[alsa-devel] Radeon unconnected HDMI eats samples at 280 kHz

[David Henningsson]

On 2014-09-24 23:37, Deucher, Alexander wrote:
>> But it makes sense that the audio is turned off when the video is, right?
>
> It's a bit complicated.  X effectively just blanks (dpms off) the display when you disconnect it or xrandr --off.  The resources are not actually reclaimed and "disabled" until the next modeset.  I don't think we really want to turn audio off when the display goes into dpms as that will be reported as a disconnect on the audio side even if the display has just gone to sleep.
>
>>
>> Because if first unplug HDMI (nothing happens) and then run "xrandr"
>> (without parameters), running that command causes a "re-detection" or
>> whatever the correct term is - i e, my DVI screen goes black for a
>> second, and afterwards the mouse pointer can no longer move to the other
>> non-existent display.
>> At that point, audio still remains in the "plugged in" state. Also,
>> executing "xrandr --output HDMI-0 --off" has no effect in that state.
>>
>
> The problem is, X just puts the display to sleep when you unplug or xrandr --off.  The current KMS API doesn't really have a notion of "disable".  The actual disabling happens at the next modeset when displays that are no longer in use are disabled and their resources are freed for possible use in the upcoming modeset.  Forcing another modeset with the hdmi disconnected should get the status updated properly.

I'm not sure how to force a modeset, but just for comparison and for the 
desire of consistency, I did the same test with a laptop with an Intel 
built-in card.

In this case, both plug-in and unplug are responded to immediately on 
both sides, so when the cable is unplugged, the video does a 
"re-detection" which disables the HDMI display, and the audio side 
reports unplugged as well.

Also, executing "xrandr --output HDMI1 --off" while the cable is plugged 
causes the audio side to report unplugged.

I think this behaviour makes sense, but I'm not sure my rank is high 
enough to dictate behaviour. :-)

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic