Re: [PATCH v3] drm/bridge: dw-hdmi-qp: use drm_hdmi_acr_get_n_cts() helper for audio N/CTS

[Christian Hewitt]

> On 21 May 2026, at 11:36 am, Simon Wright <simon@xxxxxxxxx> wrote:
> 
> 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@xxxxxxxxx>
> Closes: https://lore.kernel.org/linux-rockchip/ME3P282MB21960D9D68BFF520316BDFCEA83E2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Suggested-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>
> Suggested-by: Jonas Karlman <jonas@xxxxxxxxx>
> Tested-by: Simon Wright <simon@xxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-7
> Signed-off-by: Simon Wright <simon@xxxxxxxxx>
> Reviewed-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>
> Tested-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>

This resolved no HDMI audio on a Radxa Rock4D (RK3576) board using
Linux 7.2.1 while connected to an LG 64B7V (via Marantz AVR), so:

Tested-by: Christian Hewitt <christianshewitt@xxxxxxxxx>

> ---
> Changes in v3:
>  - Switch to drm_hdmi_acr_get_n_cts() helper as suggested by Jonas
>    Karlman, instead of adding the cts-from-N fallback inline.
>    Removes ~200 lines of open-coded N/CTS tables and search code,
>    and brings dw-hdmi-qp in line with msm/hdmi.
>  - Re-tested on the LG G3 OLED (the sink that originally reported
>    the mute) at all four TMDS rates (148.5 / 185.625 / 297 / 594
>    MHz); audio plays at every rate.  v3 has not been re-tested on
>    the LG C4 OLED or TCL 75P7K QLED that v2 included; those sinks
>    are expected to behave the same as G3 since the helper returns
>    spec-canonical HDMI 1.4b values.
>  - Also addresses a do_div() divisor-type style issue flagged by
>    sashiko's automated review of v2 (the offending line is removed
>    by the helper switch).
>  - Add Suggested-by: Jonas Karlman.
>  - Rebased onto current drm-misc-next.
> 
> Changes in v2:
>  - Resend after v1 was mailer-mangled and unapplyable (reported by
>    Luca Ceresoli).
>  - Body split into shorter paragraphs for readability per Luca's
>    review.
>  - Original LG G3 report (linux-rockchip 070633) moved from inline
>    prose to a Closes: trailer paired with Reported-by:, per
>    Documentation/process/submitting-patches.rst.
>  - Add Assisted-by: trailer per
>    Documentation/process/coding-assistants.rst.
> 
> v2: https://lore.kernel.org/linux-rockchip/00a34a82-213b-4b03-801c-3c10b163d643@xxxxxxxxx/
> v1: https://lore.kernel.org/linux-rockchip/92aa4191-2a2a-41e3-badb-c0a5b1fbb957@xxxxxxxxx/
> 
> drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c | 209 +------------------
> 1 file changed, 1 insertion(+), 208 deletions(-)
> 
> 
> base-commit: a48bbcc7ac739e93562d6148c6fa504c2e9f22f8
> 
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
> index 0dbb1274360..4e745e2c97b 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
> @@ -42,88 +42,6 @@
> 
> #define SCRAMB_POLL_DELAY_MS 3000
> 
> -/*
> - * Unless otherwise noted, entries in this table are 100% optimization.
> - * Values can be obtained from dw_hdmi_qp_compute_n() but that function is
> - * slow so we pre-compute values we expect to see.
> - *
> - * The values for TMDS 25175, 25200, 27000, 54000, 74250 and 148500 kHz are
> - * the recommended N values specified in the Audio chapter of the HDMI
> - * specification.
> - */
> -static const struct dw_hdmi_audio_tmds_n {
> - unsigned long tmds;
> - unsigned int n_32k;
> - unsigned int n_44k1;
> - unsigned int n_48k;
> -} common_tmds_n_table[] = {
> - { .tmds = 25175000,  .n_32k = 4576,  .n_44k1 = 7007,  .n_48k = 6864, },
> - { .tmds = 25200000,  .n_32k = 4096,  .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 27000000,  .n_32k = 4096,  .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 28320000,  .n_32k = 4096,  .n_44k1 = 5586,  .n_48k = 6144, },
> - { .tmds = 30240000,  .n_32k = 4096,  .n_44k1 = 5642,  .n_48k = 6144, },
> - { .tmds = 31500000,  .n_32k = 4096,  .n_44k1 = 5600,  .n_48k = 6144, },
> - { .tmds = 32000000,  .n_32k = 4096,  .n_44k1 = 5733,  .n_48k = 6144, },
> - { .tmds = 33750000,  .n_32k = 4096,  .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 36000000,  .n_32k = 4096,  .n_44k1 = 5684,  .n_48k = 6144, },
> - { .tmds = 40000000,  .n_32k = 4096,  .n_44k1 = 5733,  .n_48k = 6144, },
> - { .tmds = 49500000,  .n_32k = 4096,  .n_44k1 = 5488,  .n_48k = 6144, },
> - { .tmds = 50000000,  .n_32k = 4096,  .n_44k1 = 5292,  .n_48k = 6144, },
> - { .tmds = 54000000,  .n_32k = 4096,  .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 65000000,  .n_32k = 4096,  .n_44k1 = 7056,  .n_48k = 6144, },
> - { .tmds = 68250000,  .n_32k = 4096,  .n_44k1 = 5376,  .n_48k = 6144, },
> - { .tmds = 71000000,  .n_32k = 4096,  .n_44k1 = 7056,  .n_48k = 6144, },
> - { .tmds = 72000000,  .n_32k = 4096,  .n_44k1 = 5635,  .n_48k = 6144, },
> - { .tmds = 73250000,  .n_32k = 11648, .n_44k1 = 14112, .n_48k = 6144, },
> - { .tmds = 74250000,  .n_32k = 4096,  .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 75000000,  .n_32k = 4096,  .n_44k1 = 5880,  .n_48k = 6144, },
> - { .tmds = 78750000,  .n_32k = 4096,  .n_44k1 = 5600,  .n_48k = 6144, },
> - { .tmds = 78800000,  .n_32k = 4096,  .n_44k1 = 5292,  .n_48k = 6144, },
> - { .tmds = 79500000,  .n_32k = 4096,  .n_44k1 = 4704,  .n_48k = 6144, },
> - { .tmds = 83500000,  .n_32k = 4096,  .n_44k1 = 7056,  .n_48k = 6144, },
> - { .tmds = 85500000,  .n_32k = 4096,  .n_44k1 = 5488,  .n_48k = 6144, },
> - { .tmds = 88750000,  .n_32k = 4096,  .n_44k1 = 14112, .n_48k = 6144, },
> - { .tmds = 97750000,  .n_32k = 4096,  .n_44k1 = 14112, .n_48k = 6144, },
> - { .tmds = 101000000, .n_32k = 4096,  .n_44k1 = 7056,  .n_48k = 6144, },
> - { .tmds = 106500000, .n_32k = 4096,  .n_44k1 = 4704,  .n_48k = 6144, },
> - { .tmds = 108000000, .n_32k = 4096,  .n_44k1 = 5684,  .n_48k = 6144, },
> - { .tmds = 115500000, .n_32k = 4096,  .n_44k1 = 5712,  .n_48k = 6144, },
> - { .tmds = 119000000, .n_32k = 4096,  .n_44k1 = 5544,  .n_48k = 6144, },
> - { .tmds = 135000000, .n_32k = 4096,  .n_44k1 = 5488,  .n_48k = 6144, },
> - { .tmds = 146250000, .n_32k = 11648, .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 148500000, .n_32k = 4096,  .n_44k1 = 6272,  .n_48k = 6144, },
> - { .tmds = 154000000, .n_32k = 4096,  .n_44k1 = 5544,  .n_48k = 6144, },
> - { .tmds = 162000000, .n_32k = 4096,  .n_44k1 = 5684,  .n_48k = 6144, },
> -
> - /* For 297 MHz+ HDMI spec have some other rule for setting N */
> - { .tmds = 297000000, .n_32k = 3073,  .n_44k1 = 4704,  .n_48k = 5120, },
> - { .tmds = 594000000, .n_32k = 3073,  .n_44k1 = 9408,  .n_48k = 10240,},
> -
> - /* End of table */
> - { .tmds = 0,         .n_32k = 0,     .n_44k1 = 0,     .n_48k = 0,    },
> -};
> -
> -/*
> - * These are the CTS values as recommended in the Audio chapter of the HDMI
> - * specification.
> - */
> -static const struct dw_hdmi_audio_tmds_cts {
> - unsigned long tmds;
> - unsigned int cts_32k;
> - unsigned int cts_44k1;
> - unsigned int cts_48k;
> -} common_tmds_cts_table[] = {
> - { .tmds = 25175000,  .cts_32k = 28125,  .cts_44k1 = 31250,  .cts_48k = 28125,  },
> - { .tmds = 25200000,  .cts_32k = 25200,  .cts_44k1 = 28000,  .cts_48k = 25200,  },
> - { .tmds = 27000000,  .cts_32k = 27000,  .cts_44k1 = 30000,  .cts_48k = 27000,  },
> - { .tmds = 54000000,  .cts_32k = 54000,  .cts_44k1 = 60000,  .cts_48k = 54000,  },
> - { .tmds = 74250000,  .cts_32k = 74250,  .cts_44k1 = 82500,  .cts_48k = 74250,  },
> - { .tmds = 148500000, .cts_32k = 148500, .cts_44k1 = 165000, .cts_48k = 148500, },
> -
> - /* End of table */
> - { .tmds = 0,         .cts_32k = 0,      .cts_44k1 = 0,      .cts_48k = 0,      },
> -};
> -
> struct dw_hdmi_qp_i2c {
> struct i2c_adapter adap;
> 
> @@ -215,130 +133,6 @@ static void dw_hdmi_qp_set_cts_n(struct dw_hdmi_qp *hdmi, unsigned int cts,
>       AUDPKT_ACR_CONTROL1);
> }
> 
> -static int dw_hdmi_qp_match_tmds_n_table(struct dw_hdmi_qp *hdmi,
> - unsigned long pixel_clk,
> - unsigned long freq)
> -{
> - const struct dw_hdmi_audio_tmds_n *tmds_n = NULL;
> - int i;
> -
> - for (i = 0; common_tmds_n_table[i].tmds != 0; i++) {
> - if (pixel_clk == common_tmds_n_table[i].tmds) {
> - tmds_n = &common_tmds_n_table[i];
> - break;
> - }
> - }
> -
> - if (!tmds_n)
> - return -ENOENT;
> -
> - switch (freq) {
> - case 32000:
> - return tmds_n->n_32k;
> - case 44100:
> - case 88200:
> - case 176400:
> - return (freq / 44100) * tmds_n->n_44k1;
> - case 48000:
> - case 96000:
> - case 192000:
> - return (freq / 48000) * tmds_n->n_48k;
> - default:
> - return -ENOENT;
> - }
> -}
> -
> -static u32 dw_hdmi_qp_audio_math_diff(unsigned int freq, unsigned int n,
> -      unsigned int pixel_clk)
> -{
> - u64 cts = mul_u32_u32(pixel_clk, n);
> -
> - return do_div(cts, 128 * freq);
> -}
> -
> -static unsigned int dw_hdmi_qp_compute_n(struct dw_hdmi_qp *hdmi,
> - unsigned long pixel_clk,
> - unsigned long freq)
> -{
> - unsigned int min_n = DIV_ROUND_UP((128 * freq), 1500);
> - unsigned int max_n = (128 * freq) / 300;
> - unsigned int ideal_n = (128 * freq) / 1000;
> - unsigned int best_n_distance = ideal_n;
> - unsigned int best_n = 0;
> - u64 best_diff = U64_MAX;
> - int n;
> -
> - /* If the ideal N could satisfy the audio math, then just take it */
> - if (dw_hdmi_qp_audio_math_diff(freq, ideal_n, pixel_clk) == 0)
> - return ideal_n;
> -
> - for (n = min_n; n <= max_n; n++) {
> - u64 diff = dw_hdmi_qp_audio_math_diff(freq, n, pixel_clk);
> -
> - if (diff < best_diff ||
> -    (diff == best_diff && abs(n - ideal_n) < best_n_distance)) {
> - best_n = n;
> - best_diff = diff;
> - best_n_distance = abs(best_n - ideal_n);
> - }
> -
> - /*
> - * The best N already satisfy the audio math, and also be
> - * the closest value to ideal N, so just cut the loop.
> - */
> - if (best_diff == 0 && (abs(n - ideal_n) > best_n_distance))
> - break;
> - }
> -
> - return best_n;
> -}
> -
> -static unsigned int dw_hdmi_qp_find_n(struct dw_hdmi_qp *hdmi, unsigned long pixel_clk,
> -      unsigned long sample_rate)
> -{
> - int n = dw_hdmi_qp_match_tmds_n_table(hdmi, pixel_clk, sample_rate);
> -
> - if (n > 0)
> - return n;
> -
> - dev_warn(hdmi->dev, "Rate %lu missing; compute N dynamically\n",
> - pixel_clk);
> -
> - return dw_hdmi_qp_compute_n(hdmi, pixel_clk, sample_rate);
> -}
> -
> -static unsigned int dw_hdmi_qp_find_cts(struct dw_hdmi_qp *hdmi, unsigned long pixel_clk,
> - unsigned long sample_rate)
> -{
> - const struct dw_hdmi_audio_tmds_cts *tmds_cts = NULL;
> - int i;
> -
> - for (i = 0; common_tmds_cts_table[i].tmds != 0; i++) {
> - if (pixel_clk == common_tmds_cts_table[i].tmds) {
> - tmds_cts = &common_tmds_cts_table[i];
> - break;
> - }
> - }
> -
> - 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;
> - }
> -}
> -
> static void dw_hdmi_qp_set_audio_interface(struct dw_hdmi_qp *hdmi,
>   struct hdmi_codec_daifmt *fmt,
>   struct hdmi_codec_params *hparms)
> @@ -458,8 +252,7 @@ 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);
> + drm_hdmi_acr_get_n_cts(tmds_char_rate, sample_rate, &n, &cts);
> 
> dw_hdmi_qp_set_cts_n(hdmi, cts, n);
> }