linux-kernel - Re: [PATCH v2 1/3] drm/bridge: dw-hdmi-qp: Return 0 in audio prepare when disconnected

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Message-ID: <hw5grsaqfef4l55pu7ypsgojnltcynl22l4jqb7w2t6vlo3yxo@d5yhtxcmtroq>
Date: Wed, 23 Jul 2025 14:41:43 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Detlev Casanova <detlev.casanova@...labora.com>
Cc: linux-kernel@...r.kernel.org, Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        Douglas Anderson <dianders@...omium.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Sugar Zhang <sugar.zhang@...k-chips.com>,
        Cristian Ciocaltea <cristian.ciocaltea@...labora.com>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Raag Jadav <raag.jadav@...el.com>, dri-devel@...ts.freedesktop.org,
        linux-sound@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH v2 1/3] drm/bridge: dw-hdmi-qp: Return 0 in audio prepare
 when disconnected

On Tue, Jul 22, 2025 at 03:54:35PM -0400, Detlev Casanova wrote:
> To configure audio registers, the clock of the video port in use must be
> enabled.
> As those clocks are managed by the VOP driver, they can't be enabled here
> to write the registers even when the HDMI cable is disconnected.
> 
> Furthermore, the registers values are computed from the TMDS char rate,
> which is not available when disconnected.
> 
> Returning -ENODEV seemed reasonable at first, but ASoC will log an error
> multiple times if dw_hdmi_qp_audio_prepare() return an error.
> Userspace might also retry multiple times, filling the kernel log with:
> 
> hdmi-audio-codec hdmi-audio-codec.0.auto: ASoC error (-19): at snd_soc_dai_prepare() on i2s-hifi
> 
> This has become even worse with the support of the second HDMI TX port.
> 
> Activating the clocks to write fake data (fake because the TMDS char
> rate is unavailable) would require API changes to communicate between
> VOP and HDMI, which doesn't really make sense.
> 
> Using a cached regmap to be dumped when a cable is connected won't work
> because writing order is important and some data needs to be retrieved
> from registers to write others.
> 
> Returning 0 to silently fail sounds like the best and simplest solution.
> 
> Fixes: fd0141d1a8a2 ("drm/bridge: synopsys: Add audio support for dw-hdmi-qp")
> Signed-off-by: Detlev Casanova <detlev.casanova@...labora.com>
> ---
>  drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
> index 5e5f8c2f95be1..9b9d43c02e3a5 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
> @@ -458,8 +458,16 @@ static int dw_hdmi_qp_audio_prepare(struct drm_connector *connector,
>  	struct dw_hdmi_qp *hdmi = dw_hdmi_qp_from_bridge(bridge);
>  	bool ref2stream = false;
>  
> +	/*
> +	 * Silently return if tmds_char_rate is not set.
> +	 *
> +	 * Writing audio registers requires that the clock of the Video Port currently in
> +	 * use by the VOP (dclk_vp<id>) is enabled.
> +	 * That clock is guaranteed to be enabled when hdmi->tmds_char_rate is set, so we
> +	 * only configure audio when it is set.
> +	 */
>  	if (!hdmi->tmds_char_rate)
> -		return -ENODEV;
> +		return 0;

What if the cable gets diconnected _while_ this function is running?

>  
>  	if (fmt->bit_clk_provider | fmt->frame_clk_provider) {
>  		dev_err(hdmi->dev, "unsupported clock settings\n");
> -- 
> 2.50.1
> 

-- 
With best wishes
Dmitry