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... 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
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