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drm/bridge: dw-hdmi-qp: use drm_hdmi_acr_get_n_cts() helper for audio N/CTS

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dw_hdmi_qp_set_sample_rate() open-coded its own N and CTS lookup
tables and search/compute helpers, but it lacked the out-of-table
CTS fallback needed by strict HDMI sinks at TMDS rates not in the
table.  The original LG G3 OLED audio mute (linux-rockchip 070633)
was caused by dw_hdmi_qp_find_cts() returning 0 at 185.625 MHz,
leaving AUDPKT_ACR_CTS_OVR_EN clear and falling back to the
controller's internal CTS auto-measurement, which produces incorrect
timing on the wire at out-of-table rates.

The shared drm_hdmi_acr_get_n_cts() helper in
drivers/gpu/drm/display/drm_hdmi_helper.c already implements the
correct behaviour: it has the HDMI 1.4b spec N/CTS tables, and for
TMDS rates not in the table it computes CTS = (TMDS * N) / (128 * Fs)
inline (the canonical HDMI spec formula).  It is already used by
drivers/gpu/drm/msm/hdmi/hdmi_audio.c.

Convert dw_hdmi_qp_set_sample_rate() to call the helper.  This
removes ~200 lines of open-coded tables and search functions
(dw_hdmi_qp_find_n, _compute_n, _find_cts, _audio_math_diff,
_match_tmds_n_table, common_tmds_n_table[], common_tmds_cts_table[])
and fixes the strict-sink audio mute as a side effect of using the
helper's complete N+CTS path.

Tested on R76S (RK3576) running Linux 7.0.1, against the LG G3 OLED
(the sink that originally reported the mute in linux-rockchip
070633) at four TMDS rates spanning HDMI 1.4 and HDMI 2.0:

  TMDS         Mode              Audio with v3
  148.5 MHz    1080p60   8-bit   plays
  185.625 MHz  1080p60  10-bit   plays
  297 MHz      1080p100  8-bit   plays
  594 MHz      3840p60   8-bit   plays

Without this change, the LG G3 mutes audio at 185.625, 297, and
594 MHz (every rate outside dw-hdmi-qp's open-coded CTS table,
which contained only 148.5 MHz and below).  With this change,
drm_hdmi_acr_get_n_cts() supplies the correct CTS at every rate --
table-canonical at 148.5 / 297 / 594 MHz, and computed via the
HDMI 1.4b formula at 185.625 MHz.  The 148.5 MHz row is a
regression check confirming the in-table path is unchanged.

The Kogan KALED43XU9210STA (a permissive HDMI 2.0 sink that plays
audio at all rates with or without this change) was used as a
no-regression control: audio plays at 594 MHz with v3 loaded.

The open-coded N table in dw-hdmi-qp included optimised N values
for some TMDS rates that are not in the helper's table (e.g.
various non-CEA rates between 28-162 MHz).  For those rates the
helper's fallback returns N = 128 * Fs / 1000, which is the same
value dw_hdmi_qp_compute_n() returned when no table optimisation
was needed; no audio regression has been observed at the rates
tested above.

Reported-by: Simon Wright <simon@symple.nz>
Closes: https://lore.kernel.org/linux-rockchip/ME3P282MB21960D9D68BFF520316BDFCEA83E2@ME3P282MB2196.AUSP282.PROD.OUTLOOK.COM/
Suggested-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Suggested-by: Jonas Karlman <jonas@kwiboo.se>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Simon Wright <simon@symple.nz>
Tested-by: Igor Paunovic <royalnet026@gmail.com> # RK3588 Orange Pi 5 Plus, both HDMI controllers
Tested-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Tested-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/86fcf349-0a7a-4618-9001-612371b0f71b@symple.nz
This commit is contained in:
Simon Wright 2026-05-21 19:36:47 +12:00 • committed by Heiko Stuebner
commit a9f09b5ea0
1 changed files with 1 additions and 208 deletions
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