[wdmaudiodev] Re: [EXTERNAL] Understanding IMiniportWaveRtStreamNotification::RegisterNotificationEvent

  • From: wdmaudiodev@xxxxxxxxxxxxxx
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Sat, 31 Oct 2020 06:25:52 -0700

This driver creates a virtual capture endpoint ("microphone") that third-party chat apps like Zoom or Skype will consume.

So whenever the capture stream requests data, if there is a new packet available from the render stream, I copy it across. If the render stream isn't in RUN mode, then I silence-fill the capture buffer.


On 10/30/20 9:45 AM, Matthew van Eerde (Redacted sender Matthew.van.Eerde for DMARC) wrote:

Who consumes the data that Windows writes to you? Do they have a clock?

*From: *wdmaudiodev@xxxxxxxxxxxxxx <mailto:wdmaudiodev@xxxxxxxxxxxxxx>
*Sent: *Friday, October 30, 2020 9:26 AM
*To: *wdmaudiodev@xxxxxxxxxxxxx <mailto:wdmaudiodev@xxxxxxxxxxxxx>
*Subject: *[wdmaudiodev] Re: [EXTERNAL] Understanding IMiniportWaveRtStreamNotification::RegisterNotificationEvent

> Please don’t do this.

I'd rather go with the prevailing wisdom and do it 'properly', vs. produce another crappy, battery-draining Windows driver. So, OK, no timer.

SYSVAD is the only example of this that I have, and it says (paraphrasing) "in a real driver, you wouldn't use a timer like we show here; your DMA hardware and position registers would handle ...".

My driver is virtual, though, so there is no hardware. It just copies packets from RENDER[1] to CAPTURE, without buffering.

I could create a worker thread that waits on both render-data-ready and capture-buffer-available events. I don't have any driver-specific examples of this, but I've done this in desktop apps in the past.

Any suggestions/advice? Any insight on the SetWritePacket/SetReadPacket question?


[1] Or, if I can get a custom interface working, copy data passed-from-app-to-driver via backchannel

On 10/30/20 8:44 AM, Matthew van Eerde (Redacted sender Matthew.van.Eerde for DMARC) wrote:

      * I have a timer function that monitors this

    Please don’t do this.

    *From: *wdmaudiodev@xxxxxxxxxxxxxx <mailto:wdmaudiodev@xxxxxxxxxxxxxx>
    *Sent: *Friday, October 30, 2020 6:31 AM
    *To: *wdmaudiodev@xxxxxxxxxxxxx <mailto:wdmaudiodev@xxxxxxxxxxxxx>
    *Subject: *[EXTERNAL] [wdmaudiodev] Understanding


    I was a bit confused by the function of the
    IMiniportWaveRtOutputStream::SetWritePacket and
    IMiniportWaveRtInputStream::SetReadPacket methods, and the KEVENT
    passed into RegisterNotificationEvent.

    If I understand it correctly, AllocateBufferWithNotification gets
    called with notificationCount=2, and a buffer large enough for two
    'packets'. Packets seem to be sized to contain about 1/100 sec
    worth of audio frames. RegisterNotificationEvent gets called with
    a single (in my case) event.

    In the RENDER case, SetWritePacket(n, ...) is called when the OS
    has finished copying packet <n> into whichever half of the buffer.
    I have a timer function that monitors this, and transfers the
    packet to hardware (let's say). To let the OS know that packet <n>
    has been processed (i.e., that packet in the buffer is available),
    I call KeSetEvent(pNotificationEvent, 0, FALSE).

    So this part isn't clear to me:

        ...kernel event that must be notified as DMA progresses.
        Depending on the notification count parameter used with
        the registered event is signaled __one or two times__ for
        every time that the DMA passes through the audio buffer.

    I expected there to be two calls to RegisterNotificationEvent, and
    thus two KEVENT objects at any given time (one for each packet in
    the buffer). Instead, I only see one getting added. The doc quote
    above seems to suggest that a single event has to be set TWICE (or
    maybe set once, then the OS clears, then set again)? I've looked
    at the KEVENT struct and related APIs, but I'm still confused. Is
    this event supposed to signify "Buffer Complete" or "Packet Complete"?

    In the CAPTURE case, it looks like things are reversed. The OS
    calls SetReadPacket to query the driver about which packet is
    available for reading (transferring to client).
    RegisterNotificationEvent is getting called, so I think the same
    (single) event is SET/CLEARED? by the OS to signal ME that it has
    finished transferring data out of a packet, and that packet is
    available for DMA again.

    Thanks in advance for any and all help. :-)

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