ALSA: emu10k1: refactor PCM playback address handling

Pull the special handling of extra voices out of
snd_emu10k1_pcm_init_voice(), simplify snd_emu10k1_playback_pointer(),
and make the logic overall clearer. Also, add verbose comments.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230517174256.3657060-9-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/pci/emu10k1/emupcm.c b/sound/pci/emu10k1/emupcm.c
index 0639183..89f7e85 100644 (file)
--- a/sound/pci/emu10k1/emupcm.c
+++ b/sound/pci/emu10k1/emupcm.c
@@ -269,9 +269,6 @@ static void snd_emu10k1_pcm_init_voice(struct snd_emu10k1 *emu,
                memcpy(send_amount, &mix->send_volume[tmp][0], 8);
        }
 
-       if (master) {
-               evoice->epcm->ccca_start_addr = start_addr + 64 - 3;
-       }
        if (stereo) {
                // Not really necessary for the slave, but it doesn't hurt
                snd_emu10k1_ptr_write(emu, CPF, voice, CPF_STEREO_MASK);
@@ -299,12 +296,7 @@ static void snd_emu10k1_pcm_init_voice(struct snd_emu10k1 *emu,
                pitch_target = PITCH_48000; /* Disable interpolators on emu1010 card */
        else 
                pitch_target = emu10k1_calc_pitch_target(runtime->rate);
-       if (extra)
-               snd_emu10k1_ptr_write(emu, CCCA, voice, (end_addr - 3) |
-                             emu10k1_select_interprom(pitch_target) |
-                             (w_16 ? 0 : CCCA_8BITSELECT));
-       else
-               snd_emu10k1_ptr_write(emu, CCCA, voice, (start_addr + 64 - 3) |
+       snd_emu10k1_ptr_write(emu, CCCA, voice,
                              emu10k1_select_interprom(pitch_target) |
                              (w_16 ? 0 : CCCA_8BITSELECT));
        /* Clear filter delay memory */
@@ -401,6 +393,7 @@ static int snd_emu10k1_playback_prepare(struct snd_pcm_substream *substream)
        snd_emu10k1_pcm_init_voice(emu, 1, 1, epcm->extra, w_16, false,
                                   start_addr, end_addr, NULL);
        start_addr >>= stereo;
+       epcm->ccca_start_addr = start_addr;
        end_addr = start_addr + runtime->buffer_size;
        snd_emu10k1_pcm_init_voice(emu, 1, 0, epcm->voices[0], w_16, stereo,
                                   start_addr, end_addr,
@@ -428,13 +421,8 @@ static int snd_emu10k1_efx_playback_prepare(struct snd_pcm_substream *substream)
        snd_emu10k1_pcm_init_voice(emu, 1, 1, epcm->extra, true, false,
                                   start_addr, start_addr + (channel_size / 2), NULL);
 
-       /* only difference with the master voice is we use it for the pointer */
-       snd_emu10k1_pcm_init_voice(emu, 1, 0, epcm->voices[0], true, false,
-                                  start_addr, start_addr + channel_size,
-                                  &emu->efx_pcm_mixer[0]);
-
-       start_addr += channel_size;
-       for (i = 1; i < NUM_EFX_PLAYBACK; i++) {
+       epcm->ccca_start_addr = start_addr;
+       for (i = 0; i < NUM_EFX_PLAYBACK; i++) {
                snd_emu10k1_pcm_init_voice(emu, 0, 0, epcm->voices[i], true, false,
                                           start_addr, start_addr + channel_size,
                                           &emu->efx_pcm_mixer[i]);
@@ -540,13 +528,45 @@ static void snd_emu10k1_playback_prepare_voices(struct snd_emu10k1 *emu,
                                                bool w_16, bool stereo,
                                                int channels)
 {
+       struct snd_pcm_substream *substream = epcm->substream;
+       struct snd_pcm_runtime *runtime = substream->runtime;
+       unsigned eloop_start = epcm->start_addr >> w_16;
+       unsigned loop_start = eloop_start >> stereo;
+       unsigned eloop_size = runtime->period_size;
+       unsigned loop_size = runtime->buffer_size;
        u32 sample = w_16 ? 0 : 0x80808080;
 
+       // To make the playback actually start at the 1st frame,
+       // we need to compensate for two circumstances:
+       // - The actual position is delayed by the cache size (64 frames)
+       // - The interpolator is centered around the 4th frame
+       loop_start += 64 - 3;
        for (int i = 0; i < channels; i++) {
                unsigned voice = epcm->voices[i]->number;
+               snd_emu10k1_ptr_write(emu, CCCA_CURRADDR, voice, loop_start);
+               loop_start += loop_size;
                snd_emu10k1_playback_fill_cache(emu, voice, sample, stereo);
        }
 
+       // The interrupt is triggered when CCCA_CURRADDR (CA) wraps around,
+       // which is ahead of the actual playback position, so the interrupt
+       // source needs to be delayed.
+       //
+       // In principle, this wouldn't need to be the cache's entire size - in
+       // practice, CCR_CACHEINVALIDSIZE (CIS) > `fetch threshold` has never
+       // been observed, and assuming 40 _bytes_ should be safe.
+       //
+       // The cache fills are somewhat random, which makes it impossible to
+       // align them with the interrupts. This makes a non-delayed interrupt
+       // source not practical, as the interrupt handler would have to wait
+       // for (CA - CIS) >= period_boundary for every channel in the stream.
+       //
+       // This is why all other (open) drivers for these chips use timer-based
+       // interrupts.
+       //
+       eloop_start += eloop_size - 3;
+       snd_emu10k1_ptr_write(emu, CCCA_CURRADDR, epcm->extra->number, eloop_start);
+
        // It takes a moment until the cache fills complete,
        // but the unmuting takes long enough for that.
 }
@@ -746,24 +766,27 @@ static snd_pcm_uframes_t snd_emu10k1_playback_pointer(struct snd_pcm_substream *
        struct snd_emu10k1 *emu = snd_pcm_substream_chip(substream);
        struct snd_pcm_runtime *runtime = substream->runtime;
        struct snd_emu10k1_pcm *epcm = runtime->private_data;
-       unsigned int ptr;
+       int ptr;
 
        if (!epcm->running)
                return 0;
+
        ptr = snd_emu10k1_ptr_read(emu, CCCA, epcm->voices[0]->number) & 0x00ffffff;
-#if 0  /* Perex's code */
-       ptr += runtime->buffer_size;
        ptr -= epcm->ccca_start_addr;
-       ptr %= runtime->buffer_size;
-#else  /* EMU10K1 Open Source code from Creative */
-       if (ptr < epcm->ccca_start_addr)
-               ptr += runtime->buffer_size - epcm->ccca_start_addr;
-       else {
-               ptr -= epcm->ccca_start_addr;
-               if (ptr >= runtime->buffer_size)
-                       ptr -= runtime->buffer_size;
-       }
-#endif
+
+       // This is the size of the whole cache minus the interpolator read-ahead,
+       // which leads us to the actual playback position.
+       //
+       // The cache is constantly kept mostly filled, so in principle we could
+       // return a more advanced position representing how far the hardware has
+       // already read the buffer, and set runtime->delay accordingly. However,
+       // this would be slightly different for every channel (and remarkably slow
+       // to obtain), so only a fixed worst-case value would be practical.
+       //
+       ptr -= 64 - 3;
+       if (ptr < 0)
+               ptr += runtime->buffer_size;
+
        /*
        dev_dbg(emu->card->dev,
               "ptr = 0x%lx, buffer_size = 0x%lx, period_size = 0x%lx\n",