linux-kernel - Re: [PATCH 00/13] Add HD44780 Character LCD support

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Date:   Thu, 23 Mar 2017 18:37:18 +0100
From:   Willy TARREAU <wtarreau@...roxy.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Miguel Ojeda Sandonis <miguel.ojeda.sandonis@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ksenija Stanojevic <ksenija.stanojevic@...il.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/13] Add HD44780 Character LCD support

Hi again,

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!
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);
 	}