linux-kernel - RE: rtc ds3232 call trace in kernel

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Date:	Tue, 19 Apr 2016 02:54:01 +0000
From:	Qianyu Gong <qianyu.gong@....com>
To:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
CC:	"akinobu.mita@...il.com" <akinobu.mita@...il.com>,
	Mingkai Hu <mingkai.hu@....com>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: rtc ds3232 call trace in kernel


> -----Original Message-----
> From: Alexandre Belloni [mailto:alexandre.belloni@...e-electrons.com]
> Sent: Tuesday, April 19, 2016 12:22 AM
> To: Qianyu Gong <qianyu.gong@....com>
> Cc: akinobu.mita@...il.com; Mingkai Hu <mingkai.hu@....com>; rtc-
> linux@...glegroups.com; linux-kernel@...r.kernel.org
> Subject: Re: rtc ds3232 call trace in kernel
> 
> On 18/04/2016 at 06:15:40 +0000, Qianyu Gong wrote :
> > Hi Akinobu,
> >
> > I got an rtc call trace when booting 4.6 kernel on our board and I
> > found it was caused by this patch:
> >
> > commit fc1dcb0b39dbb10d3290f2fcd6e154670f699166
> > Author: Akinobu Mita <akinobu.mita@...il.com>
> > Date:   Mon Mar 7 00:27:53 2016 +0900
> >
> >     rtc: ds3232: use rtc->ops_lock to protect alarm operations
> >
> >     ds3232->mutex is used to protect for alarm operations which
> >     need to access status and control registers.
> >
> >     But we can use rtc->ops_lock instead.  rtc->ops_lock is held when most
> >     of rtc_class_ops methods are called, so we only need to explicitly
> >     acquire it from irq handler in order to protect form concurrent
> >     accesses.
> >
> >     Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> > Signed-off-by: Alexandre Belloni <alexandre.belloni@...e-electrons.com
> >
> > The problem is that rtc->ops_lock would be accessed in ds3232_irq()
> > without being initialized as rtc_device_register() is called too late.
> >
> > As I'm not familiar with rtc things, could I just revert the patch or
> > you already have a solution to this problem? Thanks in advance.
> >
> >
> 
> Well, many RTC drivers actually suffer from that. I'm planning to change the
> infrastructure to handle this case better.
> Meanwhile, this is worth fixing.
> 
> 
> --
> Alexandre Belloni, Free Electrons
> Embedded Linux, Kernel and Android engineering http://free-electrons.com

Thanks. I followed Akinobu's suggest and put devm_rtc_device_register() just
before registering the irq handler. The call trace disappeared. Looks like it's 
working well.


Regards,
Qianyu