Re: [ptxdist] RPi3b+ boot sequence with DistroKit (barebox)

[Roland Hieber <rhi@pengutronix.de>]

On Sat, Aug 31, 2019 at 10:35:05PM -0400, Jon Ringle wrote:
> I am a little confused about the boot sequence and where each boot stage
> gets configuration from. Here's my current understanding, which I am sure
> is flawed:
> 
> 1. On power on or reset, the GPU executes the 1st stage bootloader from ROM
> 2. 1st stage bootloader reads the file bootcode.bin from the 1st partition
> (fat) on SDCard and loads it to L2 cache and executes this as the 2nd stage
> bootloader.
> 3. The 2nd stage bootloader initializes and enables SDRAM and reads the
> file start.elf to ram as the 3rd stage bootloader.

I cannot tell anything before this point, this is totally opaque to me.

> 4. The 3rd stage bootloader reads the config.txt.
> [This is the part that confuses me with regards to how other RPi distros
> work vs DistroKit with barebox...]

Why? config.txt is the standard way to configure the ROM loader, and it
is even present on a standard Raspbian image:
https://www.raspberrypi.org/documentation/configuration/config-txt/boot.md

> I thought that this stage would also load the DTB and any dtoverlay before
> loading the "4th stage bootloader" (by default is the file kernel.img, but
> barebox changes this by having `kernel=barebox.img` in config.txt).
> However, it seems like barebox is also handling the loading and
> manipulating of the DTB that will then be used by the Linux kernel when
> barebox boots to it...

That's correct, DistroKit currently does not install any DTB overlays to
the boot partition, because barebox could not do anything with them in
the past. As far as I understand, barebox uses its compiled-in device
tree (which is mostly the upstream kernel device tree), and passes it to
the kernel on boot.

> 5. The 3rd stage bootloader read the "4th stage" and releases the CPU to
> execute it.
> With DistroKit, the config.txt and the GPU bootloader images, and
> barebox.img are on /dev/disk0.0 (available in barebox at /boot/), but the
> DTB is on /dev/disk0.1 (available in /mnt/disk0.1/boot/). There is neither
> a /boot/overlays/ nor a /mnt/disk0.1/boot/overlays/ directory. (Which is
> why I'm confused with my comments on 4 above.)
> 
> A) Can I create an overlays/ directory in order to affect the DTB that gets
> sent to the Linux kernel as described by the RPi docs:
> https://www.raspberrypi.org/documentation/configuration/device-tree.md and
> https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README ?

I don't know if barebox can handle DTBOs in the meantime, there were
some patches posted to the mailing list, but I don't know what exactly
needs to be done here. This seems like a question for the barebox
mailing list...
http://lists.infradead.org/mailman/listinfo/barebox

> B) If so, where should the overlays/ directory live...?

If you put the DTBOs into configs/platform-v7a/rpi-firmware/overlays/,
you can change configs/platform-v7a/rules/image-rpi3.make to pick them
up when you add the files to IMAGE_RPI3_DATA (this works by passing the
variable contents to genimage by replacing @FIRMWARE_RPI3@ in
configs/platform-v7a/config/images/rpi3.config) (and accordingly for
s/rpi3/rpi2/).

 - Roland

> C) Or does barebox handle this in a different way?
> 
> Thanks,
> -Jon

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> ptxdist@pengutronix.de


-- 
Roland Hieber                     | r.hieber@pengutronix.de     |
Pengutronix e.K.                  | https://www.pengutronix.de/ |
Peiner Str. 6-8, 31137 Hildesheim | Phone: +49-5121-206917-5086 |
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