EHCI handoff & AMI BIOS EHCI Handoff support.

[Melki Christian (consultant)]

Hi.

Im having trouble on one machine which has an AMI-BIOS with a function called "EHCI Handoff support". Sometimes the machine hangs in usb controller init.
Technically I think the function is meant to support OS without proper handoff support (windows xp pre sp2 or something). The problem is that I dont know how they do it.
USB Legacy support is meant to be disabled once OS gets EHCI control otherwise all hell will break loose with two drivers managing one EHCI controller.
This function (I think) makes BIOS set OS_OWNED bit and release the control once it thinks someone is trying to manage the controller besides BIOS.
The problem (again, I think) is that GRUB does not do anything if the OS_OWNED flag is set thereby sending no signal at all, relying on BIOS getting it right to stop playing with the controller.
A solution would be in EHCI initialization instead of doing:

if (usblegsup & GRUB_EHCI_BIOS_OWNED)

it should be:

if (usblegsup & (GRUB_EHCI_BIOS_OWNED || GRUB_EHCI_OS_OWNED))

because technically, GRUB is running and nothing really should matter.
We could even do:

if (usblegsup) { ...}

Because all cases are the same. We need to send a signal, no matter the state of the current settings.
The relevant part is to test the settings once the signal has been sent, it should contain GRUB_EHCI_OS_OWNED only.
But I think we cannot ignore to write the register even if it's already set because of the above specified function.

regards,
Christian

[Aleš Nesrsta]

Hi Christian,

yes, I have seen such BIOS feature somewhere some time ago.
I think the best workaround is to disable this feature... :-)
Of course, if you have installed some OS without proper support of EHCI
handoff, you cannot do this - but, from the opposite side, do you need
GRUB in this case?
You are doing something little bit exotic, doesn't? :-)
(OK, it may happen e.g. in the case when you do some emergency/rescue
boot from CD-ROM or USB on machine with some older OS.)


More seriously (if I good understood your e-mail...):

I am not sure that proposed change(s) will help. Did you try it?
BIOS probably will not expect that the OS will "touch"/use the USBLEGSUP
register in this case.
There is also the question, if the USBLEGSUP ownership is really set to
OS at the BIOS usage time when EHCI handoff feature is enabled.

[Melki Christian (consultant)]

Ales,

This is a little bit exotic yes. :)
I have recommended the feature be turned off for machines that have this option in BIOS. Running GRUB (with EHCI module loaded), which is capable of proper handover, this feature should not be turned on. I think regardless of OS GRUB is booting. Since we leave the controller in OS_OWNED when booting, it should be safe for any OS to start using it. GRUB does not try to do handback, even though I'd like such a feature. :) I have changed the code flow a bit and while it seems to help, I cannot guarantee that all BIOS code behaves the same. Im trying to get more information on the code from AMI. But you are right, changing this code without more information is likely to break more things than fixing it. Linux does not care about this case. The handover is quite identical as it stands now.

Regards,
Christian

Post by Aleš Nesrsta
Hi Christian,
yes, I have seen such BIOS feature somewhere some time ago.
I think the best workaround is to disable this feature... :-) Of course, if you
have installed some OS without proper support of EHCI handoff, you cannot
do this - but, from the opposite side, do you need GRUB in this case?
You are doing something little bit exotic, doesn't? :-) (OK, it may happen e.g.
in the case when you do some emergency/rescue boot from CD-ROM or USB
on machine with some older OS.)
I am not sure that proposed change(s) will help. Did you try it?
BIOS probably will not expect that the OS will "touch"/use the USBLEGSUP
register in this case.
There is also the question, if the USBLEGSUP ownership is really set to OS at
the BIOS usage time when EHCI handoff feature is enabled.