[Intel-gfx] [CI 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Michal Wajdeczko]

On Thu, May 04, 2017 at 02:08:44PM +0100, Chris Wilson wrote:
> Exploit the power-of-two ring size to compute the space across the
> wraparound using a mask rather than a if. Convert to unsigned integers
> so the operation is well defined.
> 
> References: https://bugs.freedesktop.org/show_bug.cgi?id=99671
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> Reviewed-by: Mika Kuoppala <mika.kuoppala at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 23 +++++++++++----------
>  drivers/gpu/drm/i915/intel_ringbuffer.h | 36 ++++++++++++++++++++-------------
>  2 files changed, 34 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 3ce1c87dec46..e7ef04cc071b 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -39,12 +39,16 @@
>   */
>  #define LEGACY_REQUEST_SIZE 200
>  
> -static int __intel_ring_space(int head, int tail, int size)
> +static unsigned int __intel_ring_space(unsigned int head,
> +				       unsigned int tail,
> +				       unsigned int size)
>  {
> -	int space = head - tail;
> -	if (space <= 0)
> -		space += size;
> -	return space - I915_RING_FREE_SPACE;
> +	/*
> +	 * "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."
> +	 */
> +	return (head - tail - CACHELINE_BYTES) & (size - 1);

Btw, as you exploit power-of-two ring size here, maybe it is worth to repeat

	GEM_BUG_ON(!is_power_of_2(size));

to emphase this assumption in the code (not only in the commit message)?

-Michal