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Linux - Embedded & Single-board computer This forum is for the discussion of Linux on both embedded devices and single-board computers (such as the Raspberry Pi, BeagleBoard and PandaBoard). Discussions involving Arduino, plug computers and other micro-controller like devices are also welcome.

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Old 05-29-2018, 09:45 AM   #1
Astralix
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Registered: Dec 2008
Location: Germany
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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.
 
Old 05-29-2018, 11:18 AM   #2
hazel
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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.
 
Old 05-30-2018, 09:40 AM   #3
Astralix
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Registered: Dec 2008
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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/...
 
  


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