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SubjectRe: [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);
}

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