drm/radeon/audio: improve ACR calculation

In order to have any realistic chance of calculating proper
ACR values, we need to be able to calculate both N and CTS,
not just CTS. We still aim for the ideal N as specified in
the HDMI spec though.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=69675

Signed-off-by: Pierre Ossman <pierre@ossman.eu>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/r600_hdmi.c

@@ -24,6 +24,7 @@
  * Authors: Christian König
  */
 #include <linux/hdmi.h>
+#include <linux/gcd.h>
 #include <drm/drmP.h>
 #include <drm/radeon_drm.h>
 #include "radeon.h"
@@ -67,25 +68,47 @@ static const struct radeon_hdmi_acr r600_hdmi_predefined_acr[] = {
     {  74250,  4096,  74250,  6272,  82500,  6144,  74250 }, /*  74.25       MHz */
     { 148352,  4096, 148352,  5733, 150670,  6144, 148352 }, /* 148.50/1.001 MHz */
     { 148500,  4096, 148500,  6272, 165000,  6144, 148500 }, /* 148.50       MHz */
-    {      0,  4096,      0,  6272,      0,  6144,      0 }  /* Other */
 };
 
+
 /*
- * calculate CTS value if it's not found in the table
+ * calculate CTS and N values if they are not found in the table
  */
-static void r600_hdmi_calc_cts(uint32_t clock, int *CTS, int N, int freq)
+static void r600_hdmi_calc_cts(uint32_t clock, int *CTS, int *N, int freq)
 {
-	u64 n;
-	u32 d;
-
-	if (*CTS == 0) {
-		n = (u64)clock * (u64)N * 1000ULL;
-		d = 128 * freq;
-		do_div(n, d);
-		*CTS = n;
-	}
-	DRM_DEBUG("Using ACR timing N=%d CTS=%d for frequency %d\n",
-		  N, *CTS, freq);
+	int n, cts;
+	unsigned long div, mul;
+
+	/* Safe, but overly large values */
+	n = 128 * freq;
+	cts = clock * 1000;
+
+	/* Smallest valid fraction */
+	div = gcd(n, cts);
+
+	n /= div;
+	cts /= div;
+
+	/*
+	 * The optimal N is 128*freq/1000. Calculate the closest larger
+	 * value that doesn't truncate any bits.
+	 */
+	mul = ((128*freq/1000) + (n-1))/n;
+
+	n *= mul;
+	cts *= mul;
+
+	/* Check that we are in spec (not always possible) */
+	if (n < (128*freq/1500))
+		printk(KERN_WARNING "Calculated ACR N value is too small. You may experience audio problems.\n");
+	if (n > (128*freq/300))
+		printk(KERN_WARNING "Calculated ACR N value is too large. You may experience audio problems.\n");
+
+	*N = n;
+	*CTS = cts;
+
+	DRM_DEBUG("Calculated ACR timing N=%d CTS=%d for frequency %d\n",
+		  *N, *CTS, freq);
 }
 
 struct radeon_hdmi_acr r600_hdmi_acr(uint32_t clock)
@@ -93,15 +116,16 @@ struct radeon_hdmi_acr r600_hdmi_acr(uint32_t clock)
 	struct radeon_hdmi_acr res;
 	u8 i;
 
-	for (i = 0; r600_hdmi_predefined_acr[i].clock != clock &&
-	     r600_hdmi_predefined_acr[i].clock != 0; i++)
-		;
-	res = r600_hdmi_predefined_acr[i];
+	/* Precalculated values for common clocks */
+	for (i = 0; i < ARRAY_SIZE(r600_hdmi_predefined_acr); i++) {
+		if (r600_hdmi_predefined_acr[i].clock == clock)
+			return r600_hdmi_predefined_acr[i];
+	}
 
-	/* In case some CTS are missing */
-	r600_hdmi_calc_cts(clock, &res.cts_32khz, res.n_32khz, 32000);
-	r600_hdmi_calc_cts(clock, &res.cts_44_1khz, res.n_44_1khz, 44100);
-	r600_hdmi_calc_cts(clock, &res.cts_48khz, res.n_48khz, 48000);
+	/* And odd clocks get manually calculated */
+	r600_hdmi_calc_cts(clock, &res.cts_32khz, &res.n_32khz, 32000);
+	r600_hdmi_calc_cts(clock, &res.cts_44_1khz, &res.n_44_1khz, 44100);
+	r600_hdmi_calc_cts(clock, &res.cts_48khz, &res.n_48khz, 48000);
 
 	return res;
 }