drm/i915/skl: Replace the HDMI DPLL divider computation algorithm

The HW validation team came back from further testing with a slightly
changed constraint on the deviation between the DCO frequency and the
central frequency. Instead of +-4%, it's now +1%/-6%.

Unfortunately, the previous algorithm didn't quite cope with these new
constraints, the reason being that it wasn't thorough enough looking at
the possible divider candidates.

The new algorithm looks at all dividers, which is definitely a hammer
approach (we could reduce further the set of dividers to good ones as a
follow up, at the cost of a bit more complicated code). But, at least,
we can now satisfy the +1%/+6% rule for all the "Well known" HDMI
frequencies of my test set (373 entries).

On that subject, the new code is quite extensively tested in
intel-gpu-tools (tools/skl_compute_wrpll).

v2: Fix cycling between central frequencies and dividers (Paulo)
    Properly choose the minimal deviation between postive and negative
    candidates (Paulo).

    On the 373 test frequencies, v2 computes better dividers than v1 (ie
    more even dividers and lower deviation on average):

    v1: average deviation: 206.52
    v2: average deviation: 194.47

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ddi.c

@@ -1104,6 +1104,103 @@ hsw_ddi_pll_select(struct intel_crtc *intel_crtc,
 	return true;
 }
 
+struct skl_wrpll_context {
+	uint64_t min_deviation;		/* current minimal deviation */
+	uint64_t central_freq;		/* chosen central freq */
+	uint64_t dco_freq;		/* chosen dco freq */
+	unsigned int p;			/* chosen divider */
+};
+
+static void skl_wrpll_context_init(struct skl_wrpll_context *ctx)
+{
+	memset(ctx, 0, sizeof(*ctx));
+
+	ctx->min_deviation = U64_MAX;
+}
+
+/* DCO freq must be within +1%/-6%  of the DCO central freq */
+#define SKL_DCO_MAX_PDEVIATION	100
+#define SKL_DCO_MAX_NDEVIATION	600
+
+static void skl_wrpll_try_divider(struct skl_wrpll_context *ctx,
+				  uint64_t central_freq,
+				  uint64_t dco_freq,
+				  unsigned int divider)
+{
+	uint64_t deviation;
+
+	deviation = div64_u64(10000 * abs_diff(dco_freq, central_freq),
+			      central_freq);
+
+	/* positive deviation */
+	if (dco_freq >= central_freq) {
+		if (deviation < SKL_DCO_MAX_PDEVIATION &&
+		    deviation < ctx->min_deviation) {
+			ctx->min_deviation = deviation;
+			ctx->central_freq = central_freq;
+			ctx->dco_freq = dco_freq;
+			ctx->p = divider;
+		}
+	/* negative deviation */
+	} else if (deviation < SKL_DCO_MAX_NDEVIATION &&
+		   deviation < ctx->min_deviation) {
+		ctx->min_deviation = deviation;
+		ctx->central_freq = central_freq;
+		ctx->dco_freq = dco_freq;
+		ctx->p = divider;
+	}
+
+}
+
+static void skl_wrpll_get_multipliers(unsigned int p,
+				      unsigned int *p0 /* out */,
+				      unsigned int *p1 /* out */,
+				      unsigned int *p2 /* out */)
+{
+	/* even dividers */
+	if (p % 2 == 0) {
+		unsigned int half = p / 2;
+
+		if (half == 1 || half == 2 || half == 3 || half == 5) {
+			*p0 = 2;
+			*p1 = 1;
+			*p2 = half;
+		} else if (half % 2 == 0) {
+			*p0 = 2;
+			*p1 = half / 2;
+			*p2 = 2;
+		} else if (half % 3 == 0) {
+			*p0 = 3;
+			*p1 = half / 3;
+			*p2 = 2;
+		} else if (half % 7 == 0) {
+			*p0 = 7;
+			*p1 = half / 7;
+			*p2 = 2;
+		}
+	} else if (p == 3 || p == 9) {  /* 3, 5, 7, 9, 15, 21, 35 */
+		*p0 = 3;
+		*p1 = 1;
+		*p2 = p / 3;
+	} else if (p == 5 || p == 7) {
+		*p0 = p;
+		*p1 = 1;
+		*p2 = 1;
+	} else if (p == 15) {
+		*p0 = 3;
+		*p1 = 1;
+		*p2 = 5;
+	} else if (p == 21) {
+		*p0 = 7;
+		*p1 = 1;
+		*p2 = 3;
+	} else if (p == 35) {
+		*p0 = 7;
+		*p1 = 1;
+		*p2 = 5;
+	}
+}
+
 struct skl_wrpll_params {
 	uint32_t        dco_fraction;
 	uint32_t        dco_integer;
@@ -1189,90 +1286,56 @@ skl_ddi_calculate_wrpll(int clock /* in Hz */,
 	uint64_t dco_central_freq[3] = {8400000000ULL,
 					9000000000ULL,
 					9600000000ULL};
-	uint32_t min_dco_deviation = 400;
-	uint32_t min_dco_index = 3;
-	uint32_t P0[4] = {1, 2, 3, 7};
-	uint32_t P2[4] = {1, 2, 3, 5};
-	bool found = false;
-	uint32_t candidate_p = 0;
-	uint32_t candidate_p0[3] = {0}, candidate_p1[3] = {0};
-	uint32_t candidate_p2[3] = {0};
-	uint32_t dco_central_freq_deviation[3];
-	uint32_t i, P1, k, dco_count;
-	bool retry_with_odd = false;
-
-	/* Determine P0, P1 or P2 */
-	for (dco_count = 0; dco_count < 3; dco_count++) {
-		found = false;
-		candidate_p =
-			div64_u64(dco_central_freq[dco_count], afe_clock);
-		if (retry_with_odd == false)
-			candidate_p = (candidate_p % 2 == 0 ?
-				candidate_p : candidate_p + 1);
-
-		for (P1 = 1; P1 < candidate_p; P1++) {
-			for (i = 0; i < 4; i++) {
-				if (!(P0[i] != 1 || P1 == 1))
-					continue;
-
-				for (k = 0; k < 4; k++) {
-					if (P1 != 1 && P2[k] != 2)
-						continue;
-
-					if (candidate_p == P0[i] * P1 * P2[k]) {
-						/* Found possible P0, P1, P2 */
-						found = true;
-						candidate_p0[dco_count] = P0[i];
-						candidate_p1[dco_count] = P1;
-						candidate_p2[dco_count] = P2[k];
-						goto found;
-					}
-
-				}
-			}
-		}
-
-found:
-		if (found) {
-			dco_central_freq_deviation[dco_count] =
-				div64_u64(10000 *
-					  abs_diff(candidate_p * afe_clock,
-						   dco_central_freq[dco_count]),
-					  dco_central_freq[dco_count]);
-
-			if (dco_central_freq_deviation[dco_count] <
-				min_dco_deviation) {
-				min_dco_deviation =
-					dco_central_freq_deviation[dco_count];
-				min_dco_index = dco_count;
+	static const int even_dividers[] = {  4,  6,  8, 10, 12, 14, 16, 18, 20,
+					     24, 28, 30, 32, 36, 40, 42, 44,
+					     48, 52, 54, 56, 60, 64, 66, 68,
+					     70, 72, 76, 78, 80, 84, 88, 90,
+					     92, 96, 98 };
+	static const int odd_dividers[] = { 3, 5, 7, 9, 15, 21, 35 };
+	static const struct {
+		const int *list;
+		int n_dividers;
+	} dividers[] = {
+		{ even_dividers, ARRAY_SIZE(even_dividers) },
+		{ odd_dividers, ARRAY_SIZE(odd_dividers) },
+	};
+	struct skl_wrpll_context ctx;
+	unsigned int dco, d, i;
+	unsigned int p0, p1, p2;
+
+	skl_wrpll_context_init(&ctx);
+
+	for (d = 0; d < ARRAY_SIZE(dividers); d++) {
+		for (dco = 0; dco < ARRAY_SIZE(dco_central_freq); dco++) {
+			for (i = 0; i < dividers[d].n_dividers; i++) {
+				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 (min_dco_index > 2 && dco_count == 2) {
-                        /* oh well, we tried... */
-                        if (retry_with_odd)
-                                break;
-
-			retry_with_odd = true;
-			dco_count = 0;
-		}
 	}
 
-	if (WARN(min_dco_index > 2,
-		 "No valid parameters found for pixel clock: %dHz\n", clock))
+	if (!ctx.p) {
+		DRM_DEBUG_DRIVER("No valid divider found for %dHz\n", clock);
 		return false;
+	}
 
-	skl_wrpll_params_populate(wrpll_params,
-				  afe_clock,
-				  dco_central_freq[min_dco_index],
-				  candidate_p0[min_dco_index],
-				  candidate_p1[min_dco_index],
-				  candidate_p2[min_dco_index]);
+	/*
+	 * gcc incorrectly analyses that these can be used without being
+	 * initialized. To be fair, it's hard to guess.
+	 */
+	p0 = p1 = p2 = 0;
+	skl_wrpll_get_multipliers(ctx.p, &p0, &p1, &p2);
+	skl_wrpll_params_populate(wrpll_params, afe_clock, ctx.central_freq,
+				  p0, p1, p2);
 
 	return true;
 }
 
-
 static bool
 skl_ddi_pll_select(struct intel_crtc *intel_crtc,
 		   struct intel_crtc_state *crtc_state,