kern_yield vs ukbd_yield

[Andriy Gapon]

on 17/12/2011 19:06 Hans Petter Selasky said the following:
> If the problem is only in UKBD driver, I don't think this is a big problem to 
> solve. The reason for the auto-magic locking, is that I've sometimes seen 
> callers in non-polling mode call into UKBD without Giant locked. And this 
> causes a panic when starting USB transfers, because the code expects Giant to 
> be locked.

Hmm, do you have an example of such a panic? I couldn't find how this could be
possible in my examination of non-polling entry points into syscons and kbd drives.

> Requirement: I would say that to use UKBD polling mode, the scheduler must be 
> stopped. Else you should not use polling mode at all. I think that mountroot 
> is getting characters the wrong way in this regard.

I think that this is a too strong / unnecessary requirement.  I think that now
(after my recent syscons commits) ukbd should work correctly and optimally in
this context.

I have written a small article about my analysis of locking in syscons,
underlying kbd drivers and ukbd in particular:
http://wiki.freebsd.org/AvgSyscons
I could have misunderstood or missed things, so I will appreciate a review and
discussion of my observations.  Thank you!

-- 
Andriy Gapon