audio: make playback packet length calculation exact

Introduce the new function st_rate_frames_in() to calculate the
exact number of audio input frames needed to get a given number
of audio output frames. The exact number of frames depends only
on the difference of opos - ipos and the number of output frames.
When downsampling, this function returns the maximum number of
input frames needed.

This new function replaces the audio_frontend_frames_out() function,
which calculated the average number of input frames rounded down
to the nearest integer. Because audio_frontend_frames_out() also
limited the number of input frames to the size of the resample
buffer, st_rate_frames_in() is not a direct replacement and two
additional MIN() functions are needed. One to prevent resample
buffer overflows and one to limit the available bytes for the audio
frontends.

After this patch the audio packet length calculation for playback is
exact. When upsampling, it's still possible that the audio frontends
can't write the last audio frame. This will be fixed later.

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20230224190555.7409-9-vr_qemu@t-online.de>

1 files changed, 39 insertions, 0 deletions

diff --git a/audio/mixeng.c b/audio/mixeng.c
index fe454e0..a24c8c4 100644
--- a/audio/mixeng.c
+++ b/audio/mixeng.c
@@ -440,6 +440,45 @@ void st_rate_stop (void *opaque)
     g_free (opaque);
 }
 
+/**
+ * st_rate_frames_in() - returns the number of frames needed to
+ * get frames_out frames after resampling
+ *
+ * @opaque: pointer to struct rate
+ * @frames_out: number of frames
+ *
+ * When downsampling, there may be more than one correct result. In this
+ * case, the function returns the maximum number of input frames needed.
+ */
+uint32_t st_rate_frames_in(void *opaque, uint32_t frames_out)
+{
+    struct rate *rate = opaque;
+    uint64_t opos_start, opos_end;
+    uint32_t ipos_start, ipos_end;
+
+    if (rate->opos_inc == 1ULL << 32) {
+        return frames_out;
+    }
+
+    if (frames_out) {
+        opos_start = rate->opos;
+        ipos_start = rate->ipos;
+    } else {
+        uint64_t offset;
+
+        /* add offset = ceil(opos_inc) to opos and ipos to avoid an underflow */
+        offset = (rate->opos_inc + (1ULL << 32) - 1) & ~((1ULL << 32) - 1);
+        opos_start = rate->opos + offset;
+        ipos_start = rate->ipos + (offset >> 32);
+    }
+    /* last frame written was at opos_start - rate->opos_inc */
+    opos_end = opos_start - rate->opos_inc + rate->opos_inc * frames_out;
+    ipos_end = (opos_end >> 32) + 1;
+
+    /* last frame read was at ipos_start - 1 */
+    return ipos_end + 1 > ipos_start ? ipos_end + 1 - ipos_start : 0;
+}
+
 void mixeng_clear (struct st_sample *buf, int len)
 {
     memset (buf, 0, len * sizeof (struct st_sample));