Dne 24. 05. 21 v 18:09 Pavel Hofman napsal(a):
Dne 24. 05. 21 v 17:40 Jerome Brunet napsal(a):On Mon 24 May 2021 at 14:29, Pavel Hofman <pavel.hofman@xxxxxxxxxxx> wrote:Dne 30. 04. 21 v 16:26 Jerome Brunet napsal(a):From: Ruslan Bilovol <ruslan.bilovol@xxxxxxxxx>This adds interface between userspace and feedback endpoint to report realfeedback frequency to the Host. Current implementation adds new userspace interface ALSA mixer control"Capture Pitch 1000000" (similar to aloop driver's "PCM Rate Shift 100000"mixer control)Value in PPM is chosen to have correction value agnostic of the actual HW rate, which the application is not necessarily dealing with, while still retaining a good enough precision to allow smooth clock correction on theplayback side, if necessary. Similar to sound/usb/endpoint.c, a slow down is allowed up to 25%. Thishas no impact on the required bandwidth. Speedup correction has an impacton the bandwidth reserved for the isochronous endpoint. The default allowed speedup is 500ppm. This seems to be more than enough but, ifnecessary, this is configurable through a module parameter. The reservedbandwidth is rounded up to the next packet size.Usage of this new control is easy to implement in existing userspace toolslike alsaloop from alsa-utils. Signed-off-by: Ruslan Bilovol <ruslan.bilovol@xxxxxxxxx> Signed-off-by: Jerome Brunet <jbrunet@xxxxxxxxxxxx>Hi, the existing patches solve the Host -> Gadget -> capturing application direction, controlling the host playback rate. The other direction (playback app -> gadget -> capturing host) is still paced by the host USB controller. Packet size is pre-calculated in u_audio_start_playback as rate/p_intervalhttps://github.com/pavhofman/linux-rpi/blob/rpi-5.10.y/drivers/usb/gadget/function/u_audio.c#L441and this fixed value is used for copying the audio data in u_audio_iso_completehttps://github.com/pavhofman/linux-rpi/blob/rpi-5.10.y/drivers/usb/gadget/function/u_audio.c#L124. That means if the gadget has a physical duplex audio device with single clock and runs a duplex operation, the path gadget-> host will not run synchronously with the physical audio device (the host -> gadget has already the feedback control implemented). How about "duplicating" the existing ALSA mixer control "Capture Pitch 1000000" to "Playback Pitch 1000000" and using pitch-adjusted p_srate in the above-linked calculations? That should make the playback side run at the playback pitch requested by gadget userspace, IIUC. For the duplex operation with single clock, the capture pitch value determined by the userspace chain (alsaloop, CamillaDSP, etc.) would be used for setting both the capture and playback pitch controls, making both directions run synchronously. I can prepare patches based on Jerome's patchset should you find this solution acceptable.Well I have experimented with pitch on the playback path. It does work but I'm not sure if it actually makes sense which is why I have not not included it in RFC If you need the playback and capture to run synchronously, you'd be better off with implicit feedback (In which the host will provide the same number of samples it received from the device) With explicit feedback, we are (supposed) to tell the host to speed up or slow down to match the device clock. This means the device run asynchronously, and the host does the adaptation (if necessary). In such case, I'm not sure adding pitch control on the playback path is a good idea. Having pitch control on the playback side allows to forward the audio captured by the physical interface of the device (like I2S) to USB without performing any resampling between the two (when USB and I2S are not in sync). It's nice and convenient ... but I wonder if this is a feature or a hack ;)Jerome, thanks a lot for your reply. The current implementation of the EP IN audio direction is timed by the USB host controller. Let's consider 48Khz samplerate and bInterval=1 fullspeed (8k packets per second). Every IN packet will always carry 6 audio frames. No matter how fast the real master clock (i.e. e.g. I2S of the gadget) runs. Until an xrun occurs, unfortunately. Even if the gadget configuration used implicit feedback, the incoming stream would still provide no synchronization information from the I2S hardware clock as the data stream is clocked by the USB host which controls the timing on the USB bus (and thus the moment of iso completion). Plus the stock usb-audio driver in Windows 10 does not support implicit feedback, according to https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/usb-2-0-audio-drivers#audio-streaming .The problem is the fixed "slicing" of the samples into the packets which cannot be controlled. The same situation was on the host side before the feedback EP was introduced. Here the one preparing the slices (the "transmitter") is the gadget now. And the slicing pace must be controllable just like the slicing pace on the host is via the feedback EP.Because the gadget supports different playback and capture rates (which I find nice), I suggest a separate control element (Playback Pitch, Capture Pitch).Of course the motivation is to avoid adaptive resampling in the gadget in the direction I2S -> gadget interface -> USB host. But the very same motivation lead to implementation of the async feedback EP in the opposite direction. IMO it is not a hack, but an indispensable feature making the gadget usable for duplex operation with hardware (i.e. the real master) clock at the backend (all the other "clocks" are just software-generated slices/blocks of audio frames).With regards, Pavel.
Jerome, please do you still have your playback-side patches available or should I prepare them? Thanks a lot,
Pavel.
