Re: [PATCH v2 1/4] drm/i915: Don't allow ring tail to reach the same cacheline as head

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 27, 2012 at 08:34:55PM +0200, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> From BSpec:
> "If the Ring Buffer Head Pointer and the Tail Pointer are on the same
> cacheline, the Head Pointer must not be greater than the Tail
> Pointer."
> 
> The easiest way to enforce this is to reduce the reported ring space.
> 
> References:
> Gen2 BSpec "1. Programming Environment" / "1.4.4.6 Ring Buffer Use"
> Gen3 BSpec "vol1c Memory Interface Functions" / "2.3.4.5 Ring Buffer Use"
> Gen4+ BSpec "vol1c Memory Interface and Command Stream" / "5.3.4.5 Ring Buffer Use"
> 
> v2: Include the exact BSpec references in the description
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

Another small bikeshed for this one: Less magic numbers with

#define RING_FREE_SPACE 64

-Daniel
> ---
>  drivers/gpu/drm/i915/i915_dma.c         |    2 +-
>  drivers/gpu/drm/i915/intel_ringbuffer.c |    4 ++--
>  drivers/gpu/drm/i915/intel_ringbuffer.h |    2 +-
>  3 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
> index 86b93b2..79089aa 100644
> --- a/drivers/gpu/drm/i915/i915_dma.c
> +++ b/drivers/gpu/drm/i915/i915_dma.c
> @@ -141,7 +141,7 @@ void i915_kernel_lost_context(struct drm_device * dev)
>  
>  	ring->head = I915_READ_HEAD(ring) & HEAD_ADDR;
>  	ring->tail = I915_READ_TAIL(ring) & TAIL_ADDR;
> -	ring->space = ring->head - (ring->tail + 8);
> +	ring->space = ring->head - (ring->tail + 64);
>  	if (ring->space < 0)
>  		ring->space += ring->size;
>  
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index b0a83ce..cf11f8a 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -45,7 +45,7 @@ struct pipe_control {
>  
>  static inline int ring_space(struct intel_ring_buffer *ring)
>  {
> -	int space = (ring->head & HEAD_ADDR) - (ring->tail + 8);
> +	int space = (ring->head & HEAD_ADDR) - (ring->tail + 64);
>  	if (space < 0)
>  		space += ring->size;
>  	return space;
> @@ -1260,7 +1260,7 @@ static int intel_ring_wait_request(struct intel_ring_buffer *ring, int n)
>  		if (request->tail == -1)
>  			continue;
>  
> -		space = request->tail - (ring->tail + 8);
> +		space = request->tail - (ring->tail + 64);
>  		if (space < 0)
>  			space += ring->size;
>  		if (space >= n) {
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 5af65b8..2f6d9b1 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -191,7 +191,7 @@ void intel_cleanup_ring_buffer(struct intel_ring_buffer *ring);
>  int __must_check intel_wait_ring_buffer(struct intel_ring_buffer *ring, int n);
>  static inline int intel_wait_ring_idle(struct intel_ring_buffer *ring)
>  {
> -	return intel_wait_ring_buffer(ring, ring->size - 8);
> +	return intel_wait_ring_buffer(ring, ring->size - 64);
>  }
>  
>  int __must_check intel_ring_begin(struct intel_ring_buffer *ring, int n);
> -- 
> 1.7.8.6
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch