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I'm trying to setup as touchscreen for a raspberry pi.

My screen only displayed a white screen when kernel started. So I wondered how boot messages are displayed on my screen during startup. Bios, Bootloader, kernel are able to show some things on the screen. How is it possible?

Let's come back to my white touchscreen. The screen is not a "standard" one because it is connected over a given SPI, not over HDMI, DVI. From logs I can see frame buffer is ready 8s after kernel startup :

[    8.026032] graphics fb1: flexfb frame buffer, 480x320, 300 KiB video memory, 4 KiB buffer memory, fps=20, spi0.0 at 16 MHz

So, if screen was correctly configured,

  • Would logs from kernel startup be displayed on screen?
  • OR Would I get kernel log displayed on screen after some seconds (time to load screen driver)?
  • OR I won't get any kernel logs. But would I get something displayed on X startup?
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The reason the kernel messages/logs do not appear in your rpi screen early in the boot process is because the framebuffer screen is activated relatively "late" in the boot process.

Actually, it is a given in the raspberry/Banana/Orange PI plataform that you won't see the boot process/kernel messages/errors before the framebuffer is activated. There is no way around that as you ask.

So to follow up the boot process from an earlier time, you can configure u-boot and the kernel to use a serial output, and follow the process from a much earlier point on the boot process.

For that, you need to buy a USB to TTL Serial Cable PL2303HX, which you connect in the console/JTAG pins of the raspberry.

You then follow the boot process in a remote terminal in your laptop/computer, with a serial remote terminal such as minicom, until the point the framebuffer output is activated.

P.S. Obviously nothing prevents you to later on see the dmesg output. The dmesg output does not show the output of ALL the previous system activity, but it does show at least the kernel messages. However,there is no way around needing some extra point of output before the framebuffer starts displaying the output of the system.

cable rpi

image credits: Adafruit

For how to set up the RS-232 output, see related Console setting in initramfs (ARM)

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