Version 1 (modified by trac, 7 years ago) ( diff )



The intended goal of this article is to provide the user with enough information so they can display an image onto a screen of their choosing.

For information regarding Audio / Video breakout adapters for Ventana see:

Video Output Drivers

There are two different display driver API's in Linux today:

The i.MX6 Video output framework drivers are currently only available in the Yocto BSP using the Freescale 3.0.35 kernel. This driver framework has not made it fully into mainline linux yet and no support is available for OpenWrt.

Displays Devices

The IMX6Q (Quad Core) CPU has 2 IPU's (image processing units) where as the IMX6DL (Dual-Lite 2-core) has 1 IPU. Each IPU can have 2 display output paths and can therefore output to 4 different displays simultaneously (either the same or unique data). Each of these display output paths is represented by a 'mxcfb' device.

Various Ventana boards have different output display possibilities (listed in the order the devices are registered):

  • GW51xx: HDMI (Digital)
  • GW52xx: HDMI (Digital), LVDS (Digital)
  • GW53xx: HDMI (Digital), LVDS (Digital)
  • GW54xx: HDMI (Digital), CVBS (Analog), LVDS (Digital)
  • GW551x: HDMI (Digital)
  • GW552x: HDMI (Digital)
  • GW553x: HDMI (Digital)

These four devices get registered in the kernel in a specific order (per model, see above), but can be overwritten in the bootloader (see below). This order dictates the 'framebuffer' and 'v4l' output device numbers. By convention, the first framebuffer is used for bootup and is known as the 'Primary Display'.

As an example, the GW5400's default display order is HDMI, CVBS, then LVDS. The table below shows how this ordering will translate in linux:

Device Registration Display Framebuffer Video
First HDMI fb0, fb1 video16, video17
Second CVBS fb2, fb3 video18, video19
Third LVDS fb4 video20
Fourth Off - -

Notice how the first two displays get two framebuffers devices and two video devices. This is because the first two displays that were registered have overlay support (subtitles, picture-in-picture etc.). Keep this in mind when determining your order.

How to Change Device Registration Order

In order to change how the devices get registered in the kernel, we must first understand the 'video=' argument.

There are several components to the 'video=' argument in the form of: <BR> video=mxcfb<n>:dev=<devicename>,<format>[,if=<interface-fmt>][,bpp=<bits-per-pixel>][,fbpix=<fb-pix-format>] where:

Component What it stands for Definition
<n> Framebuffer Device specifies the ordering of displays in Linux
dev=<device> Device Name (adv739x,hdmi,ldb) specifies the interface driver by name
if=<pixelfmt> Interface Pixel format the pixel format from the device to the IPU (ie RGB666 is for a 18-bit display)
fbpix=<pixelfmt> Framebuffer Pixel Format the format the /dev/fb* device accepts
<format> A modedb format - see modedb.txt The resolution, framerate, and pixeldepth of the framebuffer device

Interface examples:

  • CVBS:
    • video=mxcfb0:dev=adv739x,BT656-NTSC,if=BT656,fbpix=RGB565
      • the BT656-NTSC dictates the interconnect format between the CVBS decoder chip and the IPU (hardware-specific) and is the only thing currently supported
  • LVDS:
    • For Yocto v1.6 and Yocto v1.7 the device should be specified as: video=mxcfb0:dev=ldb,LDB-XGA,if=RGB666 where the LDB-XGA defines the display 'mode' detailing the resolution and timings:
      • LDB-XGA (1024x768 suitable for Freescale LVDS1 or DLC800FIGT3 displays)
      • LDB-WSVGA (1024x600 suitable for DLC700JMGT4 display)
    • For Yocto v1.8 the display mode is omitted and can be specified via the device-tree 'display' env var
  • HDMI:
    • video=mxcfb0:dev=hdmi,1280x720M@60,if=RGB24
      • the 1280x720M defines the resolution and is somewhat flexible depending on your display. See Linux mode.db for details

The instance of the 'mxcfb' device determines the framebuffer order. For example, mxcfb0 is the first display, mxcfb1 is the second, mxcfb2 is the third, and mxcfb3 is the fourth. If you are not using all four output devices be sure to disable them by setting them to 'off' to avoid mis-allocating resources (see examples below).

Here are the steps to make HDMI the primary display, LVDS have overlay support, and completely turn off CVBS and the fourth display lane (and thus save power)

Making sure that the jtag dongle is connected, power on the board and hit any key to break into the bootloader and type the following:

Ventana> setenv video 'video=mxcfb0:dev=hdmi,1280x720M@60,if=RGB24 video=mxcfb1:dev=ldb,LDB-XGA,if=RGB666 video=mxcfb2:off video=mxcfb3:off'
Ventana> saveenv
Ventana> reset

Here are some other combinations:

Displays Configuration bootloader setenv command
1 HDMI setenv video 'video=mxcfb0:dev=hdmi,1280x720M@60,if=RGB24 video=mxcfb1:off video=mxcfb2:off video=mxcfb3:off'
2 HDMI+LVDS setenv video 'video=mxcfb0:dev=hdmi,1280x720M@60,if=RGB24 video=mxcfb1:dev=ldb,LDB-XGA,if=RGB666 video=mxcfb2:off video=mxcfb3:off'
2 HDMI+CVBS setenv video 'video=mxcfb0:dev=hdmi,1280x720M@60,if=RGB24 video=mxcfb1:dev=adv739x,BT656-NTSC,if=BT656,fbpix=RGB565 video=mxcfb2:off video=mxcfb3:off'
2 LVDS+CVBS setenv video 'video=mxcfb0:dev=ldb,LDB-XGA,if=RGB666 video=mxcfb1:dev=adv739x,BT656-NTSC,if=BT656,fbpix=RGB565 video=mxcfb2:off video=mxcfb3:off'
3 CVBS+LVDS+HDMI (default) setenv video 'video=mxcfb0:dev=adv739x,BT656-NTSC,if=BT656,fbpix=RGB565 video=mxcfb1:dev=ldb,LDB-XGA,if=RGB666 video=mxcfb2:dev=hdmi,1280x720M@60,if=RGB24 video=mxcfb3:off'

U-Boot Video Script

In our Yocto images with video out support, we include a bootscript. Below are links to each of these scripts (per release):

Our most recent bootscript (v1.06) allows for displays to be configured for use within Linux. It currently supports the following:

Name Description
Hannstar-XGA Freescale MCIMX-LVDS1 10" XGA Touchscreen Display
DLC700JMGT4 GW17029 DLC700JMGT4 7" WSVGA Touchscreen Display
DLC800FIGT3 GW17030 DLC800FIGT3 8" XGA Touchscreen Display
CVBS Analog Video Out
none Turn all displays off

Setting the display environmental variable will allow video to be automatically configured. Please see below for examples:

  • For example, the following would place HDMI on the first framebuffer, and CVBS on the second:
    setenv display "HDMI CVBS"
    saveenv # Only if you want the setting to persist
  • This example will configure an LVDS display on the first framebuffer:
    setenv display "Hannstar-XGA"
    saveenv # Only if you want the setting to persist
  • This example will mark all displays to 'off':
    setenv display "none"
    saveenv # Only if you want the setting to persist

Some things to note:

  • If the environmental variable displays is set, it will take precedent over display
  • If neither display or displays are set, the bootscript will try to detect a HDMI monitor and use it
  • Only one type of display is allowed to be set, i.e. two LVDS displays cannot be set. If this is the case, the first one specified is chosen.
  • The environmental variable hdmi can be set to dictate the HDMI output resolution: 1080p, 720p, 480p. If unset, it will default to 1080p

Finally, this bootscript also configures how much CMA to allocate for the GPU/VPU on the imx6. If the environmental variable mem is unset, the bootscript will attempt to dynamically configure CMA. If you want to manually configure mem, you can set it to something like setenv mem "cma=96M". However, if you do not want to configure mem for anything, you can set it to setenv mem 'NA' and the bootscript will gloss over it and allow kernel default.

HDMI Resolutions Supported

The mxc_hdmi driver by default supports the following progressive CEA modes:

  • #1: 640x480p@59.94/60Hz 4:3
  • #2: 720x480p@59.94/60Hz 4:3
  • #3: 720x480p@59.94/60Hz 16:9
  • #4: 1280x720p@59.94/60Hz 16:9
  • #8: 720(1440)x240pH@59.94/60Hz 4:3
  • #9: 720(1440)x240pH@59.94/60Hz 16:9
  • #14: 1440x480p@59.94/60Hz 4:3
  • #15: 1440x480p@59.94/60Hz 16:9
  • #16: 1920x1080p@60Hz 16:9
  • #17: 720x576pH@50Hz 4:3
  • #18: 720x576pH@50Hz 16:9
  • #19: 1280x720p@50Hz
  • #23: 720(1440)x288pH@50Hz 4:3
  • #24: 720(1440)x288pH@50Hz 16:9
  • #29: 720(1440)x576pH@50Hz 4:3
  • #30: 720(1440)x576pH@50Hz 16:9
  • #31: 1920x1080p@50Hz
  • #32: 1920x1080p@23.98/24Hz
  • #33: 1920x1080p@25Hz
  • #34: 1920x1080p@30Hz
  • #41: 1280x720p@100Hz 16:9
  • #47: 1280x720p@119.88/120Hz 16:9

Out of the above list of supported modes only those present in the monitor's Extended Display Identification Data (EDID) are considered.

If your monitor supports a non-CEA mode or mode not listed above you can set the only_cea=0 kernel commandline parameter to allow it, however you may find that this disables HDMI audio output. This can be set in the bootloader for example:

setenv extra 'mxc_hdmi.only_cea=0'

The default mode is specified in the kernel device-tree (for example, here for the GW52xx) as '1920x1080M@60' for 1080p@60Hz. This can be overridden with a video= kernel parameter (see above). For example, if you monitor is defaulting to 1920x1080p@60Hz and you want it instead to be 1680x1050@59Hz from a matching mode of 'D:1680x1050p-59' in /sys/class/graphics/fb0/modes you could use:

setenv video 'video=mxcfb0:dev=hdmi,1680x1050M@59 video=mxcfb1:off'

Note that the Yocto BSP has a bootloader script which will set the kernel commandline (unless the video bootloader env variable is set to override it). The script will use the hdmi env variable (and defaults to 1080p if not set) to determine the resolution and use this for HDMI if an HDMI monitor is connected at bootup. For example:

  • specify 480p
    setenv video # make sure video var is blank for auto-configuration to work
    setenv hdmi 480p # set to 720x480M@60
  • specify 720p
    setenv video # make sure video var is blank for auto-configuration to work
    setenv hdmi 720p # set to 1280x720M@60
  • specify 1080p
    setenv video # make sure video var is blank for auto-configuration to work
    setenv hdmi 1080p # set to 1920x1080M@60

Note that if you specify a video mode that the monitor's EDID does not support (or supports per the EDID but doesn't appear in /sys/class/graphics/fb0/modes because only_cea=1) the mxc_hdmi driver will try to find the nearest supported resolution to what you specified.

Once booted, you can see the list of available video modes via /sys/class/graphics:

  • see current video mode:
    # cat /sys/class/graphics/fb0/mode
  • see available modes:
    # cat /sys/class/graphics/fb0/modes
    • 'S' refers to an EDID 'Standard Timing' in EDID
    • 'V' refers to a VESA mode from 'Established Timings' in EDID
    • 'D' refers to the 'Detailed mode' in EDID (which should be the monitors preferred or native mode)
  • change the resolution (must provide a mode matching those show from modes):
    echo "V:1024x768p-60" > /sys/class/graphics/fb0/mode

Tested Resolutions and Pipelines

All of these tests were run using gstreamer's videotestsrc to display a color bar on the video device.

OS Display Type Resolution & Framerate result Yocto 1.7 Pipeline Command
Yocto 1.3 HDMI 1920x1080p @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 1920x1080p @ 50hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 1280x720p @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 1280x720p @ 50hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 720x576p @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 720x480p @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 640x480p @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 HDMI 720x480p @ 59hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video16
Yocto 1.3 LVDS 1024x768p @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video20
Yocto 1.3 CVBS 720x480i @ 60hz Success gst-launch videotestsrc ! mfw_v4lsink device=/dev/video18

The pipeline command uses the gst-launch action to use the source element[videotestsrc] and pipe it into the sink element[mfw_v4lsink] and output it through the the device specified.

Note: The pipeline commands are based on the default configuration[HDMI, then CVBS, and finally LVDS].

Analog Video Output Format

The Ventana Analog Video output is provided via the ADV7393 Analog Video encoder chip. This chip has 3 outputs (DAC1, DAC2, DAC3) which are all provided on the Ventana board Video Output connector. These can be used in the following modes:

  • Composite (CVBS)
  • Y/C (S-Video)
  • Component RGB
  • Component YPrPb

The adv7393 driver currently does not support setting the output format via software as this is outside the scope of the V4l2 API, but you can access its registers on /dev/i2c-2 slave address 0x2a. See ADV7393 datasheet Table 37 for details.


Framebuffer device API and Screen Blanking

A framebuffer device drives a display from a memory buffer containing raw pixel information. This means it can run regardless of the hardware present.

In linux, the framebuffers are located at /dev/fb*. For the configuration above (HDMI as primary, LVDS as second, rest off), the /dev/ folder will contain fb0, fb1, fb2, fb3. Remember, the reason there are four framebuffers is because the first two devices get two distinct framebuffers each for overlay.

Framebuffer information is located at /sys/class/graphics/fb*. Here, you can get display properties and do things such as blanking and unblanking a screen.

If we wanted to blank the primary display and unblank the secondary screen, we would do the following:

echo 1 > /sys/class/graphics/fb0/blank
echo 0 > /sys/class/graphics/fb2/blank


  • echoing a 1 into a framebuffer's blank value will make it blank the screen it is associated with.
  • echoing a 0 into a framebuffer's blank value will unblank the screen associated with it.
  • The default linux framebuffer support will blank a console after 600 seconds of no activity. You can specify this with a kernel parameter 'consoleblank=<n>' where <n> is the number of seconds (0 to disable auto-blanking completely), or by using the setterm userspace application if available.

How to Output a Test Pattern

In order to display something, you can run a utility called fb-test. To display a variety of colors, making sure the display is unblanked, type the following and hit return: fb-test -f 0

This utility is useful as you can test a variety of colors and patterns.

Also note: Since framebuffers are just raw pixel information in memory, you can perform many low levels tasks such as directly copying the raw data with a cp /dev/fb0 /tmp/image. Or writing raw data back with cp /tmp/image /dev/fb0. This contributes to their continued fame.

You can also use gstreamer's fbdevsink element to sink to a framebuffer device.

Video4Linux (v4l) device API

v4l is another level of abstraction to do video in linux by providing an API. You can stream video using this driver using the mfw_v4lsink element in a gstreamer pipeline. The devices for this driver are located at /dev/videoXX.

Output to Display

You can download a small shell script here that will unblank/blank displays of your choosing to make typing tedious commands easier.

At the heart of the script is a gstreamer pipeline that pipe's a "video test source" to a video4linux sink device of your choosing.


These steps are for HDMI output only.

Confirm that the modes are properly showing from the command console with the following commands:

root@ventana:~# cat /sys/class/graphics/fb0/mode

root@ventana:~# cat /sys/class/graphics/fb0/modes

Confirm a monitor is detected on the I2C bus from the bootloader and that the EDID can be read:

Ventana > i2c dev 2 && i2c probe && i2c edid 0x50
Setting bus to 2
Valid chip addresses: 1E 20 37 3A 50
EDID version: 1.3
Product ID code: d015
Manufacturer: DEL
Serial number: 30323853
Manufactured in week: 14 year: 2008
Video input definition: digital signal, voltage level 0
Monitor is RGB
Maximum visible display size: 47 cm x 30 cm
Power management features: active off, suspend, standby
Estabilished timings:
        720x400         70 Hz (VGA 640x400, IBM)
        640x480         60 Hz (VGA)
        640x480         75 Hz (VESA)
        800x600         60 Hz (VESA)
        800x600         75 Hz (VESA)
        1024x768        60 Hz (VESA)
        1024x768        75 Hz (VESA)
        1280x1024       75 (VESA)
Standard timings:
        1152x864        75 Hz
        1280x1024       60 Hz
        1680x1050       59 Hz (detailed)
Monitor serial number: KU311844028S
Monitor range limits, horizontal sync: 30-83 kHz, vertical refresh: 56-75 Hz, max pixel clock: 140 MHz
Monitor name: DELL E228WFP
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