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

  • From: wdmaudiodev@xxxxxxxxxxxxxx
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Fri, 30 Oct 2020 09:25:12 -0700

> 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 IMiniportWaveRtStreamNotification::RegisterNotificationEvent


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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