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@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
