Dual Display - Android Application Tech Note

Dual Display on VAR-SOM-SOLO/DUAL / DART-MX6: Android Application Tech Note

Overview

i.MX6 platforms are capable of handling multiple displays.
You can drive two displays simultaneously.
See page: 15 - 19 http://cache.freescale.com/files/training/doc/ftf/2014/FTF-CON-F0119.pdf
Starting with Android 4.3 multiple displays are supported.
Android Application can leverage the use of multiple displays for rendering content

DART-MX6 / Solo both have LVDS0 and LVDS1 output available on the development board.

VAR-DT6CustomBoard:

For both channels: LVDS0/1 - LVDS Clock +4 Data lanes are accessible via Carrier board Headers.

VAR-SOLOCustomBoard:

LVDS0 - LVDS Clock +4 Data lanes are accessible via Carrier board Headers.
LVDS1 - LVDS Clock +3 Data lanes are accessible via Carrier board Headers (only 18 bit LCD can be interfaced)
For maximum, supported resolution and combinations refer to http://cache.freescale.com/files/training/doc/ftf/2014/FTF-CON-F0119.pdf page no 16 for more details and combination.

Note:

Not all the display modes combinations tested / verified.

The dual display setup can be in different combinations, the configuration can be like
1. Clone display (Primary is always cloned)
2. Independent display(s)

In the case of 1. The kernel will not register another framebuffer for the 2nd panel.
However, will clone internally from the SOC itself via IPU framebuffer driver from the interface specific drivers. Aka LDB, HDMI, RGB.
In the case of 2. The kernel registers 2nd FB to the platform.
This combination allows user/application to render different content on the different display(s).

Use case like Two LVDS - LVDS0 and LVDS1 port on SOLO

Hardware Setup

2 x LVDS Panel: https://www.variscite.com/products/accessories/vlcd-cap-gld-lvds/

Reference: https://www.variscite.com/wp-content/uploads/2017/12/VLCD-CAP-GLD-LVDS.pdf

Connect 1st panel to LVDS1 - which is enabled by default with the software.

Connect straight LVDS panel header to LVDS1 as this doesn’t require special handling.

Connect 2nd panel to LVDS0 - which requires changes in the software.

This example uses an identical LVDS panel however you can have an asymmetric combination as well.

Top level setup should look like below

In the schematic, you will see connectors like below.

Using wires connect LVDS0 to LVDS header in the LCD panel with the following combination.
Pin to pin as shown in the below with color coded.

PIN	LVDS0 PIN - J16		PIN	PIN	LVDS LCD Panel Header		PIN
1	3.3 V	3.3 V	2	1	VCC - 3V	VCC - 3V	2
3	GND	GND	4	3	GND	GND	4
5	LVDS0_TX0_N	LVDS0_TX0_P	6	5	RIN0_N	RIN0_P	6
7	GND	LVDS0_TX1_N	8	7	GND	RIN1_N	8
9	LVDS0_TX1_P	GND	10	9	RIN1_P	GND	10
11	LVDS0_TX2_N	LVDS0_TX2_P	12	11	RIN2_N	RIN2_P	12
13	GND	LVDS0_CLK_N	14	13	GND	RCLK_N	14
15	LVDS0_CLK_P	GND	16	15	RCLK_P	GND	16
17	LVDS0_TX3_N	LVDS0_TX3_P	18	17	5V_CC_Backlight	5V_CC_Backlight	18
19	VCC_5V	PWM_BACK_LIGHT	20	19	PWM_INPUT	GND	20

Note:

LVDS0 provides 3 lanes but LCD just needs 2 lane LVDS input.
Warning! Be careful about PIN 19 of LVDS0 header, please make sure it is connected to PIN17 or PIN18 of the LCD panel header only.

Connecting this pin to other pins of LCD or shorting to GND may cause permanent damage to LCD or board.

Software Changes for Multiple Displays

Android Oreo 8.0 and Android 7.1.2:
Note: Other Android versions may work, but not verified.

Software Preparation for Dual Display (Two LVDS):

U-Boot Changes:

On U-Boot you need below patch to be applied

For Android 8.0:

vendor/variscite/uboot-imx/

For Android 7.1.2:

bootable/bootloader/uboot-imx/

diff --git a/board/variscite/mx6var_som/mx6var_som.c b/board/variscite/mx6var_som/mx6var_som.c
index 526a11f..729c966 100644
--- a/board/variscite/mx6var_som/mx6var_som.c
+++ b/board/variscite/mx6var_som/mx6var_som.c
@@ -907,7 +907,7 @@ static void setup_display(void)
              | IOMUXC_GPR2_BIT_MAPPING_CH1_SPWG
              | IOMUXC_GPR2_DATA_WIDTH_CH1_18BIT
              | IOMUXC_GPR2_BIT_MAPPING_CH0_SPWG
-             | IOMUXC_GPR2_DATA_WIDTH_CH0_24BIT
+             | IOMUXC_GPR2_DATA_WIDTH_CH0_18BIT
              | IOMUXC_GPR2_LVDS_CH0_MODE_ENABLED_DI0
              | IOMUXC_GPR2_LVDS_CH1_MODE_ENABLED_DI0;
         writel(reg, &iomux->gpr[2]);
diff --git a/include/configs/mx6var_som.h b/include/configs/mx6var_som.h
index 1db7afb..b27f7ee 100644
--- a/include/configs/mx6var_som.h
+++ b/include/configs/mx6var_som.h
@@ -195,7 +195,7 @@
                                 "video=mxcfb0:dev=ldb; " \
                 "fi; " \
                 "setenv bootargs ${bootargs} " \
-                        "video=mxcfb1:off video=mxcfb2:off video=mxcfb3:off;\0"
+                        "video=mxcfb1:dev=ldb video=mxcfb2:off video=mxcfb3:off;\0"

 #define CONFIG_EXTRA_ENV_SETTINGS \

Note: Here we enable channel 0 for solo, as 18 bit and enable fb1 set the device to LDB(LVDS).

Kernel Changes:

On kernel side you require below patch in device tree to make it work.

For Android 8.0:

~/var_o_800_100/imx-o8.0.0_1.0.0_ga/android_build/kernel_imx

For Android 7.1.2:

~/var_n_712_200/n_712_200_build/kernel_imx

diff --git a/arch/arm/boot/dts/imx6qdl-var-solocb-dt6cb-displays.dtsi b/arch/arm/boot/dts/imx6qdl-var-solocb-dt6cb-displays.dtsi
index 8876245..b329e9d 100644
--- a/arch/arm/boot/dts/imx6qdl-var-solocb-dt6cb-displays.dtsi
+++ b/arch/arm/boot/dts/imx6qdl-var-solocb-dt6cb-displays.dtsi
@@ -12,6 +12,14 @@
        interface_pix_fmt = "RGB666";
 };

+&mxcfb2 {
+       disp_dev = "ldb";
+       interface_pix_fmt = "RGB666";
+       default_bpp = <16>;
+       int_clk = <0>;
+       late_init = <0>;
+};
+
 &i2c3 {
        touch: ft5x06_ts@38 {
                compatible = "edt,edt-ft5x06";
@@ -28,21 +36,21 @@

        lvds-channel@0 {
                fsl,data-mapping = "spwg";
-               fsl,data-width = <24>;
+               fsl,data-width = <18>;
                status = "okay";

                display-timings {
                        native-mode = <&timing0>;
                        timing0: hsd100pxn1 {
-                               clock-frequency = <35714000>;
+                               clock-frequency = <32000000>;
                                hactive = <800>;
                                vactive = <480>;
-                               hback-porch = <24>;
-                               hfront-porch = <15>;
-                               vback-porch = <13>;
-                               vfront-porch = <20>;
-                               hsync-len = <20>;
-                               vsync-len = <13>;
+                               hback-porch = <39>;
+                               hfront-porch = <39>;
+                               vback-porch = <29>;
+                               vfront-porch = <13>;
+                               hsync-len = <47>;
+                               vsync-len = <2>;
                        };
                };
        };

Software Preparation for Dual Display (LVDS + HDMI):

Kernel:

For Android 8.0:

~/var_o_800_100/imx-o8.0.0_1.0.0_ga/android_build/kernel_imx

Compile the kernel.

For Android 7.1.2:

~/var_n_712_200/n_712_200_build/kernel_imx

Compile the kernel.

Install the boot image to respective platform and then at U-Boot prompt change the bootargs

U-Boot prompt commands:

=> setenv videoargs 'setenv bootargs ${bootargs} video=mxcfb0:dev=hdmi,1920x1080M@60,if=RGB24 video=mxcfb1:dev=ldb video=mxcfb2:off video=mxcfb3:off;'
=> saveenv
=> reset

Android Framework changes (Optional for Android 8.0 only):

If you wish to have activities to be launched on secondary panel add the following line to tablet_core_hardware.xml

<feature name="android.software.activities_on_secondary_displays" />

The above mentioned file is located at "/system/etc/permissions/" on the system partition.
And in the source side, the file is located at

For Android 8.0:

android_build/variscite/var_mx6/tablet_core_hardware.xml

This feature is required for setLaunchDisplayId(<display number>) to work.

Compiling Boot Image:

Compile the Kernel boot image for your platform following below wiki pages.

For Android 8.0:

https://variwiki.com/index.php?title=VAR-SOM-MX6_Android&release=RELEASE_O8.0.0_1.0.0_VAR-SOM-MX6#Build_Android_Images

For Android 7.1.2:

https://variwiki.com/index.php?title=VAR-SOM-MX6_Android&release=RELEASE_N7.1.2_2.0.0_VAR-SOM-MX6#Build_Android_Images
and program the image to VAR-SOM-SOLO/DUAL or DART-MX6

Testing Multiple Displays on Android

By default, Android operates in clone mode, but if you wish to have two different content on two different panels;
your Android application must explicitly specify which display it wants to render the particular surface.
You can use the presentation mode. Customers are advised to use the presentation mode for testing/prototyping.

More information about the presentation mode can be found here: https://developer.android.com/reference/android/app/Presentation

You can find the reference code for application as below
Reference:

<Your_Android_Build>/development/samples/ApiDemos->./src/com/example/android/apis/app/PresentationWithMediaRouterActivity.java

If you wish to build apk for ApiDemos use below steps.

For Android 8.0:

$ cd ~/var_o_800_100/imx-o8.0.0_1.0.0_ga/android_build
$ source build/envsetup.sh
$ lunch var_mx6-eng
$ or
$ lunch var_mx6-userdebug
$ cd development/samples/ApiDemos
$ mm

This should generate

out/target/product/var_mx6/testcases/ApiDemos/ApiDemos.apk

copy apk to your present directory from which you can install on the platform

Connect OTG USB cable and use adb to install on host machine (Windows / Linux) using below command.

$ adb install ApiDemos.apk

For Android 7.1.2:

$ cd ~/var_n_712_200/n_712_200_build
$ source build/envsetup.sh
$ lunch var_mx6-eng
$ or
$ lunch var_mx6-user
$ cd development/samples/ApiDemos
$ mm

This will generate apk on

out/target/product/var_mx6/data/app/ApiDemos/ApiDemos.apk

path which you can install on the platform.

Copy apk to your present directory from which you can install on the platform.

$ adb install ApiDemos.apk

Note: For LVDS + HDMI case, at command prompt run below commands

# echo 0 > /sys/class/graphics/fb2/blank

By default, Android will clone the primary display to secondary unless you specify in the application layer for which display to use using presentation mode.