GPIO 3.5"
How i installed the 3.5" (A) type of screen on RaspberryOS Bookworm (Debian 12)
Update: I've tested Bullseye (Debian 11) and it works, but rotation was not working
Disclaimer
This is based on my own experience, and it's provided for general information and does not constitute as advice of any kind.
Initial install
This screens connect to the GPIO they were usually installed via repositories named LCD-show (1, 2), however in the bookworm release there is a simpler solution
at the bottom add:Info
in this context drm stands for Direct Rendering Manager
I had to comment out (add # at the start) vc4 or it would boot into a blackscreen
Close the nano editor using ctrl+x (exit), then y for yes (save).
Wrong colors or graphical corruption
Some screens will not behave correctly and/or display wrong colors, this usually due to the wrong SPI speed In my case the maximum i could achieve is 22.22Mhz
About speed
As you may predict speed has a big impact in usability as it dictates how often the display will refresh
If i understand the involved drivers
correctly, the spi speed is calculated as:
core_clock / core_divisor
The divisor must be a multiple of 2, that ranges between 2 and 65536
Core clock in the case of Pi 3 would be 400mhz
So even if you can enter any number, it will be approximated to a value from that formula
That's why i used 22.222.222 (400 / 18)
reboot to test any changes.
Rotation
You can rotate the screen adding a rotate line with the degrees [0, 90, 180, 270]
Screen rotation will require adjusting the touch matrix see Touch issuesBug
Raspberry linux Kernel v6.1.77 has a bug in the dtb and rotate doesn't work
The fix has been merged in linux v6.6
you can copy the dtb from the new kernel into the old one if needed
Console
To make the console work:
add at the start or end:
Important
Do not add newlines to the file, it must be 1 line with the options separated by spaces