Re: [PATCH] drm/i915: Empty the ring before disabling

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Oct 27, 2017 at 12:56:39AM +0100, Chris Wilson wrote:
> An interesting snippet from Sandybridge's prm:
> 
> "Although a Ring Buffer can be enabled in the non-empty state, it must
> not be disabled unless it is empty. Attempting to disable a Ring Buffer
> in the non-empty state is UNDEFINED."

However elsewhere it still says:
"Writing the Head Offset while the RB is enabled is UNDEFINED"
and
"It can be enabled or disabled regardless of whether there
 are valid instructions pending."

And gen2/3 bspec says this:
"Ring buffers can be disabled with valid instructions pending.
When disabled, the ring buffer will be removed from command 
stream arbitration at the next arbitration point (i.e.,
instruction boundary)."

> 
> Let's avoid the undefined behaviour as we disable the rings prior to
> reset and resume.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 4 +++-
>  drivers/gpu/drm/i915/intel_uncore.c     | 4 +++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 05e01446b00b..3f2073a9d37a 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -480,10 +480,12 @@ static bool stop_ring(struct intel_engine_cs *engine)
>  		}
>  	}
>  
> -	I915_WRITE_CTL(engine, 0);
>  	I915_WRITE_HEAD(engine, 0);
>  	I915_WRITE_TAIL(engine, 0);
>  
> +	/* The ring must be empty before it is disabled */
> +	I915_WRITE_CTL(engine, 0);
> +
>  	return (I915_READ_HEAD(engine) & HEAD_ADDR) == 0;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index 20e3c65c0999..ac688ee6fe4e 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -1387,10 +1387,12 @@ static void gen3_stop_engine(struct intel_engine_cs *engine)
>  		DRM_DEBUG_DRIVER("%s: timed out on STOP_RING\n",
>  				 engine->name);
>  
> -	I915_WRITE_FW(RING_CTL(base), 0);
>  	I915_WRITE_FW(RING_HEAD(base), 0);
>  	I915_WRITE_FW(RING_TAIL(base), 0);
>  
> +	/* The ring must be empty before it is disabled */
> +	I915_WRITE_FW(RING_CTL(base), 0);
> +
>  	/* Check acts as a post */
>  	if (I915_READ_FW(RING_HEAD(base)) != 0)
>  		DRM_DEBUG_DRIVER("%s: ring head not parked\n",
> -- 
> 2.15.0.rc2
> 
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-- 
Ville Syrjälä
Intel OTC
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