Minutes below. Comments, corrections and omissions welcomed.
We are missing some links to some of the slides presented. Please reply with a link if you have one.
Audio Mini Conference Minutes - Santa Fe 2016 =============================================
Present: Mark, Rakesh, Liam, Vinod, Mengdong, Pierre, Patrick, Takashi S, Javilars, Takashi I, Eric, Dylan, Mauro, Shuah, Charles, Matthew, Lars, Sagar and others.
Android USB Audio - Eric ========================
USB audio is becoming more important as analog jack can be removed. USB stack is difficult to optimise for low power/latency audio (too many wake ups with USB 1ms tick). Aim to add no_irq (no IRQ for each period) and mmap support in USB 3.0 stack to improve power and latency.
USB playback and capture with synchronised start API is missing from stack/ALSA. Some patches were posted for synchronised endpoint but never made it to the list, Pierre is going to resend.
USB currently defines simple audio endpoints. Support is needed to define more complex USB audio endpoints rather than just "playback" and "capture".
Sub 10ms latency (round trip ?) is difficult to achieve on USB due to stack and drivers. Mmap may help reduce latency by removing buffer copies.
Inter Device Dependencies =========================
Kernel Summit joined for this discussion. There is complexity between devices, more than just a parent/child relationship for some devices making it difficult to order probing and PM ops.
Rafael, has created patch set to solve dependences for PM ops in Gregs tree, the patches could also be used for probing. The patches add an API to allow device relationships to be established as a graph. This API supports non parent/child relationships to and allows PM ops to run in a deterministic sequence. Relationship dependency discovery needs to come from DT/ACPI and will need standardised bindings.
Mark described the current audio workarounds where the machine driver specifies relationships and deferred probe is used to manage the probing sequence. Some people do not like the high number deferred probe messages in dmesg.
Relation graphs must be ordered so that cycles to do not exist.
Next steps are to use Rafaels patches for probe ordering.
AV Sync - Eric ===============
A few frame counters are 32 bit (compressed API) and should be 64 bit. Vinod will update compressed API to 64 bit.
Timers should also consider boot time and suspend/resume time as it's currently difficult to keep AV syn over suspend/resume cycles.
AV sync on TV's hacky atm, we need AV sync API in ALSA. Takashi noted this was long time problem that needs solution. Some options for AV sync were pseudo transport streams (like existing mpeg format) but these were difficult for mmap(). Other devices had used runtime PLL updating with patches sent in the past ?
Compressed API could be used to send transport stream for AV sync.
HDMI/DP glue to GFX - Vinod ===========================
Currently several interlaces exist to pass DP/HDMI events and information between audio and GFX stacks/drivers. ARM devices using pdata interface, i915 has another solution, BYT uses another interface. Needs to be consistent for user space ELD notification, jack detection/mapping. Currently HDA drivers have hack to assist userspace (PCM5).
There are also difficulties mapping PCMs, channels and jacks to multiple HDMI/DP devices.
Tikashi and Vinod to investigate best solutions.
DP-MST - Vinod ===============
Jack notifications more difficult with MST (extra complexity). How do we map multiple jacks from multiple monitors, disable unused pins when not rendering ? The 5.1 use case for gaming has to split channels to multiple monitors. Decision is that new devices would export topology to userpace so that userspace know the jack, channel mappings.
Soundwire Support - Rakesh and Pierre =====================================
Soundwire new hardware interface for audio devices. Needs new subsystem support for soundwire master and slave devices. Hardik has posted initial RFC to the list for review.
Rakesh presented slides . Link to the slides ?
Some people had reviewed the RFC. Some changes to strct device/driver usage required to make it more like other subsystems upstream. There is also need to deal with devices that can power off after or before enumeration. Some confusion over the terminology used too, compared to other bus systems like I2C. Master should be device and not driver. Top level feedback is that it does not look like bus code.
There is also a need to consider objects in DT/ACPI. AR Vinod, to look at STIO.
Topology and UCM file deployment - Vinod, Mengdong, Liam ========================================================
Difficult to release and deploy topology/ucm updates with alsa-lib cadence. Distros to be contacted for best deployment where cadence can be faster than alsa-lib. Liam and Vinod to contact distro package maintainers.
UCM - Liam, Mengdong ====================
Which driver card name string to change, there are 3 - long, short and driver. CRAS uses the string in the []. Not sure about PA.
Liam to create patch that includes machine name in card name string.
Mengdong has added #include and #define like support to topology. Mengdong will port this to UCM too so that a library of codecs can be built up and included by the machine ucm files. e.g.
Asus T100 UCM would include rt5640 codec and baytrail DSP UCM configs.
Tracepoint for audio - Lars, Takashi S ======================================
Lars and Takashi S have similar patchsets. Easy to make tooling for parsing traceout. This could replace printk and snd_printk, but builds bigger binary. Option to disable can be added to reduce size. Uses ftrace output and timestamp. Used in the PCM operations. Patches are still WiP and will be posted as RFC when ready.
Dropping Dice Driver - Takashi S ================================
Dice hardware has serious problems making it unusable. Takashi S to discuss with Clemens regarding any potential work around for HW issue. If no work around is found then it could be marked as broken or moved to staging ?
Sound Open Firmware - Liam ==========================
Liam introduced SOF and took QA. Liam is also working on an ELF executable firmware image that can run on the developers host PC.
http://alsa-project.org/main/index.php/Firmware
Paravirtualisation of Audio Drivers - Rakesh ============================================
Safety critical systems have requirements to play safety critcal sounds (like alarms, warnings) and regular non safety critical sound like media. Rakesh proposed a Xen hypervisor method to guarantee the safety critical audio would always take precedence over the non safety critical audio.
Xen would run a privileged OS (for safety critical) and unprivileged OS (for non safety critical audio). A para virtualised audio driver would allow the Unprivileged OS to play audio on the privileged OS.
Lars suggested Libvirt and KVM may make this task far easier and less restricted than using Xen. Rakesh to investigate further.
Userspace driver for Firewire - Takashi S =========================================
Need to represent mixers for some USB and FW devices in userspace. Why ?. Possibility of using topology for userspace. Nobody keen on exposing kernel APIs as userspavce ABIs. Possibility of looking at libvirt interfaces for userspace drivers many similarities. Kernel component, already use FW sound card.
Mixer API not suitable, many bus so use control API.
ALSA Stream Configuration Domains - Lars ========================================
ALSA has powerful but complex system to configure and constrain stream configurations at runtime but it does not scale well on modern ASoC based sound devices with multiple components and DAIs.
Lars presented a solution that can break complex sound cards down to simple domains where configuration can be set by userspace to exactly match each use case. Userspace would also be able to read the configuration in each domain. New APIs calls would be needed in alsa-lib to achieve this.
Lars also requested that ASoC become a first class citizen with ALSA since most new sound cards are ASoC based. Proposal was accepted.