LVDS configuration for HDMI 1920x1080@60

Technical Support
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1

I am trying to connect two 1080p standard monitors to the Apalis iMX8 using the Ixora carrier board. One on the HDMI connector, and one on the LVDS connector, going through a LVDS → DVI converter board made by westar, and then a DVI to HDMI adapter.

I created a device tree overlay for it, and weston sees both HDMI-A-1 and LVDS-1 outputs and I can run applications on both outputs, but nothing shows up on the connected monitor. No error messages show in the kernel. On the LVDS panel configuration, I used what an internet search said were “standard” 1920x1080@60 settings (although I’m not sure about the data-mapping). Should those generally work or will I need to find the specific configuration for the monitor I have attached? Here is the overlay I ended up with:

/dts-v1/;
/plugin/;
/ {
        compatible = "toradex,apalis-imx8";
};

&ldb2 {
        status = "okay";
        fsl,dual-channel;
};

&ldb2_phy {
        status = "okay";
};

&panel_lvds {
        status = "okay";
        data-mapping = "vesa-24";
        width-mm = <598>;
        height-mm = <336>;
        panel-timing {
                clock-frequency = <148500000>;
                hactive = <1920>;
                hback-porch = <148>;
                hfront-porch = <88>;
                hsync-len = <44>;
                vactive = <1080>;
                vback-porch = <36>;
                vfront-porch = <4>;
                vsync-len = <5>;
        };
};
2

Hi @aaronb,

Can you provide more information about your environment, like which O.S. are you using (Yocto Build BSP, TorizonCore…)?

Can you confirm that you don’t see anything on both outputs, or only in the LVDS?

Best regards,
André Curvello

3

@andrecurvello.tx We are using Yocto BSP. Everything is working fine on the board HDMI output, it is only the LVDS output we are having trouble with. LVDS-1 shows up in weston without error and it runs applications on it, they just don’t appear on the monitor. It could be something going on with the LVDS → DVI board. I was just wondering if the device tree overlay looks ok for connecting to an off-the-shelf 1080p monitor.

4

Your overlay appears to be mostly correct from what I can see. Since Weston is seeing the LVDS-1 interface, that at least guarantees the interface is enabled and working. I guess then let’s focus on the timings.

I can’t say I know exactly how the timings are being translated over the adapters to the HDMI display. Though looking at your current timings there’s a couple of variables we can play with. This is an overlay for our 10’’ LVDS display: display-lt170410_overlay.dts « overlays - device-tree-overlays.git - Sources for Device Tree Overlays

Your current overlay isn’t using 3 timing variables, hsync-active, vsync-active, and pixelclk-active. I’m not entirely sure what the values here should be for your sitaution, but they’re binary variables (0 or 1) so there’s not a lot of choice. I would say start by adding these to see if it changes anything in your situation.

What would also be nice is if you had some kind of LVDS display, to at least sanity check that the timings you are providing are valid. That way we could at least isolate issues with the timings or issues with the adapters.

Best Regards,
Jeremias