[bug #36740] FTDI USB Serial Device not recognized and "insmod ehci" freezes grub

[Ales Nesrsta]

Follow-up Comment #19, bug #36740 (project grub):

Hi once again,

I made some investigation about boot freezing today and I found this:
GRUB DOES NOT FREEZE ON BOOT when module for companion USB host controller
(UHCI in my case) is loaded together with EHCI module.

So, I think that the freezing is not real bug (e.g. some memory corruption
etc.) but "feature" which can happen in some case of BIOS properties/settings,
USB HW configuration and "partial" USB handling by GRUB.
Maybe is good to mention this case somewhere in GRUB documentation, e.g. by
something like following text:
   If PC has EHCI controller with companion controller(s) UHCI/OHCI, than
should be loaded both modules ehci + uhci/ohci (or none of them). In another
case some USB related things can behave unexpectedly (because, in such case,
some parts of the common USB HW are handled by GRUB and some by BIOS).

Conclusion:
I think the subject of this bug is solved by patch
https://savannah.gnu.org/support/download.php?file_id=27349

The second "bug" I found is probably not a bug - and it can be avoided by
this:
1. Don't load any GRUB USB driver :-)
2. Load drivers for all types of USB controllers on PC, not EHCI only.
(There could be one potential problem - GRUB doesn't have driver for xHCI...)

BR,
Ales

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