>> >> It looks like in the future the ALSA drivers for some Intel hardware >> will dynamically create a new PCM device when a DisplayPort monitor is >> plugged in. This is being discussed in this thread (part of the thread >> is also cross-posted to pulseaudio-discuss): >> <a href="http://thread.gmane.org/gmane.comp.freedesktop.xorg.drivers.intel/62703/focus=63002">http://thread.gmane.org/gmane.comp.freedesktop.xorg.drivers.intel/62703/focus=63002</a> >> > > No objections to the plan below, but some nitpicks. > > >> >> PulseAudio doesn't currently support dynamic PCM devices, so work is >> needed to add that support. I may work on that, or it may be someone >> else from Intel. I'll describe here what code changes I think we should >> make. That serves two purposes: to get feedback about the plan before >> any code gets written, and to help with the implementation work if >> someone else than me is going to write the code. >> >> This is a long mail, but I'm sure I still didn't think of every issue >> that will arise when implementing this... >> >> >> Event: monitor gets plugged in >> ------------------------------ >> >> The first thing that happens should be that PulseAudio gets a wakeup >> from the alsa mixer, when a new ELD control for the monitor is added. >> This is important, because the mixer should be ready when PulseAudio >> starts to use the new PCM device. It's up to the driver developer to do >> this right. This wakeup can be ignored, so no code changes are needed in >> PulseAudio to handle this. >> >> The second thing that happens is that udev notifies PulseAudio about a >> new PCM device. Interfacing with udev is done in >> src/modules/module-udev-detect.c. Currently PulseAudio only cares about >> new and removed cards, so module-udev-detect has to be modified to also >> keep track of what PCM devices each card has, so that new devices can be >> noticed. > > > I am not sure whether this "wakeup from alsa mixer first, from udev second" ordering is something that can be relied on. But a "ELD control exists when udev notifies us" requirement looks sensible. > > >> >> When module-udev-detect sees a new PCM device, it needs to notify >> module-alsa-card about it. module-udev-detect's only interface with >> module-alsa-card is pa_module, which is not useful for adding the >> notification. I think we should move the bulk of the code in >> module-alsa-card.c to a new class: pa_alsa_card. module-udev-detect >> would then create pa_alsa_card objects instead of loading >> module-alsa-card instances. module-alsa-card would still exist as a >> wrapper around pa_alsa_card, but the module would not be used by >> module-udev-detect. With pa_alsa_card in place, we can add a >> pa_alsa_card_pcm_added() function to its API. > > > OK. > > >> pa_alsa_card should be defined in src/modules/alsa/alsa-card.[ch] and >> included in the <a href="http://libalsa-util.la">libalsa-util.la</a> helper library. >> >> When moving the code from module-alsa-card to pa_alsa_card, some changes >> to the sink and source error handling is needed. Currently, if something >> fails in the IO thread of an alsa sink or source, the sink/source >> unloads the module that owns the sink/source (see the end of >> thread_func() in alsa-sink.c and alsa-source.c). Now the owner module of >> alsa sinks and sources becomes module-udev-detect, and we certainly >> don't want to unload that if a single sink or source fails. The >> sink/source should notify the pa_alsa_card object of the failure (new >> functions pa_alsa_card_sink_failed() and pa_alsa_card_source_failed()), >> and pa_alsa_card should free the failed pa_alsa_sink or pa_alsa_source >> object. > > > The case of manually-loaded module-alsa-sink is not described here, but should be. > > >> Let's get back to the pa_alsa_card_pcm_added() function. What should it >> do? We shouldn't support dynamic PCMs for arbitrary hw PCMs, because >> that's not compatible with relying on logical device names like "front", >> "surround51" etc. At this point we only need to support dynamic PCMs >> that are dedicated to hotplugged HDMI/DisplayPort/Thunderbolt devices. >> The current proposal to detect such PCMs is to add a new HDMI class to >> snd_pcm_class_t, which can be queried with snd_pcm_info_get_class(). >> >> Currently when opening HDMI devices, we use "hdmi:x,y" as the device >> string, where x is the card index and y is the device index. The device >> index may be different than the hw device index, but the mapping between >> "hdmi:x,y" to "hw:x,z" is static. The mapping can be different with >> different drivers, AFAIK. We currently blindly try all device indexes >> from 0 to 7 when probing the card. Takashi Iwai told that such behaviour >> won't be compatible with drivers that create dynamic PCMs for HDMI. I >> guess the reason is that the dynamically allocated hw device indexes can >> (and usually do) fall outside the index range that is used in "hdmi:x,y". >> >> Since the new HDMI PCM class is new, old kernels and old alsa-lib won't >> use that class even with PCMs that are actually dedicated to HDMI. We >> need to tell apart drivers that use the new HDMI PCM class and drivers >> that don't. With drivers that never use the HDMI class, we should keep >> using the "hdmi:x,y" device strings. With drivers that use the HDMI >> class, we should use "hdmi:CARD=x,SYSDEV=z", where z is the hw device >> index (the SYSDEV parameter doesn't currently exist, so alsa-lib needs >> to be updated to support it). How do we tell the two kinds of drivers >> apart? The current proposal is to add a version field to the PCM info. >> Version 0 would mean that the driver is unaware of the HDMI PCM class, >> and version 1 would mean that the driver will set the PCM class to HDMI >> when appropriate. >> >> I assume that the PCM info version will be provided separately for each >> PCM device, but I expect the version to be always the same for every >> device that belongs to the same card. We definitely need to make the >> decision between the two models at the card level in any case, because >> we can't really mix the "hdmi:x,y" model with the "hdmi:CARD=x,SYSDEV=z" >> model within the same card. When probing a new card, we should check the >> PCM info version of the first device that we probe, and choose the HDMI >> model for the card based on that. If the first device has PCM info >> version 0, then we won't support dynamic HDMI devices for that card, >> even if subsequent devices would somehow have version 1. >> >> The "mapping" concept in our alsa-mixer code corresponds to the PCM >> devices. What mappings exist is configured in >> src/modules/alsa/mixer/profile-sets/default.conf. We have many HDMI >> entries, here are a couple of examples: >> >> [Mapping hdmi-stereo] >> description = Digital Stereo (HDMI) >> device-strings = hdmi:%f >> paths-output = hdmi-output-0 >> channel-map = left,right >> priority = 4 >> direction = output >> >> [Mapping hdmi-stereo-extra1] >> description = Digital Stereo (HDMI 2) >> device-strings = hdmi:%f,1 >> paths-output = hdmi-output-1 >> channel-map = left,right >> priority = 2 >> direction = output >> >> The "device-strings" option doesn't suit the HDMI case very well, if we >> sometimes have to use "hdmi:x,y" and sometimes "hdmi:CARD=x,SYSDEV=z". >> Also, the path configuration files assume a particular mapping from >> "hdmi:x,y" to "hw:x,z" when dealing with ELD information and jack >> detection, and that assumed mapping is incorrect with dynamic HDMI >> devices. (This assumption probably also means that our HDMI ELD and jack >> detection functionality is currently broken on non-HDA cards.) >> >> I propose that we add a new boolean option for mappings, which would >> indicate that the mapping represents more than one PCM device, and that >> the PCM and mixer handling is performed according to the complex HDMI >> specific rules that I've explained above. The option name could be e.g. >> "dynamic-hdmi". When "dynamic-hdmi" is be set for a mapping, the >> "description", "device-strings", "paths-output" and "direction" options >> in the configuration file would be ignored, and the appropriate values >> for those would be hardcoded. That would allow us to shorten >> default.conf quite a bit, and also remove all the hdmi-output-N.conf >> path files (the generated paths would be hardcoded too). >> >> All that hardcoding isn't nice from flexibility point of view, but I >> don't think the lost flexibility is very important in this case. I >> believe that any alternative solution that would keep everything in the >> configuration files would introduce a significant amount of complexity >> to deal with the variance in the "hdmi:x,y" -> "hw:x,z" mapping (and I'm >> not sure it's even possible to have non-HDA-specific ELD information and >> jack detection support with the old-style "hdmi:x,y" device strings, >> since we don't have a reliable method to figure out what hw PCM device >> index y corresponds to). >> >> In case of dynamic HDMI mappings, the HDMI profiles need to become >> dynamic too. So, if a profile definition in a configuration file >> references a mapping that has the "dynamic-hdmi" flag set, that profile >> definition will then represent multiple profiles, one for each HDMI >> device. I'm not sure how to deal with the "description" option. Should >> it be ignored, or should we append a number to the description when >> there are multiple HDMI devices? When profiles are autogenerated (which >> is the common case), then the existing profile description logic should >> work fine. >> >> So, when pa_alsa_card_pcm_added() sees that a new HDMI PCM device >> appeared, it should create a new mapping for the device, a new path for >> the mapping, and a new profile containing the mapping. Then >> pa_alsa_card_pcm_added() needs to call pa_card_add_profile(), and >> hopefully it will just work. >> >> >> Event: monitor gets unplugged >> ----------------------------- >> >> When a monitor gets unplugged, module-udev-detect gets a notified, and >> it should figure out that a PCM device has disappeared. It should then >> call pa_alsa_card_pcm_removed(), which is a new function that needs to >> be implemented. > > > Again, the case of manually loaded module-alsa-sink is not considered. > > >> >> pa_alsa_card_pcm_removed() should mirror pa_alsa_card_pcm_added(), just >> undoing the things that _added() did, in reverse order. So, first it >> should call pa_card_remove_profile(). That function doesn't exist >> currently, so it needs to be implemented. Next >> pa_alsa_card_pcm_removed() should free the pa_alsa_profile, >> pa_alsa_mapping and pa_alsa_path objects corresponding to the removed >> device. I think that's it for pa_alsa_card_pcm_removed(). >> >> If the profile that is being removed is active, pa_card_remove_profile() >> should change the card profile to something else. I suppose it should >> use the same logic as what pa_card_new() uses when choosing the default >> profile. >> > > What else is missing is the big picture of the available multi-monitor use cases. > > Currently, if one has a card with multiple HDMI outputs, one can use audio from one monitor at a time, using profiles. I.e. there are no profiles like "HDMI Digital Stereo Output + HDMI 3 Digital Surround Output" (with the possibility to move individual streams between monitors), presumably due to the combinatorial explosion. Will this limitation be lifted? > > Also, AFAIR, there was a discussion of a possibility to send the same PCM stream to two monitors. How would that work? <a href="http://www.intel.com/support/graphics/sb/CS-031040.htm">http://www.intel.com/support/graphics/sb/CS-031040.htm</a> The multi display monitors can be configured by graphic driver in clone mode or extended display mode Are there hardware limitation (e.g. bandwidth ) which limit the highest sample rate and 32bits cannot be used by multi monitors at same time ? Are there any different in primary display , second display and third display ? Will th power off/on of the monitor in the middle of dasiy chain affect the other displays of the dasiy chain ? <a href="http://www.intel.com/support/graphics/sb/CS-033714.htm">http://www.intel.com/support/graphics/sb/CS-033714.htm</a> How do pulseaudio/driver know multi displays are in clone node or independent display mode ?