WINEALSA: remove crackling in DirectSound/ALSA combination

[Alex Villací­s Lasso]

For a long time, I have been annoyed by a faint crackling that can be 
heard on the sound output of any DirectSound program when full hardware 
acceleration is enabled on the ALSA driver. Even after a patch I sent 
earlier (which corrected a bigger artifact), this crackling remained. 
Last night, I finally set upon removing this crackling, based on a 
comment in IDsDriverBufferImpl_GetPosition() in 
dlls/winmm/winealsa/audio.c, line 3302:

    //* /*FIXME*/: snd_pcm_mmap_hw_ptr() should not be accessed by a user app. *//
    //*        It will NOT return what why want anyway. *//
    hw_ptr = _snd_pcm_mmap_hw_ptr(wwo->pcm);

I thought, maybe it is not returning the correct information for 
GetPosition, so that is why the crackling exists, since new samples are 
off-position from what they should be in order to guarantee a smooth 
sound. So I deduced, maybe the IDsDriverBufferImpl object can keep a 
simulation of a hardware pointer and use that instead of the (slightly 
incorrect) hardware pointer. My original intention was to remove 
entirely the need to reference _snd_pcm_mmap_hw_ptr(), since this is 
obviously an internal alsa-lib function and should not be used from 
inside the app (as the comment rightly indicates). However, I found out 
that the simulated hardware pointer tends to lag behind the real 
hardware pointer, and horrible mixups happen if the difference 
approaches or exceeds the length of the mmap buffer. So the simulated 
pointer is loosely kept close to the hardware pointer in the supplied 
patch.

The good news: the patch sort of works (in my setup, at least, with 
Fedora Core 4). All the games I have (Japanese RPGs) now have smooth 
sound, unless the CPU load is too high.
The bad news: the patch does nothing to make the dsound tests pass in 
Wine (but they were already failing before the patch :-)

What I would like to know is the following:
* Is there any ALSA user for which the DirectSound tests have *actually* 
passed? In full acceleration mode?
* Can somebody check if the DirectSound tests actually pass in Windows? 
The test code seems very straightforward, but (in my setup at least) 
they trigger many more errors and FIXMEs than my RPGs, and that is even 
without taxing the CPU with graphics and bitmap manipulation, as the 
RPGs do.
* Can somebody using the ALSA driver acknowledge whether they have the 
crackling problem too, and whether this patch actually solves it for 
them too?
* Can somebody please test this patch (even if they don't have the 
crackling issue) and check whether any previously working game breaks 
because of it?

Also, can some ALSA or DirectSound guru comment on the specifics of 
*why* the patch actually works? I am not entirely convinced that a mere 
loose updating of the simulated pointer from the hardware pointer is 
enough to have a smooth sound, but it works. I also don't understand how 
did the crackling appear in the first place, even though the direct 
hardware pointer was used.

Any comments or reports are welcome (especially why the patch works).

Alex Villacís Lasso

Changelog:
* Remove crackling in DirectSound/ALSA playback with full hardware 
acceleration

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