Message-ID: <20150115062436.GA6615@saruman>
Date:	Thu, 15 Jan 2015 00:24:36 -0600
From:	Felipe Balbi <balbi@...com>
To:	Paul Zimmerman <Paul.Zimmerman@...opsys.com>
CC:	"balbi@...com" <balbi@...com>,
	Alan Stern <stern@...land.harvard.edu>,
	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

Hi,

On Wed, Jan 14, 2015 at 11:04:27PM +0000, Paul Zimmerman 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.
> > > >
> > > > Alright, I'll apply this patch but for 3.20 with a stable tag as I have
> > > > already sent my last pull request to Greg. Unless someone has a really
> > > > big complaint about doing things as such.
> > >
> > > It should go to 3.19-rc shouldn't it? It's a fix, and Robert's platform
> > > is broken without it, IIUC.
> > 
> > It can also be categorized as "has-never-worked-before" before the code
> > has been like this forever. Since we don't really have a git bisect
> > result pointing to a commit that went in v3.19 merge window, I'm not
> > sure how I can convince myself that this absolutely needs to be in
> > v3.19.
> > 
> > At a minimum, I need a proper bisection with a proper commit being
> > blamed (even if it's a commit from months ago). From my point of view,
> > debugging of this "regression" has not been finalized and we're just
> > "assuming" it's caused by GSNPSID because moving that inside the
> > spin_lock seems to fix the problem.
> 
> On further investigation, I was wrong about "this code has been
> basically the same since at least November 2013". Prior to commit
> db8178c33db "usb: dwc2: Update common interrupt handler to call gadget
> interrupt handler" from November 2014, the gadget interrupt handler
> did not read from the GSNPSID register.

right, but the common IRQ always did. So unless Robert's SoC has always
been used only for peripheral, then I agree with you that behavior did,
in fact, change.

> So likely the bug in Robert's hardware has been there all along, and
> that commit just caused it to manifest itself.

Robert, out of curiosity, which SoC are you using ? Is it UP or SMP ?

I guess we need a mention on commit log that at least SoC XYZ is known
to break unless the register access is done with locks held.

-- 
balbi

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