[Intel-gfx] [PATCH 2/2] drm/i915: Recover all available ringbuffer space following reset

[Chris Wilson]

On Fri, Oct 23, 2015 at 02:07:35PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Having flushed all requests from all queues, we know that all
> > ringbuffers must now be empty. However, since we do not reclaim
> > all space when retiring the request (to prevent HEADs colliding
> > with rapid ringbuffer wraparound) the amount of available space
> > on each ringbuffer upon reset is less than when we start. Do one
> > more pass over all the ringbuffers to reset the available space
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Arun Siluvery <arun.siluvery at linux.intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> > Cc: Dave Gordon <david.s.gordon at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c         | 14 ++++++++++++++
> >  drivers/gpu/drm/i915/intel_lrc.c        |  1 +
> >  drivers/gpu/drm/i915/intel_ringbuffer.c | 13 ++++++++++---
> >  drivers/gpu/drm/i915/intel_ringbuffer.h |  2 ++
> >  4 files changed, 27 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 41263cd4170c..3a42c350fec9 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -2738,6 +2738,8 @@ static void i915_gem_reset_ring_status(struct drm_i915_private *dev_priv,
> >  static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
> >  					struct intel_engine_cs *ring)
> >  {
> > +	struct intel_ringbuffer *buffer;
> > +
> >  	while (!list_empty(&ring->active_list)) {
> >  		struct drm_i915_gem_object *obj;
> >  
> > @@ -2783,6 +2785,18 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
> >  
> >  		i915_gem_request_retire(request);
> >  	}
> > +
> > +	/* Having flushed all requests from all queues, we know that all
> > +	 * ringbuffers must now be empty. However, since we do not reclaim
> > +	 * all space when retiring the request (to prevent HEADs colliding
> > +	 * with rapid ringbuffer wraparound) the amount of available space
> > +	 * upon reset is less than when we start. Do one more pass over
> > +	 * all the ringbuffers to reset last_retired_head.
> > +	 */
> > +	list_for_each_entry(buffer, &ring->buffers, link) {
> > +		buffer->last_retired_head = buffer->tail;
> > +		intel_ring_update_space(buffer);
> > +	}
> 
> This is all in vain as the i915_gem_context_reset() ->
> intel_lr_context_reset still sets head and tail to zero.
> 
> So your last_retired_head will still dangle in a pre-reset
> world when the rest of the ringbuf items will be set to post
> reset world.

It's only setting that so that we computed the full ring space as
available and then we set last_retired_head back to -1. So what's
dangling?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre