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

[Detlev Casanova <detlev.casanova@collabora.com>]

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.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
---
 drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c | 2 +-
 1 file changed, 1 insertion(+), 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..7d3d7b5084c8d 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi-qp.c
@@ -459,7 +459,7 @@ static int dw_hdmi_qp_audio_prepare(struct drm_connector *connector,
 	bool ref2stream = false;
 
 	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