The current minimum target latency is partially based upon the default qemu
behaviour whereby audio packets are delivered in a sawtooth pattern, with
packet timestamps drifting between 5ms above and below the measured clock.
This 5ms error is baked into the minimum target latency to avoid
underrunning.
This sawtooth pattern can be reduced by specifying a lower timer period in
the qemu configuration, so remove it from the hardcoded minimum latency and
add it to the default configurable buffer latency instead. This allows
users that have configured their VM appropriately to reduce the overall
latency.
The best quality resampler has an intrinsic latency of about 3ms, and the
processing itself takes another 1-2ms per 10ms block. The faster setting
has an intrinsic latency of about 0.4ms, with about 0.04ms processing time.
This makes for an overall saving of about 4ms, with negligible loss in
quality.
This adds a new `audio:bufferLatency` option which allows the user to
adjust the amount of buffering LG does over the absolute bare minimum. By
default, this is set large enough to absorb typical timing jitter from
Spice. Users may reduce this if they care more about latency than audio
quality.
This adds a new `audio:periodSize` option which defaults to 2048 frames.
For PipeWire, this controls the `PIPEWIRE_LATENCY` value. For PulseAudio,
the controls the target buffer length (`tlength`) value.
If the audio device starts earlier than required, we slew the read pointer
backwards to avoid underrunning. We need to apply this same offset to the
recorded device position, otherwise the Spice thread will think playback is
further ahead than it really is and inject unnecessary latency to
compensate.
When the 'keep alive' playback times out, playback is stopped from the
audio callback, resulting in an assertion failure inside PulseAudio as we
try to lock the main loop thread while already inside it.
The actual time between opening the device and the device starting to pull
data can range anywhere between nearly instant and hundreds of
milliseconds. To minimise startup latency, open the device as soon as the
first playback data is received from Spice. If the device starts earlier
than required, insert a period of silence at the beginning of playback to
avoid underrunning. If it starts later, just accept the higher latency and
let the adaptive resampling deal with it.
We can set the startup latency for the next playback far more precisely if
we have the device open already.
Only keep the device open with no playback for 30 seconds to avoid keeping
the device open unnecessarily forever.
Underruns can still happen quite easily at the beginning of playback,
particularly at very low latency settings. Further increase the startup
latency to avoid this.
Many X11 window managers will present an application on their
taskbar as a combination of the application name and an icon
imagery pulled from the X-Property _NET_WM_ICON. Applications
built under frameworks such as Qt or GTK have this property
populated by the framework. This commit adds the Atom _NET_WM_ICON
and populates it with a 64x64 icon of Looking Glass.
The desktop doesn't need its own sampler, there is already an identically
configured one in the `desktop->texture`.
For some reason, using the texture sampler fixes a black screen issue
with my GTX 660 using the 470.86 driver. Maybe hitting some limit
for how many samplers can be allocated?
PipeWire startup latency varies wildly depending on what else is, or was
last using the audio device. In the worst case, PipeWire can request two
full buffers within a very short period of time immediately at the start of
playback, so make sure we've got enough data in the buffer to support this.
The target latency is now based upon the device maximum period size
(which may be configured by setting the `PIPEWIRE_LATENCY` environment
variable if using PipeWire), with some allowance for timing jitter from
Spice and the audio device.
PipeWire can change the period size dynamically at any time which must be
taken into account when selecting the target latency to avoid underruns
when the period size is increased. This is explained in detail within the
commit body.
The recent `pwnkit` exploit brought this to my attention, not that we
are a setuid process we should still do this properly... who knows where
this code might get used in the future.
