Fixed assignment of /dev/fb0 and /dev/fb1 possible?

Hi there!

I am developing a unit, based on Freescale iMX6 SOC, that has an internal OLED display connected via a SPI interface and is driven by the simple LCD / framebuffer driver framework.

In addition the user can optionally connect a HDMI monitor, mouse and keyboard to the unit and is then able to control the unit on that way.

Unfortunately the internal display switches its framebuffer device number, depending if a monitor has been connected on boot off the unit.

As a side-effect two things happen now, if HDMI is not used on boot. On one hand, the application does not run anymore, as it fails to get hold of /dev/fb1. Instead the graphical desktop is starting on /dev/fb0, showing a little bit of a desktop and a mouse pointer on a 480x128 OLED grayscale display.

System platform is Armbian (Debian / Ubuntu remix) with kernel 4.15.15.

Two things are needed for me:
- how to either lock /dev/fb0 and /dev/fb1 in place or how to identify the SPI connected device in a safe way.
- how to avoid the desktop running if no HDMI is connected or how to run the desktop even when HDMI is not connected.

Any help welcomed.

That kind of thing is usually done by udev. You can find out the pci device paths of the two devices and then make a udev rule using the DEVPATH variable to assign a fixed fb device to each. But writing udev rules is very tricky.

Thanks hazel!

I wonder if there is an option to find the OLED display by using sys tree? There are many options to find devices from different directions. But I can't quite remember that I have seen any links between /dev/spi and /dev/fb using paths inside /sys/class/...