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AM4378: Save and restore backlight across shutdowns

Rajat Rao
Intellectual 760 points
Part Number: AM4378


Hello,

I'm using the AM4378's ECAP PWM module to control the backlight of my LCD and everything works fine.

My problem is that the backlight doesn't get saved and restored after a power-off. Looking at the systemd-backlight@.service, a save is supposed to happen on a shutdown and a restore on boot. In our case, we perform a "hot" shutdown where the power is just removed, not a "safe" shutdown where all processes are terminated. Can any systemd expert look at this and help us out here? We would like to save the backlight on change instead of on shutdown.

  • 0
    TI__Guru** 102170 points

    Hello Rajat,

    Have you tried changing the device tree settings for the PWM backlight node? for example, the default-brightness-level?

    Regards,

    Nick

  • +1 in reply to Nick Saulnier
    Intellectual 760 points

    Hi Nick,

    Yes, that works. But, different users might find different settings suitable for them in the field. Basically, it should remember the value across power-on's.

    We have managed to solve this by installing an interrupt on the backlight sysfs entry and forcing a save using a separate script. I was looking to see if there is a "cooler" systemd way to make the backlight service script do this automatically.