[alsa-devel] Questions regarding workaround for VMware

[Bankim Bhavsar]

On Fri, Aug 28, 2009 at 12:51 PM, Takashi Iwai <tiwai at suse.de> wrote:
> At Fri, 28 Aug 2009 11:29:52 -0700,
> Bankim Bhavsar wrote:
>>
>> Takashi I came across a workaround for VMware in ALSA kernel code
>> In the comments you mention about inaccurate timer source.
>> Could you elaborate on the problem?
>
> The problem is that the irq timing emulated on vmware isn't always
> accurate.  The sound hardware is supposed to issue an irq at the
> programmed buffer period.  On VMware, this irq generation seems to be
> done based on the system timer (or alike), thus it's not generated
> at the accurate timing that the hardware should give.
>
> The driver has no idea whether it's on VM, thus assumed that the
> IRQ must come accurately more or less.  In the recent code, there are
> some additions to correct the DMA position more to believe the accuracy
> of the IRQ than the position calculated from the hardware register.
> This caused a regression on VMware.  My patch fixed that, at least,
> for VMware in the stance of previous versions.

Thanks for the explanation, Takashi. Sorry for the delayed response.

I tested with the fix for VMware with 2.6.31rc9 kernel. Sound playback
suffers from underruns and app terminates prematurely.
I've attached the output of "aplay sample.wav -vvv".

I compiled kernel with following config options turned on
CONFIG_SND_DEBUG=y
CONFIG_SND_DEBUG_VERBOSE=y
CONFIG_SND_PCM_XRUN_DEBUG=y

and set /proc/asound/card0/pcm0p/xrun_debug to 1.

I can't seem to find  ALSA xrun_debug log messages with dmesg. Am I
looking in the wrong place?

As far as the timing of sound IRQ goes, with our emulation of ENS1371
delay in firing interrupt ranges from 50-500 usecs. Is that large
enough to cause xruns with
recent changes?

Linux guests with kernel version 2.6.31+ running under VMware are
affected currently.

Let me know if you need more information.

>> Is there something specific you would like us to fix in our virtual
>> sound device or timer source?
>
> Well, the only question is how can we get the programmed sound IRQ
> more accurately...  If hrtimer with the fine timer source is available,
> this could be emulated well, I guess.

I'll consult our timer experts in VMware and get back to you on this question.

-- Bankim.
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