Re: ALSA-LIB: Support for format IEC958_SUBFRAME_LE in the plug plugin?

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Dne 02. 02. 24 v 9:00 Pavel Hofman napsal(a):

Dne 30. 01. 24 v 20:22 Pavel Hofman napsal(a):

Dne 30. 01. 24 v 13:30 Jaroslav Kysela napsal(a):

It looks like a way to go. The hdmi_mode can be set using ALSA
configuration use snd_config_search (with global config - snd_config
pointer) and snd_config_get_bool functions for that. The variable may be
named like 'defaults.pcm.plug.iec958.hdmi_mode' or so (see alsa.conf).

That's interesting. IIUC such parameter would globally switch all plugs
instances to the hdmi_mode. Would a user-based ~/.asoundrc with such
variable be applied for the hard-coded plughw:XX device? Maybe it would
be enough for most use cases, eliminating the need for an app-specific
environment variable.

Or maybe a prioritized sequence getenv('ALSA_PCM_PLUG_IEC958_HDMI_MODE') -> snd_config_search('defaults.pcm.plug.iec958.hdmi_mode') could be used.
We usually don't use getenv hacks for alsa-lib features. The global configuration path can be redirected using ALSA_CONFIG_DIR and per user (~/.asoundrc or $XDG_CONFIG_HOME/alsa/asoundrc) and per card configurations (/var/lib/alsa/card#.conf.d) are also loaded from the global alsa.conf file.

Perphaps, the iec958 plug code may include card number / card driver name to the configuration tree lookup - like 'defaults.pcm.plug.iec958.0.hdmi_mode' or 'defaults.pcm.plug.iec958.vc4-hdmi.hdmi_mode' . With this extension, this value can be set in global src/conf/cards/vc4-hdmi.conf for this hw.

Actually, looking at pcm_iec958.c and the commit which introduced the hdmi_mode param I am not sure the the hdmi_mode is of any value for the plug plugin.

IIUC the hdmi_mode value gets used only if the status is IEC958_AES0_NONAUDIO:

int single_stream = iec->hdmi_mode &&
                 (iec->status[0] & IEC958_AES0_NONAUDIO) &&
                 (channels == 8);

But the plug plugin would pass NULL as status_bits which in snd_pcm_iec958_open will be replaced with default_status_bits:

static const unsigned char default_status_bits[] = {
         IEC958_AES0_CON_EMPHASIS_NONE,
         IEC958_AES1_CON_ORIGINAL | IEC958_AES1_CON_PCM_CODER,
         0,
         IEC958_AES3_CON_FS_NOTID, /* will be set in hwparams */
         IEC958_AES4_CON_WORDLEN_NOTID /* will be set in hwparams */
     };

Logically no IEC958_AES0_NONAUDIO bit is set in the default status bits.

IMO we can safely pass hdmi_mode=false to snd_pcm_iec958_open because using plug for non-audio stream would not make sense anyway.


A patch related to the conversion could look like this, IMO.

===================================================================
diff --git a/src/pcm/pcm_plug.c b/src/pcm/pcm_plug.c
--- a/src/pcm/pcm_plug.c    (revision ffed4f342678c31bf0b9edfe184be5f3de41603a)
+++ b/src/pcm/pcm_plug.c    (date 1706857992580)
@@ -490,6 +490,20 @@
  }
  #endif

+#ifdef BUILD_PCM_PLUGIN_IEC958
+static int snd_pcm_plug_iec958_open(snd_pcm_t **pcmp, const char *name, snd_pcm_format_t sformat, snd_pcm_t *slave, int close_slave)
+{
+    return snd_pcm_iec958_open(pcmp, name, sformat, slave, close_slave,
+        /* using default status bits defined in the iec958 plugin*/
+        NULL,
+        /* default preamble values as used in the iec958 plugin */
+        {0x08, 0x02, 0x04 /* Z, X, Y */},
+        /* hdmi_mode=0 because it is applied only for IEC958_AES0_NONAUDIO which is not in the default status bits */
+        0
+    );
+}
+#endif
+
 static int snd_pcm_plug_change_format(snd_pcm_t *pcm, snd_pcm_t **new, snd_pcm_plug_params_t *clt, snd_pcm_plug_params_t *slv)
  {
      snd_pcm_plug_t *plug = pcm->private_data;
@@ -565,6 +579,12 @@
  #ifdef BUILD_PCM_PLUGIN_ADPCM
          case SND_PCM_FORMAT_IMA_ADPCM:
              f = snd_pcm_adpcm_open;
+            break;
+#endif
+#ifdef BUILD_PCM_PLUGIN_IEC958
+        case SND_PCM_FORMAT_IEC958_SUBFRAME_LE:
+        case SND_PCM_FORMAT_IEC958_SUBFRAME_BE:
+            f = snd_pcm_plug_iec958_open;
              break;
  #endif
          default:

===================================================================


Unfortunately I am afraid I do not understand fully the code in snd_pcm_plug_hw_refine_schange which calls method snd_pcm_plug_slave_format where the IEC958 formats should also be checked.


I am trying to understand the code logic in snd_pcm_plug_hw_refine_cchange and snd_pcm_plug_hw_refine_schange as there are no comments at all, to no avail.

Please what is the meaning of local variable f in this snippet of snd_pcm_plug_hw_refine_cchange https://github.com/alsa-project/alsa-lib/blob/2a736a0d2543f206fd2653aaae8a08a4c42eb917/src/pcm/pcm_plug.c#L909-L921 ?

for (format = 0; format <= SND_PCM_FORMAT_LAST; format++) {
	snd_pcm_format_t f;
	if (!snd_pcm_format_mask_test(format_mask, format))
		continue;
	if (snd_pcm_format_mask_test(sformat_mask, format))
		f = format;
	else {
		f = snd_pcm_plug_slave_format(format, sformat_mask);
		if (f == SND_PCM_FORMAT_UNKNOWN)
			continue;
	}
	snd_pcm_format_mask_set(&fmt_mask, format);
}

The snd_pcm_plug_hw_refine_schange method sets the f to the fmt_mask in https://github.com/alsa-project/alsa-lib/blob/2a736a0d2543f206fd2653aaae8a08a4c42eb917/src/pcm/pcm_plug.c#L831 , but not in https://github.com/alsa-project/alsa-lib/blob/2a736a0d2543f206fd2653aaae8a08a4c42eb917/src/pcm/pcm_plug.c#L920

Please what is the meaning/contract of the method snd_pcm_plug_slave_format https://github.com/alsa-project/alsa-lib/blob/2a736a0d2543f206fd2653aaae8a08a4c42eb917/src/pcm/pcm_plug.c#L252 ? What is the relation between input format and output format?

Thanks a lot in advance.




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