linux-kernel - RE: [PATCH v2] usb: dwc2: call dwc2_is_controller

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Date:	Wed, 14 Jan 2015 22:28:54 +0000
From:	Paul Zimmerman <Paul.Zimmerman@...opsys.com>
To:	"balbi@...com" <balbi@...com>,
	Alan Stern <stern@...land.harvard.edu>
CC:	Robert Baldyga <r.baldyga@...sung.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dinguyen@...nsource.altera.com" <dinguyen@...nsource.altera.com>,
	"yousaf.kaukab@...el.com" <yousaf.kaukab@...el.com>,
	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>
Subject: RE: [PATCH v2] usb: dwc2: call dwc2_is_controller_alive() under
 spinlock

> From: Felipe Balbi [mailto:balbi@...com]
> Sent: Wednesday, January 14, 2015 1:46 PM
> 
> On Wed, Jan 14, 2015 at 04:41:23PM -0500, Alan Stern wrote:
> > > > > This is really, really odd. Register accesses are atomic, so the lock
> > > > > isn't really doing anything. Besides, you're calling
> > > > > dwc2_is_controller_alive() from within the IRQ handler, so IRQs are
> > > > > already disabled.
> > > >
> > > > Spinlocks sometimes do more than you think.  For instance, here the
> > > > lock prevents the register access from happening while some other CPU
> > > > is holding the lock.  If a silicon quirk causes the register access to
> > > > interfere with other activities, this could be important.
> > >
> > > readl() (which is used by dwc2_is_controller_alive()) adds a memory
> > > barrier to the register accesses, that should force all register
> > > accesses the be correctly ordered.
> >
> > Memory barriers will order accesses that are all made on the same CPU
> > with respect to each other.  They do not order these accesses against
> > accesses made from another CPU -- that's why we have spinlocks.  :-)
> 
> a fair point :-) The register is still read-only, so that shouldn't
> matter either :-)
> 
> > >  I fail to see how a silicon quirk
> > > could cause this and if, indeed, it does, I'd be more comfortable with a
> > > proper STARS tickect number from synopsys :-s
> >
> > Maybe accessing this register somehow resets something else.  I don't
> > know.  It seems unlikely, but at least it explains how adding a
> > spinlock could fix the problem.
> 
> I would really need Paul (or someone at Synopsys) to confirm this
> somehow. Maybe it has something to do with how the register is
> implemented, dunno.
> 
> Paul, do you have any idea what could cause this ? Could the HW into
> some weird state if we read GSNPSID at random locations or when data is
> being transferred, or anything like that ?

Only thing I can think of is that there is some silicon bug in Robert's
platform. But I am not aware of any STARs that mention accesses to the
GSNPSID register as being problematic.

Funny thing is, this code has been basically the same since at least
November 2013. So I think some other recent change must have modified
the timing of the register accesses, or something like that. But that's
just handwaving, really.

-- 
Paul

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