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So, I'm building a device involving a raspberry pi (Pi4) and an e-ink display, with SPI communication. I'm wondering what the soonest would be, I could communicate with the display. Main idea here is, to show something on the display as soon as possible, informing user device is actually booting. Since a getty terminal is available very shortly (UART), I assume SPI should also be available pretty much right after a few seconds, once the bcm2711 (or older) is initialized.

Side note: I access SPI directly through mem using go-rpio, and not using the bcm2835 lib (mostly the go-rpio driver is a limited low-level adaptation of the bcm2835 lib), so it requires access to /proc/device-tree/soc/ranges to obtain the mem base range, then access to /dev/mem or /dev/gpiomem to memmap the ranges.

Follow-up question: what's the best way to run my code after boot so I have the earliest possible access: creating a systemd service? what would be the After/Before targets to use?

Edit: added follow-up question

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  • perhaps a cheap microcontroller could be added to display the initial image
    – jsotola
    Commented May 30 at 16:15
  • "I'm wondering what the soonest would be ...". Why don't you try it?
    – Seamus
    Commented May 30 at 18:30
  • I did try and the soonest I get is around 20s post power on, which I believe is largely improvable, since I only need to be able to access SPI (not loading anything from disk). I just don't know if I'm overlooking something, since I see the GPIO driver is loaded around 5s in... Would I need to move my code to the /boot/firmware partition?
    – Phil H
    Commented May 31 at 14:24
  • Add a line to /etc/rc.local and by the time the system runs that SPI will be more than alive.
    – Dougie
    Commented May 31 at 17:02
  • I want to take advantage of systemd so the app can be restarted in case of crash and handled properly on shutdown or reboot
    – Phil H
    Commented Jun 2 at 0:29

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