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

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Message-ID: <20250722195437.1347865-2-detlev.casanova@collabora.com>
Date: Tue, 22 Jul 2025 15:54:35 -0400
From: Detlev Casanova <detlev.casanova@...labora.com>
To: linux-kernel@...r.kernel.org
Cc: 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>,
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
	Detlev Casanova <detlev.casanova@...labora.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: [PATCH v2 1/3] drm/bridge: dw-hdmi-qp: Return 0 in audio prepare when disconnected

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