Re: [PATCH 1/2] drm/i915: Initialize seqno for VECS too

[Ben Widawsky <ben@bwidawsk.net>]

On Mon, Aug 12, 2013 at 04:53:03PM -0700, Ben Widawsky wrote:
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65387
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67198
> Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 10c2aaa..665602f 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1592,6 +1592,8 @@ void intel_ring_init_seqno(struct intel_ring_buffer *ring, u32 seqno)
>  	if (INTEL_INFO(ring->dev)->gen >= 6) {
>  		I915_WRITE(RING_SYNC_0(ring->mmio_base), 0);
>  		I915_WRITE(RING_SYNC_1(ring->mmio_base), 0);
> +		if (HAS_VEBOX(ring->dev))
> +			I915_WRITE(RING_SYNC_2(ring->mmio_base), 0);
>  	}
>  
>  	ring->set_seqno(ring, seqno);

We require n-1 mailboxes for proper semaphore synchronization. All
semaphore synchronization code relies on proper values in these
mailboxes. The fact that we failed to touch the vebox ring by itself was
unlikely to be an issue since the HW should be initializing the values
to 0. However the error framework for testing seqno wrap introduced by
Mika, in addition to the hangcheck via seqno, and
i915_error_first_batchbuffer() combined caused a nice explosion.

The problem is caused by seqno wrap because the wrap condition is not
properly setup. The wrap code attempts to set the sync mailboxes all to
0, and then set the current seqno to one less than 0. In all cases, the
vebox mailbox wasn't properly being initialized. This caused a wrap to
not occur. When hangcheck kicks in with the bogus seqno values, the rest
just doesn't work. It makes me wonder if we shouldn't consider a dumber
version of hangcheck...

How we messed this up:
VECS support was written before the aforementioned other features. Upon
VECS being rebased, these facts were missed.

-- 
Ben Widawsky, Intel Open Source Technology Center