Re: [PATCH v3 3/3] input: gpio-keys: Use hrtimer for software debounce, if possible

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Tony Lindgren <tony@atomide.com>
Cc: Paul Cercueil <paul@crapouillou.net>,
	od@zcrc.me, linux-input@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/3] input: gpio-keys: Use hrtimer for software debounce, if possible
Date: Tue, 6 Apr 2021 21:34:17 -0700	[thread overview]
Message-ID: <YG02SZA22Jd71B2t@google.com> (raw)
In-Reply-To: <YGwds97OjXIdZF3L@atomide.com>

On Tue, Apr 06, 2021 at 11:37:07AM +0300, Tony Lindgren wrote:
> Hi,
> 
> * Dmitry Torokhov <dmitry.torokhov@gmail.com> [700101 02:00]:
> > On Sun, Mar 07, 2021 at 10:22:40PM +0000, Paul Cercueil wrote:
> > > We want to be able to report the input event as soon as the debounce
> > > delay elapsed. However, the current code does not really ensure that,
> > > as it uses the jiffies-based schedule_delayed_work() API. With a small
> > > enough HZ value (HZ <= 100), this results in some input events being
> > > lost, when a key is quickly pressed then released (on a human's time
> > > scale).
> > > 
> > > Switching to hrtimers fixes this issue, and will work even on extremely
> > > low HZ values (tested at HZ=24). This is however only possible if
> > > reading the GPIO is possible without sleeping. If this condition is not
> > > met, the previous approach of using a jiffies-based timer is taken.
> > > 
> > > Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> > 
> > Applied with minor edits to make more use of debounce_use_hrtimer flag.
> 
> While testing Linux next I noticed that this patch causes a null pointer
> dereference at least when unbinding a gpio-keys instance, see below.

Ugh, my "minor edits" did screw things up ;( as I mixed up release and
debounce timers. I'll fix it up.

> 
> Regards,
> 
> Tony
> 
> 8< -----------------
> Unable to handle kernel NULL pointer dereference at virtual address 0000000c
> ...
> PC is at hrtimer_active+0xc/0x98
> LR is at hrtimer_try_to_cancel+0x24/0x140
> ...
> [<c01c43b8>] (hrtimer_active) from [<c01c50f4>] (hrtimer_try_to_cancel+0x24/0x140)
> [<c01c50f4>] (hrtimer_try_to_cancel) from [<c01c5224>] (hrtimer_cancel+0x14/0x4c)
> [<c01c5224>] (hrtimer_cancel) from [<bf1cae24>] (gpio_keys_attr_store_helper+0x1b8/0x1d8 [gpio_keys])
> [<bf1cae24>] (gpio_keys_attr_store_helper [gpio_keys]) from [<bf1cae80>] (gpio_keys_store_disabled_keys+0x18/0x24 [gpio_keys])
> [<bf1cae80>] (gpio_keys_store_disabled_keys [gpio_keys]) from [<c038ec7c>] (kernfs_fop_write_iter+0x10c/0x1cc)
> [<c038ec7c>] (kernfs_fop_write_iter) from [<c02df858>] (vfs_write+0x2ac/0x404)
> [<c02df858>] (vfs_write) from [<c02dfaf4>] (ksys_write+0x64/0xdc)
> [<c02dfaf4>] (ksys_write) from [<c0100080>] (ret_fast_syscall+0x0/0x58)
> 

-- 
Dmitry