[pulseaudio-discuss] [PATCH v2 4/8] core: move sink-inputs conditionally when update default_sink

Fri Jul 12 11:13:20 UTC 2019

On Wed, 2019-07-10 at 22:03 +0200, Georg Chini wrote:
> On 10.07.19 16:03, Tanu Kaskinen wrote:
> > On Fri, 2019-07-05 at 10:57 +0200, Georg Chini wrote:
> > > On 05.07.19 09:41, Tanu Kaskinen wrote:
> > > > On Tue, 2019-07-02 at 09:08 +0200, Georg Chini wrote:
> > > > > On 02.07.19 08:43, Tanu Kaskinen wrote:
> > > > > > On Mon, 2019-07-01 at 08:03 +0200, Georg Chini wrote:
> > > > > > > On 01.07.19 07:08, Tanu Kaskinen wrote:
> > > > > > > When a new sink appears, pa_core_update_default_sink() is called. Since
> > > > > > > PulseAudio 11.1, bluetooth and USB sinks have higher priority than the
> > > > > > > internal sound card, so pa_core_update_default_sink() will switch to
> > > > > > > the BT speakers in your example. The main benefit of module-switch-on-
> > > > > > > connect was that it moved existing streams to the new sink, but after
> > > > > > > this patch set that's handled by the core. Therefore there's much less
> > > > > > > need for module-switch-on-connect.
> > > > > > > 
> > > > > This is true, but only relevant if there is no configured default
> > > > > sink. If the configured default sink is set, there will be no switch
> > > > > to a newly appearing sink because the configured default sink
> > > > > is prioritized over all other sinks. In this case you still need
> > > > > module-switch-on-connect.
> > > > My estimation is that this is very rarely a problem.
> > > Mh, my estimation is, that this will be the normal case. At some
> > > point the user will set a default sink manually, and from that point
> > > on, automatic switching will no longer work. The configured default
> > > sink is never unset once it is set.
> > Yes, sure, but I tried to explain why this is rarely a problem. (In
> > another thread you described a valid use case where this becomes a
> > problem, but I don't think that's a "normal case".)
> > 
> > > > If you have manually selected an external sound card as the default
> > > > sink and you then plug in another external sound card, then you may or
> > > > may not want to automatically switch to the new sound card. If you
> > > > fiddle a lot with two external sound cards and you always want to use
> > > > the last one plugged in, then module-switch-on-connect is still useful,
> > > > but in this case it doesn't really matter that your old default device
> > > > choice is forgotten, because PulseAudio will anyway prefer the
> > > > remaining external sound card.
> > > This topic is not about forgetting the last sink, but about switching
> > > to a new sink at all. Once you manually set the default sink, it will
> > > never switch automatically without module-switch-on-connect,
> > > because the default sink selection process prefers the configured
> > > default sink over all other sinks.
> > You're assuming that we always want to switch to a newly plugged in
> > device. Let's say that you have just one USB device, no other external
> > devices. If you for some reason set the internal sound card as the
> > default device, I think it's a good thing that we don't automatically
> > switch to that USB device when it's plugged in again. Therefore I think
> > it's a good thing that we don't load module-switch-on-connect by
> > default.
> > 
> > If you in this situation plug in another external device, it would
> > probably make sense to switch to it, but I don't what the full policy
> > would look like that would allow this without breaking anything. I
> > believe it's a good compromise to stick to the current configured
> > default sink until the user changes it or the device goes away.
> > 
> You said you will write another mail tomorrow, so I skip most
> of your comments, but while reading your mails I realized
> what is really behind my dislike of the current switching logic:
> 
> I believe it is a bad idea to make switching to new sinks
> dependent on the configured default sink. Switching to
> new sinks is something a user should be able to turn on
> and off separately. Behavior could be something like that:
> 
> If switching to new sinks is turned off and a new sink turns up,
> we should never switch to it, even if it has higher priority and
> even if the configured default sink is not set. The only exception
> would be if the configured default sink is set and the sink that
> turns up is the configured default sink.
> 
> If switching to new sinks is turned on, we always switch to a
> new device. The configured default sink is the one we always
> return to when the last added device is removed. This means
> if two devices have been added, we would already go back to
> the configured default when the second device is removed. If
> no default is configured, we would use the highest priority sink
> to return to.

As I said in IRC, I changed my mind about always switching to new
devices regardless of the current configured default sink. I now think
that's a good idea. As you've said, if the user ever configures the
default sink, PulseAudio will from then on never switch to plugged in
devices. I first thought that's OK, because we should respect the
user's explicit preference if the configured default device remains
available, and if it isn't available, then automatic switching to the
plugged in device will work fine.

But if we implement default device history as I've suggested, and the
internal sound card has ever been configured as the default and is
therefore in the history database, it will always be preferred over
never-before-seen plugged in devices, which is bad. In this situation
automatic switching truly is entirely disabled, without the user
requesting such policy change and without a way to re-enable the
policy.

You suggested above that if automatic switching to plugged in devices
is disabled, we shouldn't switch even if there's no configured default
sink. That has the problem that if you plug in a USB device but keep
using the internal sound card, PulseAudio will change the routing after
reboot, because PulseAudio has to choose some sink as the default sink,
and it will choose the USB sink, because it has higher priority.
Changing the default sink on reboot seems bad enough that we should
keep switching to higher priority sinks even if automatic switching to
new devices is otherwise disabled.

I don't know if you have some plans to enable module-switch-on-connect
(or similar policy) by default, but if you do, note the following
complications:

 * We should only switch to newly plugged in hardware, not to new
virtual sinks or sinks that are created as part of a profile change.
I'm not sure about network sinks (tunnels, raop). At least if the
network sinks are created via automatic discovery, they shouldn't be
automatically switched to. Also, we shouldn't switch to HDMI
automatically.

 * We should treat plugging in headphones to an internal sound card the
same way as plugging in USB headphones. That is, we should make the
internal sound card the default sink. This can perhaps be implemented
later, though (or even earlier, by modifying module-switch-on-connect
without loading it by default).

-- 
Tanu

https://www.patreon.com/tanuk
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