On Mon, Sep 30, 2013 at 10:45:03AM -0600, Alex Williamson wrote: > On Mon, 2013-09-30 at 18:09 +0300, Ville Syrjälä wrote: > > On Mon, Sep 30, 2013 at 08:43:51AM -0600, Alex Williamson wrote: > > > On Mon, 2013-09-30 at 17:08 +0300, ville.syrjala@xxxxxxxxxxxxxxx wrote: > > > > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > > > > > > > We have several problems with out VGA handling: > > > > - We try to use the GMCH control VGA disable bit even though it may > > > > be locked > > > > - If we manage to disable VGA throuh GMCH control, we're no longer > > > > able to correctly disable the VGA plane > > > > - Taking part in the VGA arbitration is too expensive for X [1] > > > > > > > > So let's treat the GMCH control VGA disable bit as read-only and leave > > > > it for the BIOS to set, as it was intended. To disable VGA we will use > > > > the VGA misc register, and to disable VGA IO we will disable IO space > > > > completely via the PCI command register. > > > > > > > > But we still need VGA register access during resume (and possibly during > > > > lid event on insane BIOSen) to disable the VGA plane. Also we need to > > > > re-disable VGA memory decode via the VGA misc register on resume. > > > > > > > > Luckily up to gen4, VGA registers can be accessed through MMIO. > > > > Unfortunately from gen5 onwards only the legacy VGA IO port range > > > > works. So on gen5+ we still need IO space to be enabled during those > > > > few special moments when we need to access VGA registers. > > > > > > > > We still want to opt out of VGA arbitration on gen5+, so we have keep > > > > IO space disabled most of the time. And when we do need to poke at VGA > > > > registers, we enable IO space briefly while no one is looking. To > > > > guarantee that no one is looking we will use stop_machine(). > > > > > > What?! Why would we not simply wait for the arbiter lock? > > > > Well, there are the X problems which I really don't want to > > attempt solving. > > > > Also the arbiter looks a lot like deadlock heaven to me. > > > > What if the other guy doesn't release the arbiter lock in a timely > > fashion? It could be some userspace process that's stopped inside > > gdb or something. > > > > What if we're doing the restore thing in intel_lid_notify() > > and we've already locked the modeset locks and are now waiting > > for the arbiter lock, but the other guy who is holding the arbiter > > lock is doing a modeset ioctl at the same time and gets stuck > > waiting for a modeset lock? > > > > I guess we might be able to solve those problems by killing > > the userspace client after a while. But I'd rather just hide > > and go code up something more productive ;) > > So in summary, ignore the infrastructure intended to solve this problem > because it's hard and may have corner cases and instead stop the entire > machine so we can be sure our access is exclusive. If this is the > solution then VGA arbiter has failed us. Thanks, Well, if you want to fix the vga arbiter (assuming it's possible) feel free to send patches. I can even give a vague promise of reviewing them. But I guess we need to fix this ASAP or face the wrath of the angry mob who lost their dri capabilities. The other option is to move the stop_machine infrastructure inside vgaarb and then we can say we're using vgaarb as *deity* intended ;) -- Ville Syrjälä Intel OTC _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
