linux-kernel - Re: [Letux-kernel] [PATCH 5/5] input: twl6040-vibra: remove mutex

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Date:	Wed, 20 Apr 2016 11:15:48 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	"H. Nikolaus Schaller" <hns@...delico.com>
Cc:	Fabio Estevam <fabio.estevam@...escale.com>,
	linux-input@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Peter Ujfalusi <peter.ujfalusi@...com>,
	kernel@...a-handheld.com,
	Discussions about the Letux Kernel 
	<letux-kernel@...nphoenux.org>
Subject: Re: [Letux-kernel] [PATCH 5/5] input: twl6040-vibra: remove mutex

On Wed, Apr 20, 2016 at 08:10:28PM +0200, H. Nikolaus Schaller wrote:
> Hi,
> 
> > Am 20.04.2016 um 19:49 schrieb Dmitry Torokhov <dmitry.torokhov@...il.com>:
> > 
> > On Wed, Apr 20, 2016 at 11:22:53AM +0200, H. Nikolaus Schaller wrote:
> >> 
> >>> Am 19.04.2016 um 10:08 schrieb H. Nikolaus Schaller <hns@...delico.com>:
> >>> 
> >>> 
> >>>> Am 19.04.2016 um 10:01 schrieb Dmitry Torokhov <dmitry.torokhov@...il.com>:
> >>>> 
> >>>> On Tue, Apr 19, 2016 at 09:49:01AM +0200, H. Nikolaus Schaller wrote:
> >>>>> 
> >>>>>> Am 18.04.2016 um 23:20 schrieb Dmitry Torokhov <dmitry.torokhov@...il.com>:
> >>>>>> 
> >>>>>> On Mon, Apr 18, 2016 at 09:55:41PM +0200, H. Nikolaus Schaller wrote:
> >>>>>>> The mutex does not seem to be needed.
> >>>>>> 
> >>>>>> twl6040_vibra_suspend() and vibra_play_work() may run concurrently, no?
> >>>>> 
> >>>>> Hm. I don't know about the rule that would give an answer to this question...
> >>>> 
> >>>> Sorry, that was actually a statement, not really a question.
> >>> 
> >>> Indeed. In doubt about the answer we should take measures for the worst case.
> >>> 
> >>>> It is
> >>>> possible (although very unlikely) that userspace posts play request and
> >>>> workqueue will not run until after suspend callback.
> >>>> 
> >>>> Thinking about it some more I wonder if we better do what
> >>>> twl6040_vibra_close() does and cancel the work before shutting off the
> >>>> device, so that there is no chance of work executing after suspend
> >>>> callback and reenabling the device. This way we can indeed remove the
> >>>> mutex.
> >>> 
> >>> Ok, I am fine with this.
> >>> 
> >>> Will post an update ASAP.
> >> 
> >> While doing testing I did run again into another locking related issue which I
> >> had not yet tried to address with my patch set. It is a:
> >> 
> >> 	BUG: scheduling while atomic
> >> 
> >> report which sometimes ends in a kernel panic.
> >> 
> >> I have attached such a log. vibra.py is here:
> >> 
> >> 	http://git.goldelico.com/?p=gta04-kernel.git;a=blob;f=Letux/root/vibra.py;hb=refs/heads/letux-4.6-rc4
> >> 
> >> Basically it does an ioctl(EVIOCSFF) which triggers vibra_play.
> >> 
> >> Maybe, can you decipher from the log what the reason could be?
> >> 
> >> I only conjecture that it happens when vibra_play tries to read the regmap
> >> of the twl6040 to get twl6040_get_vibralr_status (which has no pendant
> >> in the twl4030 driver).
> >> 
> >> Do we have to configure the twl6040 regmap differently?
> > 
> > Right, vibra_play is called with interrupts disabled (that's why we have
> > workqueue to enable/disable regulators, etc, when we start or stop
> > vibration), so twl6040_get_vibralr_status() should not sleep, but
> > apparently it does.
> 
> Yes, regmap using i2c communication may sleep.
> 
> > Maybe the check for audio configuration should be
> > moved into vibra_play_work().
> 
> Hm. It is there to disable while in audio routing mode, but
> a workqueue can't report error values back to the scheduling thread...

Nothing checks result of ->play() anyways, so that -EBUSY was completly
useless.

> 
> So we can either silently make vibra not work (or just report
> in the kernel log) if "Vibra is configured for audio".

We might want to ratelimit that message.

> 
> Or we need to get a different mechanism to know the vibra status.
> 
> Hm.
> 
> Research shows that it regmap was introduced long ago but between 3.11 and
> 3.12 a private cache for these control registers was removed.
> 
> http://lxr.free-electrons.com/source/drivers/mfd/twl6040.c?v=3.11#L66
> http://lxr.free-electrons.com/source/drivers/mfd/twl6040.c?v=3.11#L85
> http://lxr.free-electrons.com/source/drivers/mfd/twl6040.c?v=3.11#L462
> 
> This had been non-blocking and was probably there exactly to protect
> against this locking issue!
> 
> After removing the private cache the code must rely on twl6040_get_vibralr_status
> not fetching from the chip.
> 
> Ah, this correlates to that I see this issue only once and then everything
> works.
> 
> This means we have to fetch the current vibra control registers once
> outside of vibra_play(). Probably during probing by a single call to
> twl6040_get_vibralr_status() and ignoring the result.
> 
> After it has been fetched once (to know any status from the last reboot)
> the regmap should track all changes arriving through the sound subsystem
> (audio vibra enable) and the call to twl6040_get_vibralr_status in interrupt
> context should no longer block.
> 
> Does this sound reasonable?

Yes, as long as you document this so that it does not get removed by
mistake later.

Thanks.

-- 
Dmitry