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From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Thu, 23 Mar 2017 19:49:53 +0100
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Subject: Re: [PATCH 00/13] Add HD44780 Character LCD support
To: Willy TARREAU <wtarreau@haproxy.com>
Cc: Miguel Ojeda Sandonis <miguel.ojeda.sandonis@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ksenija Stanojevic <ksenija.stanojevic@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Hi Willy,

On Thu, Mar 23, 2017 at 6:37 PM, Willy TARREAU <wtarreau@haproxy.com> wrote:
> On Thu, Mar 23, 2017 at 06:15:01PM +0100, Willy TARREAU wrote:
>> So I finally had the time to give it a try by applying your patchset
>> on top of our 4.9 prod kernel. For now I'm getting a panic at boot :
>>
>> BUG: unable to handle kernel NULL pointer dereference at           (null)
>> IP: [<ffffffff813e203d>] __mutex_lock_slowpath+0x6d/0x100
> (...)
>
> It was indeed related to the backlight mutex. It's initialized
> after charlcd_init_display() succeeds, but the backlight is
> touched within this function. I just moved it upwards and that
> solved the issue. Feel free to re-integrate this (or an equivalent)
> into your patch.
>
> With only this fix, everything works fine here without even
> having to touch the configuration, good job!

Thanks for testing!

>
> Thanks!
> Willy
> --
>
> diff --git a/drivers/auxdisplay/charlcd.c b/drivers/auxdisplay/charlcd.c
> index 6aea900..e7f1bfe 100644
> --- a/drivers/auxdisplay/charlcd.c
> +++ b/drivers/auxdisplay/charlcd.c
> @@ -703,6 +703,8 @@ static int charlcd_init(struct charlcd *lcd)
>         struct charlcd_priv *priv = to_priv(lcd);
>         int ret;
>
> +       mutex_init(&priv->bl_tempo_lock);
> +
>         /*
>          * before this line, we must NOT send anything to the display.
>          * Since charlcd_init_display() needs to write data, we have to
> @@ -713,7 +715,6 @@ static int charlcd_init(struct charlcd *lcd)
>                 return ret;
>
>         if (lcd->ops->backlight) {
> -               mutex_init(&priv->bl_tempo_lock);
>                 INIT_DELAYED_WORK(&priv->bl_work, charlcd_bl_off);
>         }

I take it you're using v1 of the patchset?

v2 had this change:

  - Move backlight mutex initialization before call to
    charlcd_init_display(), to prevent locking an uninitialized mutex
    (reported by 0day).

https://lkml.org/lkml/2017/3/10/1151

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds