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

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

Hi Dmitry
On Wednesday, 23 July 2025 07:41:43 EDT Dmitry Baryshkov wrote:
> 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@collabora.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?

Unplugging the cable will not deactivate the clocks, that's the job of the 
VOP's vop2_crtc_atomic_disable() function. My understanding is that that 
function will not be called until this one has returned, but I could be wrong.

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

Regards,
Detlev.