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

[Sebastian Reichel <sebastian.reichel@collabora.com>]

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Hi,

On Tue, Sep 01, 2026 at 11:25:41AM +0000, Christian Hewitt wrote:
> 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@gmail.com>
> ---
> 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.

You are looking for this series:

https://lore.kernel.org/linux-rockchip/86fcf349-0a7a-4618-9001-612371b0f71b@symple.nz/

Greetings,

-- Sebastian

>  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
> 

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