Re: [9fans] pi4 USB '/boot/kfs' error

[Richard Miller <9fans@hamnavoe.com>]

I said:
> If you want to boot the pi4 completely from a USB drive, with no
> SD card present (I've never tried this), you'll need an alternative
> place for a fs(3) configuration (maybe in the built-in bootdir in
> the kernel?), or the boot code will need tweaking to get the partfs(8)
> partitions re-mounted at the right time.

I've done a bit of experimenting, and did manage to boot a pi4 with
only a usb drive and no SD card. It turns out that you can use the
first part of the mbr sector on the usb drive to store the fs(3)
configuration, ie fsconfig=/dev/sdU0.0/data in cmdline.txt -- as
long as you don't have so many partitions that the config text
overlaps the DOS partition information in the mbr (at offset 0x1be),
the pi4 bootrom code will happily boot from the usb drive. I wouldn't
suggest that PC users try this - a PC bios might be more strict about
what's in the mbr.

Booting without an SD card seems to have some rough edges. You get
irritating error messages when startup procedures include /dev/sdM0
in the list of disks to look at, and the reboot(8) command doesn't
work because the usb disk interface probably needs some soft-reset
to be done before the pi bootrom can initialise it again.  Even if
you are primarily using a usb drive, booting from SD (or netbooting?)
might still be preferable.


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