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Boot GRUB from USB - potential problem

From: Aleš Nesrsta
Subject: Boot GRUB from USB - potential problem
Date: Sun, 16 Dec 2012 17:54:45 +0100


I found something not really bad but probably also not good, related to
booting of GRUB from USB device.
I expect it is probably (very) low priority thing.
Sorry if it was discussed before, I didn't notice it.

Consider this situation:
- You have ordinary PC which (BIOS) supports boot from USB drive (such
as flash disk etc.)
- You boot GRUB on this PC from some USB device into command line
- You do "insmod ehci" (or load any other USB module - uhci or ohci - it
depends what USB host controller Your PC has)
- immediately after issuing the command above the GRUB freezes

Because loading of GRUB USB driver stops BIOS support of USB device from
which You have booted GRUB, i.e. GRUB has no longer access to any of its
(It cannot be done in any other way because USB host controller cannot
be shared by BIOS and GRUB together.)
(I don't know how it happens on EFI boot but I am afraid it will result
in the same situation.)

In fact, it is logical behavior - but:

Somebody may see it as a bug - maybe it should be mentioned somewhere in
GRUB documentation - ?

This behavior of GRUB prevents using of any additional USB device in
GRUB if GRUB is booted from USB device and the additional device is not
supported by BIOS. Such device could be e.g. USB/serial converter.
 -> I have no idea how to solve it if somebody will need such situation,
e.g. to debug something via serial line or connect some modem/terminal
via serial line.
(Maybe some RAM disk could be used where all GRUB files will be loaded
before real "start" of GRUB? I.e. do boot of GRUB in the little bit
similar way like Linux does? Maybe something like this is currently
possible in GRUB - ? - sorry, I don't know GRUB too deeply...)


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