multi-channel playback regression

[Wu Fengguang <fengguang.wu@intel.com>]

Hi Takashi,

When doing multi-channel playback tests on IbexPeak, I found that the
following patch makes the playback enter an infinite loop, repeatedly
playing a range of ~0.5s audio content. (Seems that some buffer
pointer can never advance.)

Thanks,
Fengguang
---

79452f0a28aa5a40522c487b42a5fc423647ad98
Author: Takashi Iwai <tiwai@suse.de>
Date:   Wed Jul 22 12:51:51 2009 +0200

    ALSA: pcm - Fix regressions with VMware

    VMware tends to report PCM positions and period updates at utterly
    wrong timing.  This screws up the recent PCM core code that tries
    to correct the position based on the irq timing.

    Now, when a backward irq position is detected, skip the update
    instead of rebasing.  (This is almost the old behavior before
    2.6.30.)
    
    Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index 333e4dd..3b673e2 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -244,18 +244,27 @@ static int snd_pcm_update_hw_ptr_interrupt(struct snd_pcm_substream *substream)
                        delta = new_hw_ptr - hw_ptr_interrupt;
        }
        if (delta < 0) {
-               delta += runtime->buffer_size;
+               if (runtime->periods == 1)
+                       delta += runtime->buffer_size;
                if (delta < 0) {
                        hw_ptr_error(substream,
                                     "Unexpected hw_pointer value "
                                     "(stream=%i, pos=%ld, intr_ptr=%ld)\n",
                                     substream->stream, (long)pos,
                                     (long)hw_ptr_interrupt);
+#if 1
+                       /* simply skipping the hwptr update seems more
+                        * robust in some cases, e.g. on VMware with
+                        * inaccurate timer source
+                        */
+                       return 0; /* skip this update */
+#else                  
                        /* rebase to interrupt position */
                        hw_base = new_hw_ptr = hw_ptr_interrupt;
                        /* align hw_base to buffer_size */
                        hw_base -= hw_base % runtime->buffer_size;