[Intel-gfx] [PATCH] drm/i915/skl: Skip remaining dividers when deviation is 0

[Damien Lespiau]

We can't improve a 0 deviation, so when we find such a divider, skip the
remaining ones they won't be better.

This short-circuit the search for 34 of the 373 test frequencies in the
corresponding i-g-t test (tools/skl_compute_wrpll)

Suggested-by: Paulo Zanoni <paulo.r.zanoni at intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau at intel.com>
---
This patch has to be applied after patch 12 and 13 for the recent SKL DPLL
series.

 drivers/gpu/drm/i915/intel_ddi.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
index f6b3ccc..45116d8 100644
--- a/drivers/gpu/drm/i915/intel_ddi.c
+++ b/drivers/gpu/drm/i915/intel_ddi.c
@@ -1122,7 +1122,11 @@ static void skl_wrpll_context_init(struct skl_wrpll_context *ctx)
 #define SKL_DCO_MAX_PDEVIATION	100
 #define SKL_DCO_MAX_NDEVIATION	600
 
-static void skl_wrpll_try_divider(struct skl_wrpll_context *ctx,
+/*
+ * Returns true if we're sure to have found the definitive divider (ie
+ * deviation == 0).
+ */
+static bool skl_wrpll_try_divider(struct skl_wrpll_context *ctx,
 				  uint64_t central_freq,
 				  uint64_t dco_freq,
 				  unsigned int divider)
@@ -1141,6 +1145,10 @@ static void skl_wrpll_try_divider(struct skl_wrpll_context *ctx,
 			ctx->dco_freq = dco_freq;
 			ctx->p = divider;
 		}
+
+		/* we can't improve a 0 deviation */
+		if (deviation == 0)
+			return true;
 	/* negative deviation */
 	} else if (deviation < SKL_DCO_MAX_NDEVIATION &&
 		   deviation < ctx->min_deviation) {
@@ -1150,6 +1158,7 @@ static void skl_wrpll_try_divider(struct skl_wrpll_context *ctx,
 		ctx->p = divider;
 	}
 
+	return false;
 }
 
 static void skl_wrpll_get_multipliers(unsigned int p,
@@ -1311,13 +1320,15 @@ skl_ddi_calculate_wrpll(int clock /* in Hz */,
 				unsigned int p = dividers[d].list[i];
 				uint64_t dco_freq = p * afe_clock;
 
-				skl_wrpll_try_divider(&ctx,
-						      dco_central_freq[dco],
-						      dco_freq,
-						      p);
+				if (skl_wrpll_try_divider(&ctx,
+							  dco_central_freq[dco],
+							  dco_freq,
+							  p))
+					goto skip_remaining_dividers;
 			}
 		}
 
+skip_remaining_dividers:
 		/*
 		 * If a solution is found with an even divider, prefer
 		 * this one.
-- 
2.1.0