[Intel-gfx] [PATCH 1/2] drm/i915/execlists: Wrap tail pointer after reset tweaking

[Chris Wilson]

On Mon, Mar 27, 2017 at 01:44:00PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > If the request->wa_tail is 0 (because it landed exactly on the end of
> > the ringbuffer), when we reconstruct request->tail following a reset we
> > fill in an illegal value (-8 or 0x001ffff8). As a result, RING_HEAD is
> > never able to catch up with RING_TAIL and the GPU spins endlessly. If
> > the ring contains a couple of breadcrumbs, even our hangcheck is unable
> > to catch the busy-looping as the ACTHD and seqno continually advance.
> 
> Tail is past ring size (on hw) and the ring contents has seqno writes.
> So we will replay the ring contents over and over and seqno advances
> and wraps back to the first breadcrumbs in ring?

Yup. It was most confusing to watch. The execlist_port[] was static,
RING_START was static, yet the seqno kept changing. I felt like I was
hallucinating. That or insomnia.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre