[PATCH net v5 2/5] dpll: zl3073x: reject inexact frequencies for N-divided outputs

@@ -987,6 +987,7 @@ zl3073x_dpll_output_pin_frequency_set(const struct dpll_pin *dpll_pin,
 	const struct zl3073x_synth *synth;
 	u32 new_div, synth_freq;
 	struct zl3073x_out out;
+	u64 n_period, rem;
 	u8 out_id;
 
 	guard(mutex)(&zldpll->lock);

@@ -1014,16 +1015,11 @@ zl3073x_dpll_output_pin_frequency_set(const struct dpll_pin *dpll_pin,
 	}
 
 	if (zl3073x_dpll_is_p_pin(pin)) {
-		/* We are going to change output frequency for P-pin but
-		 * if the requested frequency is less than current N-pin
-		 * frequency then indicate a failure as we are not able
-		 * to compute N-pin divisor to keep its frequency unchanged.
-		 *
-		 * Update divisor for N-pin to keep N-pin frequency.
+		/* Keep the N-pin frequency: the new P-pin frequency has to be
+		 * a multiple of it, at least twice as high.
 		 */
-		out.esync_n_period = (out.esync_n_period * out.div) / new_div;
-		if (!out.esync_n_period)
-			return -EINVAL;
+		n_period = mul_u32_u32(out.esync_n_period, out.div);
+		n_period = div64_u64_rem(n_period, new_div, &rem);
 
 		/* Update the output divisor */
 		out.div = new_div;

@@ -1031,17 +1027,31 @@ zl3073x_dpll_output_pin_frequency_set(const struct dpll_pin *dpll_pin,
 		/* For 50/50 duty cycle the divisor is equal to width */
 		out.width = out.div;
 	} else {
-		/* We are going to change frequency of N-pin but if
-		 * the requested freq is greater or equal than freq of P-pin
-		 * in the output pair we cannot compute divisor for the N-pin.
-		 * In this case indicate a failure.
-		 *
-		 * Update divisor for N-pin
+		/* The N-pin frequency has to divide the P-pin frequency and
+		 * be at most half of it.
 		 */
-		out.esync_n_period = div64_u64(synth_freq, frequency * out.div);
-		if (!out.esync_n_period)
-			return -EINVAL;
+		n_period = frequency * out.div;
+		n_period = div64_u64_rem(synth_freq, n_period, &rem);
+	}
+	if (n_period < 2) {
+		NL_SET_ERR_MSG_FMT(extack,
+				   "OUT%uN freq must be at most half of OUT%uP freq",
+				   out_id, out_id);
+		return -EINVAL;
+	}
+	if (n_period > U32_MAX) {
+		NL_SET_ERR_MSG_FMT(extack,
+				   "OUT%uN freq is too low for OUT%uP freq",
+				   out_id, out_id);
+		return -EINVAL;
+	}
+	if (rem != 0) {
+		NL_SET_ERR_MSG_FMT(extack,
+				   "OUT%uN freq must divide OUT%uP freq",
+				   out_id, out_id);
+		return -EINVAL;
 	}
+	out.esync_n_period = n_period;
 
 	/* For 50/50 duty cycle the divisor is equal to width */
 	out.esync_n_width = out.esync_n_period;