Re: [PATCH] Don't touch USB controllers with MMIO disabled in quirks

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> But an alternative strategy (which might be very sensible) is to forget 
> about the handoff entirely, and just shut down the bus master flag 
> unconditionally. Just make sure that the eventual driver will reset the 
> controller before it re-enables bus mastering.

Unfortunately, I know quite a few devices (including USB controllers)
that will react badly to the bus master flag being just cleared like
that. By badly, I mean it ranges from simply ignoring it and happily
continuing whatever bus master was being done, to completely screwing up
and crapping all over your memory/bus.

> That would seem to be the simplest possible "handoff". The only danger is 
> that I could imagine that there would be controllers out there that get 
> really confused (ie "I'm not going to play nice any more") if we shut them 
> up that way.

I suspect with the IO/MEM enable test fix we did, it shoul work fine in
practice for all cases. Let's address the "potential" issues if they
happen to show up in real life, which I doubt.

Ben.