linux-kernel - Re: omapdss: Division by zero in kernel

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Date:	Fri, 21 Aug 2015 11:42:14 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Pali Rohár <pali.rohar@...il.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Jyri Sarha <jsarha@...com>
CC:	<linux-omap@...r.kernel.org>, <linux-fbdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	Sebastian Reichel <sre@...nel.org>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	Tony Lindgren <tony@...mide.com>, Nishanth Menon <nm@...com>
Subject: Re: omapdss: Division by zero in kernel

On 24/07/15 19:03, Pali Rohár wrote:
> Hello,
> 
> when on N900 (real HW or qemu) I run this command
> 
> / # echo 0 > /sys/devices/platform/omapdss/overlay0/enabled && echo 0 > /sys/class/graphics/fb0/size
> 
> then kernel crash with this error message
> 
> / # [   29.904113] Division by zero in kernel.

The problem is that fb console uses the kernel mmapped framebuffer, but
omapfb is not aware of the fb console. So the above commands free the
framebuffer, as omapfb thinks no one is using it, and then fb console
tries to touch the fb.

omapfb tracks mmaps from userspace, and refuses to free a fb it it's
mmapped.

I don't know how to fix it straight away. Maybe there's a way for omapfb
to check if the fbcon uses the fb in question, and if so, refuses to
release/resize the memory.

 Tomi

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