Brightness control through HDMI

I bought a TX2 developer Kit and connected it to a regular LCD Display through HDMI cable. The board was flashed with version 28.2.1 via JetPack.
I found that I cannot control the screen brightness in the Ubuntu System Settings, and I cannot change the brightness from command line either. I notice that this folder /sys/class/backlight/ is empty.

My goal is to control brightness using software. Is it possible to do that just through the HDMI cable? Or what else should I do? Any pointer would be helpful to me.

Thanks!

We didn’t use any pwm to control backlight and that is why that sysfs is empty.

Thanks for the reply.
What should I do to enable backlight control? modify the device tree or recompile the Linux kernel?
I hope someone could give me a general idea and I will try to figure out the details.
Sorry, i’m really a noob here.

To be more precisely, we don’t provide backlight control in our HDMI driver so even you add something in device tree, it would not work.

Thank so much for the explanation.
It seems to me that pwm backlight control is only available via J23 (Display Expansion Connector).

I’m not sure about this, but if VESA DDC/CI is supported, the brightness of external displays can also be changed using DDC-Control on HDMI.