Re: [PATCH] usb: xhci: Fix Set TR Dequeue Pointer cycle state for quirky xHCs

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 07/17/2014 10:50 PM, Julius Werner wrote:
>> Hmm. Wouldn't it be safer to have a quirk for this, and only enable
>> the workaround if the Asmedia controller is detected? This code is so
>> complicated that it is difficult to see whether this could have a
>> harmful effect on controllers without the bug.
> 
> Sorry for making it complicated, but it kinda has been like that
> before already... I don't think the new patch adds much confusion on
> its own. I would really advise against making this a quirk: checking
> for it in every case essentially doesn't cost us anything (just one
> more register compare that is negligible against all the
> cache-coherent memory accesses of walking the ring), and hardcoding a
> quirk would mean that we have to identify and add every host
> controller that does this individually.
> 
> I still haven't seen anything in the XHCI spec that actually forbids
> this behavior, so it might be a perfectly legal interpretation and who
> knows how many host controllers chose to do it that way. Until we find
> them all, we would have some really bad and hard to track down bugs on
> those controllers (we really just got lucky this time that Maciej was
> able to catch it in a bisect). I think it's better to program the
> driver defensively and able to deal with unexpected situations in
> general.
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It's starting to get a bit too complicated.
xhci find_new_dequeue_state() now has four places where it can toggle the cycle bit
in addition to the cycle toggle in find_trb_seg().

In the end we really just want to do it max once.

I'll see if I can simplify the whole cycle bit code somehow.

If not, then we need to take this or revert the original patch

-Mathias