitable for sending across the connector

Plane: represents the overlay buffer that a CRTC can be fed with

A utility application modetest can be used to get the list of available drm blocks. All the information available for the device can be displayed by using it.

3.6.4.1. Finding Connector ID

Run the below modetest command:

# modetest -M tidss -c

Look for the display device for which the connector ID is required - such as HDMI, LCD etc.

Connectors:
id      encoder status          type    size (mm)       modes   encoders
4       3       connected       HDMI-A  480x270         20      3
  modes:
        name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot)
  1920x1080 60 1920 2008 2052 2200 1080 1084 1089 1125 flags: phsync, pvsync; type: preferred, driver
...
16      15      connected       unknown 0x0             1       15
  modes:
        name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot)
  800x480 60 800 1010 1040 1056 480 502 515 525 flags: nhsync, nvsync; type: preferred, driver

The modes displayed are the various resolutions supported by the connected display.

3.6.4.2. Finding Plane ID

To find the Plane ID, run the modetest command:

# modetest -M tidss -p

Which should show something like below:

Planes:
id      crtc    fb      CRTC x,y        x,y     gamma size
19      0       0       0,0             0,0     0
 formats: RG16 RX12 XR12 RA12 AR12 XR15 AR15 RG24 RX24 XR24 RA24 AR24 NV12 YUYV UYVY
 props:
 ...
20      0       0       0,0             0,0     0
 formats: RG16 RX12 XR12 RA12 AR12 XR15 AR15 RG24 RX24 XR24 RA24 AR24 NV12 YUYV UYVY
 props:
 ...

3.6.4.3. Using Connector ID and Plane ID

The above information may be used with some userspace applications to control which displays are rendered to. These applications are using what is known as kernel mode setting (kms). For more information about kernel mode setting see the upstream kms documentation. In this section you only need to keep 2 things in mind:

Applications that intend to interact with the kms interface usually don’t need any user input. They can query device info through the interface and will normally pick the first connected display automatically.

Only one application can manage the kms interface at a time. Weston is normally the first graphical application started out of the box and as such it will prevent you from starting any other kms applications. See Stopping Weston if you want to use another kms application.

That being said, if you wish to change rendering behavior for an application check with that applications documentation for a way to specify connector, plane, and / or crtc information. One kms application we include is kmscube. Below are some examples on how to alter it’s default behavior.

Run kmscube on the default display:

# kmscube

Run kmscube on the secondary display:

# kmscube -n <connector-id>

For example, if the connector id for the secondary display is 16:

# kmscube -n 16

1: CRTC stands for cathode-ray tube controller, a throw back to the old cathode-ray tubes TV’s which had a controller that generated video timings based on the data it is being fed by a buffer.