[wdmaudiodev] Re: SetNotificationFreq() has no effect on CopyFrom() ???
- From: Ken Cooper <Ken.Cooper@xxxxxxxxxxxxx>
- To: "wdmaudiodev@xxxxxxxxxxxxx" <wdmaudiodev@xxxxxxxxxxxxx>
- Date: Mon, 4 Jun 2007 14:44:27 -0700
First of all, CopyTo() and CopyFrom() are the same routines. The differences
you are seeing are a result of the usage differences in render versus capture.
WaveCyclic was written back in the Win98 days and uses a cyclic DMA buffer that
the audio DMA reads or writes to transfer data to or from the codec. IRPs are
used as the data transport down the audio driver stack and the data is
transferred between the IRPs and the cyclic DMA buffer using a deferred
procedure call (DPC). When run, the DPC looks at the current DMA position and,
based on knowledge of the previous buffer read/write position, determines how
many audio frames to transfer between the cyclic DMA buffer and available
streaming IRPs. The DPC calls CopyTo or CopyFrom to copy the data and then
updates its buffer read/write position info.
There are several factors that can contribute to getting multiple CopyTo or
CopyFrom calls in a single DPC cycle and for the variable transfer sizes.
*) If the desired transfer wraps around the end of the cyclic DMA buffer
the transfer will be broken into two transfers, one at the end of the buffer
and one at the beginning
*) If the desired transfer spans an IRP boundary the transfer will be
broken up into two transfers (or more depending on IRP packet sizes)
*) The DPC is scheduled to run each time the miniport notifies the port
driver. So, if you set the notification frequency to 20ms and notify the port
at that rate, the DPC will run at least every 20ms. However, the DPC is also
scheduled to run each time a new streaming IRP arrives (this reduces the chance
of starvation and glitches). As a result, you may see variable DPC scheduling
impacting DMA transfer sizes.
The asymmetry between render and capture are just artifacts of the different
data flow direction combined with the different timing of the three factors
above. I hope that this explanation answers your questions.
Regards,
Ken
-----Original Message-----
From: wdmaudiodev-bounce@xxxxxxxxxxxxx
[mailto:wdmaudiodev-bounce@xxxxxxxxxxxxx] On Behalf Of Don Bell
Sent: Monday, June 04, 2007 9:55 AM
To: wdmaudiodev@xxxxxxxxxxxxx
Subject: [wdmaudiodev] SetNotificationFreq() has no effect on CopyFrom() ???
In a driver that I have written based on the MSVAD (simple) sample, I
use the inherited IMiniportWaveCyclicStream::SetNotificationFreq() to
set the interval to 20ms.
This indeed sets correctly the interval in which the inherited
IDmaChannel::CopyFrom(), supplying exactly FrameSize bytes to read
every such call.
However, IDmaChannel::CopyFrom() arrives at a higher "rate" (up to 5
such calls every 20ms), with the ByteCount parameter being anywhere
between 16 and FrameSize (15 < ByteCount < FrameSize + 1).
As long as I understand that this behavior is consistent and in line
with Microsoft's design, I can deal with that. However, I was
surprised to discover this lack of symmetry between CopyTo and
CopyFrom (or "render" and "capture").
Could you please confirm that my observation is correct? i.e. there is
no way to set PortCls in such a way that IDmaChannel::CopyFrom()'s
third parameter (IN ULONG ByteCount) will always be caled with a
fixed value?
Also, some insight as to why this is implemented this way would very
appreciated. I am curious to understand the rational behind this.
Thanks,
Don
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