Re: [PATCH] ALSA - hda: hdmi flag to stop playback when monitor is disconnected

[David Henningsson <david.henningsson@canonical.com>]

On 2015-11-11 15:23, Yang, Libin wrote:
>
>> -----Original Message-----
>> From: Takashi Iwai [mailto:tiwai@suse.de]
>> Sent: Wednesday, November 11, 2015 10:13 PM
>> To: libin.yang@linux.intel.com
>> Cc: David Henningsson; alsa-devel@alsa-project.org;
>> mengdong.lin@linux.intel.com; Yang, Libin
>> Subject: Re: [PATCH] ALSA - hda: hdmi flag to stop playback when
>> monitor is disconnected
>>
>> On Wed, 11 Nov 2015 10:02:14 +0100,
>> Takashi Iwai wrote:
>>>
>>> On Wed, 11 Nov 2015 09:39:09 +0100,
>>> libin.yang@linux.intel.com wrote:
>>>>
>>>> From: Libin Yang <libin.yang@linux.intel.com>
>>>>
>>>> Add a flag that user can decide to stop HDMI/DP playback when
>>>> the corresponding monitor is disconnected and refuse to open PCM
>>>> if there is no monitor connected.
>>>>
>>>> Background:
>>>> When a monitor is disconnected and a new monitor is connected,
>>>> the parameters of the 2 monitors may be different. Audio driver
>>>> need handle this situation.
>>>>
>>>> Besides, stopping playback when monitor is disconnected will
>>>> help to save the power.
>>>>
>>>> Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
>>>
>>> Thanks.  Below are just nitpicking, so let's test this patch at first,
>>> especially to see whether it has any significant influence on PA, then
>>> respin with the fixes.
>>>
>>> David, care to check in your side, too?
>>
>> So I tested this with PA 7.1, and it failed, unfortunately.
>> In short:
>> - PA needs the PCM access at probe.  If it gets an error, the device
>>    will be never enumerated again

...and that's the expected behaviour given how PA works today. Which 
also means that this new parameter should not be enabled by default, 
unless we want to break PA 7.1 and earlier versions.

>
> Thanks for test.
>
> If so, should we re-write the hdmi_pcm_open() and
> generic_hdmi_playback_pcm_prepare() function to make
> the probe works?  Or not support dynamic pcm assignment?

I've already suggested how we can make it work, by assigning converter 
nodes dynamically regardless of whether or not the PCM has a monitor 
connected to it.

Keeping the power well on for up to five seconds (and often much less) 
during boot should not be a big deal power saving wise, so the power 
save argument doesn't hold IMO.

>> - PA removes the device when it's disconnected.  The PCM stop with
>>    DISCONNECT state leads to the device disappearance.
>
> If we can't use DISCONNECT, what else can we use? Or we can't
> stop PCM when monitor is disconnected?

The current behavior is to keep the PCM running, which also has the 
benefit to be able to switch monitors on the fly (i e without stopping 
the stream), if both support the stream format. At least I think one can 
do this, I've actually never tried.

While reporting -ENODEV for a disconnected monitor seems useful to some 
scenarios, there are certainly other scenarios where it doesn't work.

PA should use the ELD and/or the Jack kctl to figure out when it needs 
to reprobe the device. This has not been implemented.

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