From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pali =?utf-8?B?Um9ow6Fy?= Date: Wed, 26 Aug 2015 10:16:12 +0000 Subject: Re: omapdss: Division by zero in kernel Message-Id: <20150826101612.GI27333@pali> List-Id: References: <201507241803.42891@pali> <55D6E466.5060706@ti.com> <20150821084807.GD27391@pali> <55D6ECB5.6010708@ti.com> In-Reply-To: <55D6ECB5.6010708@ti.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Tomi Valkeinen Cc: Jean-Christophe Plagniol-Villard , Jyri Sarha , linux-omap@vger.kernel.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Pavel Machek , Sebastian Reichel , Aaro Koskinen , Tony Lindgren , Nishanth Menon On Friday 21 August 2015 12:17:41 Tomi Valkeinen wrote: > > > On 21/08/15 11:48, Pali Rohár wrote: > > On Friday 21 August 2015 11:42:14 Tomi Valkeinen wrote: > >> > >> > >> 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. > >> > > > > Hi! Thanks for explaining. > > > >> 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. > >> > > > > What about refusing those calls from fb console? So fb console will not > > know about this problem and omapfb will just ignore drawn functions? > > Hmm, I'm not sure I understand what you mean... omapfb is not drawing > anything, fbcon is doing the drawing independently to the fb. And the fb > suddenly disappears without fbcon realizing that. > > >> 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 > >> > > > > Maemo userspace (on Nokia N900) uses above commands to initialize > > graphic and Xserver. So it would be nice if disabling framebuffer would > > work even if fbcon.ko is loaded (or compiled directly into zImage). > > Ok. And N900 has fbcon enabled? I wonder how it survives... > Depends on compiled kernel. Original stock Nokia kernel 2.6.28 has it disabled, but when I recompiled it with fbcon (either static linked into zImage or external fbcon.ko) it works and I do not see any problem. So I think it survives... > fbcon can be unbound from userspace with something like: > > echo 0 > /sys/class/vtconsole/vtcon1/bind > > After that I think the memory can be freed. > > But obviously the kernel should not crash here, no question about that. > > Tomi > Maybe just adding that test for zero to prevent division by zero? -- Pali Rohár pali.rohar@gmail.com