I'm getting the impression that u-boot starts a considerable amount of activity on the GPU. Does it simply go away once an introduced kernel is started? Later on in the tutorial it's clear that bluetooth requires
its own software, which doesn't appear to be open source.
So, just how "bare" is bare metal on the Pi?
Found a somewhat readable (for a non-programmer) account of how to
load and run "bare metal" programs on a Pi4. It's at: https://isometimes.github.io/rpi4-osdev/
The author still uses u-boot to load and run his code, leaving the question of how much code he _didn't_ write remains resident and active.
I'm getting the impression that u-boot starts a considerable amount of activity on the GPU. Does it simply go away once an introduced kernel is started? Later on in the tutorial it's clear that bluetooth requires
its own software, which doesn't appear to be open source.
So, just how "bare" is bare metal on the Pi?
You can get the ARM cores to yourself if you want. It is possible to keepu-boot
in memory to provide 'firmware' services that your operating system doesn't provide, and there's also TrustZone, although I don't think the Pi environment uses that out of the box (I'm less familiar with Pi4).
If you want to do bare-metal things without U-boot, there's BakingPi: https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/os/
although that's written for the Pi1 and will need small changes for Pis 0/2/3. I'm not sure it'll work on Pi4. There are some PRs for supporting other Pis (Alex hasn't been able to work on it for a while): https://github.com/Chadderz121/bakingpi-www
Theo <theom+news@chiark.greenend.org.uk> wrote:u-boot
You can get the ARM cores to yourself if you want. It is possible to keep
doesn'tin memory to provide 'firmware' services that your operating system
provide, and there's also TrustZone, although I don't think the Pi environment uses that out of the box (I'm less familiar with Pi4).
Can the firmware services alluded to be called by a user logged in
through a "normal" connection? I.e., bypassing normal security and authentication mechanisms? That's not an issue if the firmware is removed/overwritten by the loaded operating system, but if u-boot
remains I understand much better the objections raised by open source advocates.
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