ALSA: pcm: reinvent the stream synchronization ID API

Until the commit e11f0f90a626 ("ALSA: pcm: remove SNDRV_PCM_IOCTL1_INFO
internal command"), there was a possibility to pass information
about the synchronized streams to the user space. The mentioned
commit removed blindly the appropriate code with an irrelevant comment.

The revert may be appropriate, but since this API was lost for several
years without any complains, it's time to improve it. The hardware
parameters may change the used stream clock source (e.g. USB hardware)
so move this synchronization ID to hw_params as read-only field.

It seems that pipewire can benefit from this API (disable adaptive
resampling for perfectly synchronized PCM streams) now.

Note that the contents of ID is not supposed to be used for direct
comparison with a specific byte sequence. The "empty" case is when
all bytes are zero (driver does not offer this information)
and all other cases must be only used for equal comparison among
PCM streams (including different sound cards) if they are using
identical hardware clock.

Cc: Takashi Sakamoto <takaswie@kernel.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20240625172836.589380-2-perex@perex.cz

diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 3edd7a7346daaaf30ddb44e1151d8caae8371a64..dbce137d8806bc952f7f3b13d23fbafd14fc2493 100644 (file)
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -93,6 +93,7 @@ struct snd_pcm_ops {
 #define SNDRV_PCM_IOCTL1_CHANNEL_INFO  2
 /* 3 is absent slot. */
 #define SNDRV_PCM_IOCTL1_FIFO_SIZE     4
+#define SNDRV_PCM_IOCTL1_SYNC_ID       5
 
 #define SNDRV_PCM_TRIGGER_STOP         0
 #define SNDRV_PCM_TRIGGER_START                1
@@ -401,7 +402,7 @@ struct snd_pcm_runtime {
        snd_pcm_uframes_t silence_start; /* starting pointer to silence area */
        snd_pcm_uframes_t silence_filled; /* already filled part of silence area */
 
-       union snd_pcm_sync_id sync;     /* hardware synchronization ID */
+       unsigned char sync[16];         /* hardware synchronization ID */
 
        /* -- mmap -- */
        struct snd_pcm_mmap_status *status;

diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index 628d46a0da92eb0393dd592a38e987d08dcf6db0..8bf7e8a0eb6f03a8026e07f1968568fb099e7c9f 100644 (file)
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -142,7 +142,7 @@ struct snd_hwdep_dsp_image {
  *                                                                           *
  *****************************************************************************/
 
-#define SNDRV_PCM_VERSION              SNDRV_PROTOCOL_VERSION(2, 0, 17)
+#define SNDRV_PCM_VERSION              SNDRV_PROTOCOL_VERSION(2, 0, 18)
 
 typedef unsigned long snd_pcm_uframes_t;
 typedef signed long snd_pcm_sframes_t;
@@ -334,7 +334,7 @@ union snd_pcm_sync_id {
        unsigned char id[16];
        unsigned short id16[8];
        unsigned int id32[4];
-};
+} __attribute__((deprecated));
 
 struct snd_pcm_info {
        unsigned int device;            /* RO/WR (control): device number */
@@ -348,7 +348,7 @@ struct snd_pcm_info {
        int dev_subclass;               /* SNDRV_PCM_SUBCLASS_* */
        unsigned int subdevices_count;
        unsigned int subdevices_avail;
-       union snd_pcm_sync_id sync;     /* hardware synchronization ID */
+       unsigned char pad1[16];         /* was: hardware synchronization ID */
        unsigned char reserved[64];     /* reserved for future... */
 };
 
@@ -420,7 +420,8 @@ struct snd_pcm_hw_params {
        unsigned int rate_num;          /* R: rate numerator */
        unsigned int rate_den;          /* R: rate denominator */
        snd_pcm_uframes_t fifo_size;    /* R: chip FIFO size in frames */
-       unsigned char reserved[64];     /* reserved for future */
+       unsigned char sync[16];         /* R: synchronization ID (perfect sync - one clock source) */
+       unsigned char reserved[48];     /* reserved for future */
 };
 
 enum {

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index 071c67cbc479051eca440d66bb919c75e564c79b..f64a03f79b01e07c0562c35fc67173da1227b41d 100644 (file)
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -525,10 +525,8 @@ void snd_pcm_set_sync(struct snd_pcm_substream *substream)
 {
        struct snd_pcm_runtime *runtime = substream->runtime;
        
-       runtime->sync.id32[0] = substream->pcm->card->number;
-       runtime->sync.id32[1] = -1;
-       runtime->sync.id32[2] = -1;
-       runtime->sync.id32[3] = -1;
+       *(__u32 *)runtime->sync = cpu_to_le32(substream->pcm->card->number);
+       memset(runtime->sync + 4, 0xff, sizeof(runtime->sync) - 4);
 }
 EXPORT_SYMBOL(snd_pcm_set_sync);
 
@@ -1810,6 +1808,25 @@ static int snd_pcm_lib_ioctl_fifo_size(struct snd_pcm_substream *substream,
        return 0;
 }
 
+/**
+ * is sync id (clock id) empty?
+ */
+static inline bool pcm_sync_empty(const unsigned char *sync)
+{
+       return sync[0] == 0 && sync[1] == 0 && sync[2] == 0 && sync[3] == 0 &&
+              sync[4] == 0 && sync[5] == 0 && sync[6] == 0 && sync[7] == 0;
+}
+
+static int snd_pcm_lib_ioctl_sync_id(struct snd_pcm_substream *substream,
+                                    void *arg)
+{
+       struct snd_pcm_hw_params *params = arg;
+
+       if (pcm_sync_empty(params->sync))
+               memcpy(params->sync, substream->runtime->sync, sizeof(params->sync));
+       return 0;
+}
+
 /**
  * snd_pcm_lib_ioctl - a generic PCM ioctl callback
  * @substream: the pcm substream instance
@@ -1831,6 +1848,8 @@ int snd_pcm_lib_ioctl(struct snd_pcm_substream *substream,
                return snd_pcm_lib_ioctl_channel_info(substream, arg);
        case SNDRV_PCM_IOCTL1_FIFO_SIZE:
                return snd_pcm_lib_ioctl_fifo_size(substream, arg);
+       case SNDRV_PCM_IOCTL1_SYNC_ID:
+               return snd_pcm_lib_ioctl_sync_id(substream, arg);
        }
        return -ENXIO;
 }

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index c152ccf32214fc9712bfb1aa2bf5d4281581a2a7..4057f9f10aeec250ae9b50f168f2c096f6c77e71 100644 (file)
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -533,6 +533,12 @@ static int fixup_unreferenced_params(struct snd_pcm_substream *substream,
                                          SNDRV_PCM_INFO_MMAP_VALID);
        }
 
+       err = snd_pcm_ops_ioctl(substream,
+                               SNDRV_PCM_IOCTL1_SYNC_ID,
+                               params);
+       if (err < 0)
+               return err;
+
        return 0;
 }
 

diff --git a/sound/pci/emu10k1/p16v.c b/sound/pci/emu10k1/p16v.c
index e7f097cae574e63a3af636cee8fda50f29428a16..773725dbdfbd608bd1d3fa37f74587633cab2a1e 100644 (file)
--- a/sound/pci/emu10k1/p16v.c
+++ b/sound/pci/emu10k1/p16v.c
@@ -174,10 +174,9 @@ static int snd_p16v_pcm_open_playback_channel(struct snd_pcm_substream *substrea
        if (err < 0)
                 return err;
 
-       runtime->sync.id32[0] = substream->pcm->card->number;
-       runtime->sync.id32[1] = 'P';
-       runtime->sync.id32[2] = 16;
-       runtime->sync.id32[3] = 'V';
+       *(__u32 *)runtime->sync = cpu_to_le32(substream->pcm->card->number);
+       memset(runtime->sync + 4, 0, sizeof(runtime->sync) - 4);
+       strncpy(runtime->sync + 4, "P16V", 4);
 
        return 0;
 }