[PATCH 07/20] drm/amd/display: Don't use stereo sync and audio on RGB signals

[Harry Wentland]

On 2025-07-30 04:19, Timur Kristóf wrote:
> On Tue, 2025-07-29 at 14:21 -0400, Harry Wentland wrote:
>>
>>
>> On 2025-07-23 11:58, Timur Kristóf wrote:
>>> Features like stereo sync and audio are not supported by RGB
>>> signals, so don't try to use them.
>>>
>>
>> Where does it say that?
>>
>> Harry
> 
> 1. Audio
> 
> VGA ports (and the analog part of DVI-I ports) simply cannot carry
> audio. So there is no hardware to control any audio, therefore there is
> nothing for this code to enable, which is why I added those ifs to not
> even try to enable audio on analog video signals.
> 

My bad, I was thinking RGB as opposed to YCbCr. Forgot that we use
RGB signal to refer to VGA.

> As a side note, DVI-D ports (and the digital part of DVI-I ports) may
> have a non-standard extension to carry digital audio signals, but that
> is not revelant to supporting analog displays.
> 
> 2. Stereo sync
> 
> With regards to stereo sync, I didn't find any reference to this in the
> legacy display code, so I assumed either it is unsupported or the VBIOS
> already sets it up correctly. At least, considering that the legacy
> code didn't bother setting it up, we don't lose any functionality if we
> leave it out of DC as well.
> 
> That being said, upon some further digging in the DCE register files, I
> found a register called DAC_STEREOSYNC_SELECT so maybe I could
> investigate using that. Maybe it would be better to work with the
> registers directly instead of the VBIOS? Would it be okay to
> investigate that further in a future patch series once this one is
> merged?
> 

I don't think DC supports stereo sync currently. I'm not sure there is
much value in pursuing that.

>>> diff --git a/drivers/gpu/drm/amd/display/include/signal_types.h
>>> b/drivers/gpu/drm/amd/display/include/signal_types.h
>>> index a10d6b988aab..825a08fcb125 100644
>>> --- a/drivers/gpu/drm/amd/display/include/signal_types.h
>>> +++ b/drivers/gpu/drm/amd/display/include/signal_types.h
>>> @@ -118,6 +118,11 @@ static inline bool dc_is_dvi_signal(enum
>>> signal_type signal)
>>>  	}
>>>  }
>>>  
>>> +static inline bool dc_is_rgb_signal(enum signal_type signal)

To avoid confusion with people that haven't worked on analog
signals in years (or ever) it might be better to name this
dc_is_analog_signal.

Harry

>>> +{
>>> +	return (signal == SIGNAL_TYPE_RGB);
>>> +}
>>> +
>>>  static inline bool dc_is_tmds_signal(enum signal_type signal)
>>>  {
>>>  	switch (signal) {