There's an entire pile of issues in here:
- Use the main RING_HEAD register, not ACTHD. ACTHD points at the gtt
offset of the batch buffer when a batch is executed. Semaphores are
always emitted to the main ring, so we always want to look at that.
- Mask the obtained HEAD pointer with the actual ring size, which is
much smaller. Together with the above issue this resulted us in
trying to dereference a pointer way outside of the ring mmio
mapping. The resulting invalid access in interrupt context
(hangcheck is executed from timers) lead to a full blown kernel
panic. The fbcon panic handler then tried to frob our driver harder,
resulting in a full machine hang at least on my snb here where I've
stumbled over this.
- Handle ring wrapping correctly and be a bit more explicit about how
many dwords we're scanning. We probably should also scan more than
just 4 ...
- Space out some of teh computations for readability.
This reduces hard-hangs on my snb here. Mika and QA both say that it
doesn't completel remove them, but at least for me it's a clear
improvement in stability.
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
References: https://bugs.freedesktop.org/show_bug.cgi?id=74100
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
88fe429db0