Re: omapdss: Division by zero in kernel

Pali Rohár
2015-08-26T10:16:18Z

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?