[PATCH] drm/bridge: dw-hdmi-qp: Compute ACR CTS for unlisted TMDS rates

[Christian Hewitt]

common_tmds_cts_table[] holds only six TMDS character rates (25.175,
25.2, 27, 54, 74.25 and 148.5 MHz), so dw_hdmi_qp_find_cts() returns 0
for everything else. dw_hdmi_qp_set_cts_n() then clears the CTS override
enable and programs a value of 0, leaving the sink with no CTS to
regenerate the audio clock from.

Any deep colour link falls into this gap: a 10 bpc RK3576 HDMI output
runs at 185625000 Hz (148.5 MHz * 1.25), which is absent from both
tables. N is computed dynamically and comes out correct at 6144, but
AUDPKT_ACR_CONTROL1 reads back as 0.

Give CTS the same dynamic fallback that N already has, using the formula
from the Audio chapter of the HDMI specification, and drop the -ENOENT
returned into an unsigned int for the unlisted sample rates.

Fixes: fd0141d1a8a2a ("drm/bridge: synopsys: Add audio support for dw-hdmi-qp")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Christian Hewitt <christianshewitt@xxxxxxxxx>
---
This was found after testing unrelated patches from DetlevC that rename
the RK audio cards to see the impact in Kodi. RK3588 had audio output,
while RK3576 did not. I'd not used an RK3576 board for a while so tasked
Claude to help triage the problem, and this was the finding. The problem
appears to have been exposed since Kodi reworked plane selection logic
and support for 10bpc planes; earlier Kodi/LibreELEC images were using
8bpc planes thus avoiding the problem.

 drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c | 45 ++++++++++++--------
 1 file changed, 27 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
index 5f4718c3b9db..7cf327de0249 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
@@ -12,6 +12,7 @@
 #include <linux/export.h>
 #include <linux/i2c.h>
 #include <linux/irq.h>
+#include <linux/math64.h>
 #include <linux/minmax.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
@@ -307,8 +308,15 @@ static unsigned int dw_hdmi_qp_find_n(struct dw_hdmi_qp *hdmi, unsigned long pix
 	return dw_hdmi_qp_compute_n(hdmi, pixel_clk, sample_rate);
 }
 
+static unsigned int dw_hdmi_qp_compute_cts(unsigned long pixel_clk,
+					   unsigned long sample_rate,
+					   unsigned int n)
+{
+	return div64_u64((u64)pixel_clk * n, 128ULL * sample_rate);
+}
+
 static unsigned int dw_hdmi_qp_find_cts(struct dw_hdmi_qp *hdmi, unsigned long pixel_clk,
-					unsigned long sample_rate)
+					unsigned long sample_rate, unsigned int n)
 {
 	const struct dw_hdmi_audio_tmds_cts *tmds_cts = NULL;
 	int i;
@@ -320,23 +328,24 @@ static unsigned int dw_hdmi_qp_find_cts(struct dw_hdmi_qp *hdmi, unsigned long p
 		}
 	}
 
-	if (!tmds_cts)
-		return 0;
-
-	switch (sample_rate) {
-	case 32000:
-		return tmds_cts->cts_32k;
-	case 44100:
-	case 88200:
-	case 176400:
-		return tmds_cts->cts_44k1;
-	case 48000:
-	case 96000:
-	case 192000:
-		return tmds_cts->cts_48k;
-	default:
-		return -ENOENT;
+	if (tmds_cts) {
+		switch (sample_rate) {
+		case 32000:
+			return tmds_cts->cts_32k;
+		case 44100:
+		case 88200:
+		case 176400:
+			return tmds_cts->cts_44k1;
+		case 48000:
+		case 96000:
+		case 192000:
+			return tmds_cts->cts_48k;
+		}
 	}
+
+	dev_dbg(hdmi->dev, "Rate %lu missing; compute CTS dynamically\n", pixel_clk);
+
+	return dw_hdmi_qp_compute_cts(pixel_clk, sample_rate, n);
 }
 
 static void dw_hdmi_qp_set_audio_interface(struct dw_hdmi_qp *hdmi,
@@ -471,7 +480,7 @@ static void dw_hdmi_qp_set_sample_rate(struct dw_hdmi_qp *hdmi, unsigned long lo
 	unsigned int n, cts;
 
 	n = dw_hdmi_qp_find_n(hdmi, tmds_char_rate, sample_rate);
-	cts = dw_hdmi_qp_find_cts(hdmi, tmds_char_rate, sample_rate);
+	cts = dw_hdmi_qp_find_cts(hdmi, tmds_char_rate, sample_rate, n);
 
 	dw_hdmi_qp_set_cts_n(hdmi, cts, n);
 }
-- 
2.43.0