On 15/09/2020 16:29, Dennis Lee Bieber wrote:
> On Tue, 15 Sep 2020 11:11:19 +0100, The Natural Philosopher
>  declaimed the following:
>
>> On 15/09/2020 11:07, Andy Burns wrote:
>
>>> Presume cmdline.txt is the equivalent on a pi, can you append
>>> init=/bin/bash  there?
>>
>> Well that is precisely what I am trying to find out!
>> Busybox installs must do something like that
>
>  Based on
> https://www.raspberrypi.org/documentation/configuration/cmdline-txt.md
> cmdline.txt is not the file to be modified for this purpose. Nor does
> config.txt seem to cover it. OTOH:
>
> pi@rpi3bplus-1:~$ cat /boot/cmdline.txt
> console=tty1 root=/dev/mmcblk0p7 rootfstype=ext4 elevator=deadline
> fsck.repair=yes rootwait quiet splash plymouth.ignore-serial-consoles
>
> pi@rpi3bplus-1:~$ cat /proc/cmdline
> coherent_pool=1M 8250.nr_uarts=1 snd_bcm2835.enable_compat_alsa=0
> snd_bcm2835.enable_hdmi=1 snd_bcm2835.enable_headphones=1
> bcm2708_fb.fbwidth=1920 bcm2708_fb.fbheight=1280 bcm2708_fb.fbswap=1
> vc_mem.mem_base=0x3ec00000 vc_mem.mem_size=0x40000000  console=tty1
> root=/dev/mmcblk0p7 rootfstype=ext4 elevator=deadline fsck.repair=yes
> rootwait quiet splash plymouth.ignore-serial-consoles
> pi@rpi3bplus-1:~$
>
> shows the contents of cmdline.txt concatenated to the rest of the actual
> boot parameters used, so it may work by just adding init=...
>
> https://www.debian.org/doc/manuals/debian-reference/ch03.en.html
> """
> The init program is executed as the first program with PID=1 to perform the
> main boot process of starting many programs. The default file path for the
> init program is "/sbin/init" but it can be changed by the kernel boot
> parameter as "init=/path/to/init_program".
> """
> pi@rpi3bplus-1:~$ ls -l /sbin/init
> lrwxrwxrwx 1 root root 20 May 11 03:06 /sbin/init -> /lib/systemd/systemd
> pi@rpi3bplus-1:~$
>
>  So... creating a file system where /sbin/init is a link to your desired
> init process might also be sufficient.
>
>
Yes. I had more or less got there. As  said three options now present
themselves- a device driver  under stripped down linux, a bare metal
approach and 'byte' the networking bullet, or use something other than a
Pi. Maybe an X86 would have better floating point...


Maybe an almost bare metal PC chassis is the way to go...


--
"The most difficult subjects can be explained to the most slow witted
man if he has not formed any idea of them already; but the simplest
thing cannot be made clear to the most intelligent man if he is firmly
persuaded that he knows already, without a shadow of doubt, what is laid
before him."

    - Leo Tolstoy

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