[Intel-gfx] [PATCH] drm/i915/gt: Be defensive in the face of false CS events

[Chris Wilson]

Quoting Chris Wilson (2020-07-10 13:07:17)
> If the HW throws a curve ball and reports either en event before it is
> possible, or just a completely impossible event, we have to grin and
> bear it. The first few events, we will likely not notice as we would be
> expecting some event, but as soon as we stop expecting an event and yet
> they still keep coming, then we enter into undefined state terrority.
> In which case, bail out, stop processing the events, and reset the
> engine and our set of queued requests to recover.
> 
> The sporadic hangs and warnings will continue to plague CI, but at least
> system stability should not be compromised.
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2045
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_lrc.c | 8 ++++++--
>  drivers/gpu/drm/i915/i915_gem.h     | 2 ++
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index fbcfeaed6441..f22cf8ed47ac 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -2567,6 +2567,7 @@ static void process_csb(struct intel_engine_cs *engine)
>         tail = READ_ONCE(*execlists->csb_write);
>         if (unlikely(head == tail))
>                 return;
> +       execlists->csb_head = tail;
>  
>         /*
>          * Hopefully paired with a wmb() in HW!
> @@ -2613,6 +2614,9 @@ static void process_csb(struct intel_engine_cs *engine)
>                 if (promote) {
>                         struct i915_request * const *old = execlists->active;
>  
> +                       if (GEM_WARN_ON_ONCE(!*execlists->pending))

I wonder if we should just default GEM_WARN_ON to be GEM_WARN_ON_ONCE,
CI reboots after a warning so the spam is unhelpful.
-Chris