Machine check in ohci_hub_status_data (powerpc)

Hi,

Here's something I just observed yesterday. At first I was blaming my
suspend changes for powerpc since I can't make it happen without them,
but I've gone back and forth over them and can't find the problem, so
while I'm still not sure they aren't at fault I wanted some input from
the USB side.

What happens is that apparently for whatever (legitimate or not) reason
right before suspend the rh_timer in hcd.c gets scheduled but never runs
before suspend. Then during resume, when IRQs are enabled, it gets to
run, but the device hasn't been resumed yet.

So what happens then is that in rh_timer usb_hcd_poll_rh_status is
called and calls (indirectly) ohci_hub_status_data which errors out in
line 431:

430:        /* init status */
431:        if (roothub_status (ohci) & (RH_HS_LPSC | RH_HS_OCIC))
432:                buf [0] = changed = 1;

for the obvious reason that roothub_status() can't actually access the
registers yet. I suspect this can also happen on x86 but that just
returns 0xffffffff on the read and not much more happens. Or powermac
really is special because we have a hook to turn off the USB asic.

Now, since I mentioned a timer timing clearly has an influence :) But
the odd thing is that it happens fairly consistently when I suspend from
the console right after boot, while it never happens when suspending
from within X (or even from the console when I'm logged in in X). Maybe
suspend is so much slower from X that the timer fires before the machine
is suspended or something.

It seems to me that the timer shouldn't be scheduled when we go into
suspend, but I'll leave the proper fix up to somebody else. Unless of
course my suspend changes do indeed have a bug somewhere (I sure hope
not), unfortunately I haven't been able to trigger this on an unpatched
kernel. But I haven't tried very hard either.

johannes

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