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

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Date:	Thu, 15 Jan 2015 11:23:50 +0100
From:	Robert Baldyga <r.baldyga@...sung.com>
To:	balbi@...com, Paul Zimmerman <Paul.Zimmerman@...opsys.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	"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 01/15/2015 07:24 AM, Felipe Balbi 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.

As far as I know, DWC2 at this platform was always used as peripheral.
Exynos SoC's has EHCI USB controllers, so in 99% of cases there is
simply no need to use DWC2 as host.

> 
>> 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.
> 

I'm using Exynos4412 (Odroid U3). Revision number of my DWC2 is 2.81a.

I will update commit message and send patch v3.

Thanks,
Robert Baldyga
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