[PATCH] drm/bridge: dw-hdmi-qp: compute audio CTS from N when not in TMDS table

[Simon Wright]

dw_hdmi_qp_find_cts() returns 0 for any TMDS character rate not
present in common_tmds_cts_table[] (which terminates at 148.5 MHz).
In that case dw_hdmi_qp_set_sample_rate() calls dw_hdmi_qp_set_cts_n()
with cts == 0, which leaves AUDPKT_ACR_CTS_OVR_EN clear and falls
back to the controller's internal CTS auto-measurement.
On at least the RK3576 hdptx integration, strict HDMI sinks mute
audio in this configuration.  The behaviour reproduces at any rate
not present in the table -- it is not specific to HDMI 2.x:
1920x1080 at 60 with 10-bit deep colour (185.625 MHz, HDMI 1.4) is
affected, as is 3840x2160 at 60 8-bit (594 MHz, HDMI 2.0).  Supplying
explicit CTS via the standard set_cts_n() override path resolves
the mute.
The driver already has the symmetric machinery for the N-table-miss
case: dw_hdmi_qp_compute_n() falls back to a dynamic search using
dw_hdmi_qp_audio_math_diff() ((pixel_clk * n) / (128 * freq)) when
no table entry matches.  The CTS path lacks the equivalent fallback.
Compute CTS inline in dw_hdmi_qp_set_sample_rate() from N per the
HDMI spec (CTS = TMDS * N / (128 * Fs)) when find_cts() returns 0.
The same formula appears in the legacy DesignWare HDMI driver
(drivers/gpu/drm/bridge/synopsys/dw-hdmi.c, hdmi_set_clk_regenerator())
under its AHB / GP audio paths.
Tested on R76S (RK3576) on Armbian-edge mainline 7.0.1 with Cristian
Ciocaltea's hdptx-clk-fixes v1 series applied:
   TMDS         Mode             In table?  Audio with fix
   148.5 MHz    1080p60  8-bit   yes        plays (regression check)
   185.625 MHz  1080p60 10-bit   no         plays
   297 MHz      3840p30  8-bit   no         plays
   594 MHz      3840p60  8-bit   no         plays
Sinks tested: LG G3 OLED, LG C4 OLED, TCL 75P7K QLED, Kogan
KALED43XU9210STA (Changhong-OEM 4K LED).  Without the fix, the LG
G3 mutes audio -- this is the originally reported bug
(linux-rockchip 070633).  The LG C4 was verified during this cycle
to mute at 185.625 MHz (1080p60 10-bit) with the unpatched module.
The Kogan TV plays audio with or without the fix; it is a
permissive HDMI 2.0 implementation that does not strictly cross-
check ACR timing.  The TCL 75P7K was tested with the fix loaded
only.
Audio plays at every supported sample rate (32 / 44.1 / 48 / 96 /
192 kHz) verified on the TCL at the 185.625 MHz out-of-table TMDS
rate -- the fallback's CTS = (TMDS * N) / (128 * Fs) computation is
independent of sample rate, as expected.
The LG C4's CTA-861 SVDs do not advertise 3840x2160 at 60 over TMDS
(it is signalled FRL-only on that model), so that one row is N/A
on the C4.  The same audio-path code is exercised at 297 MHz on
that sink and behaves identically to the G3.
Reported-by: Simon Wright <simon at symple.nz>
Closes: 
https://lists.infradead.org/pipermail/linux-rockchip/2026-May/070633.html
Suggested-by: Cristian Ciocaltea <cristian.ciocaltea at collabora.com>
Tested-by: Simon Wright <simon at symple.nz>
Signed-off-by: Simon Wright <simon at symple.nz>
---
  drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c | 15 +++++++++++++++
  1 file changed, 15 insertions(+)
diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c 
b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
index 0dbb1274360..25ac8d3cfc8 100644
---a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
+++b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
@@-461,6 +461,21 @@static void dw_hdmi_qp_set_sample_rate(struct 
dw_hdmi_qp *hdmi, unsigned long lo
   n = dw_hdmi_qp_find_n(hdmi, tmds_char_rate, sample_rate);
   cts = dw_hdmi_qp_find_cts(hdmi, tmds_char_rate, sample_rate);
+ /*
+  * dw_hdmi_qp_find_cts() returns 0 for any TMDS rate not in
+  * common_tmds_cts_table[].  Falling through to the controller's
+  * auto-measure path mutes audio on strict HDMI sinks at out-of-
+  * table rates (e.g. 185.625 MHz, 297 MHz, 594 MHz).  Compute CTS
+  * from N per HDMI spec instead, so the standard override path
+  * supplies it on the wire.
+  */
+ if (!cts && n) {
+   u64 computed = (u64)tmds_char_rate * n;
+
+   do_div(computed, 128ULL * sample_rate);
+   cts = (unsigned int)computed;
+ }
+
   dw_hdmi_qp_set_cts_n(hdmi, cts, n);
  }
-- 
2.53.0.windows.3