ALSA: hda/hdmi - implement mst_no_extra_pcms flag

To support the DP-MST multiple streams via single connector feature,
the HDMI driver was extended with the concept of backup PCMs. See
commit 9152085d ("ALSA: hda - add DP MST audio support").

This implementation works fine with snd_hda_intel.c as PCM topology
is fully managed within the single driver.

When the HDA codec driver is used from ASoC components, the concept
of backup PCMs no longer fits. For ASoC topologies, the physical
HDMI converters are presented as backend DAIs and these should match
with hardware capabilities. The ASoC topology may define arbitrary
PCMs (i.e. frontend DAIs) and have processing elements before eventual
routing to the HDMI BE DAIs. With backup PCMs, the link between
FE and BE DAIs would become dynamic and change when monitors are
(un)plugged. This would lead to modifying the topology every time
hotplug events happen, which is not currently possible in ASoC and
there does not seem to be any obvious benefits from this design.

To overcome above problems and enable the HDMI driver to be used
from ASoC, this patch adds a new mode (mst_no_extra_pcms flags) to
patch_hdmi.c. In this mode, the codec driver does not assume
the backup PCMs to be created.
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191029134017.18901-2-kai.vehmanen@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/hda_codec.h

@@ -254,6 +254,7 @@ struct hda_codec {
 	unsigned int force_pin_prefix:1; /* Add location prefix */
 	unsigned int link_down_at_suspend:1; /* link down at runtime suspend */
 	unsigned int relaxed_resume:1;	/* don't resume forcibly for jack */
+	unsigned int mst_no_extra_pcms:1; /* no backup PCMs for DP-MST */
 
 #ifdef CONFIG_PM
 	unsigned long power_on_acct;

sound/pci/hda/patch_hdmi.c

@@ -2072,15 +2072,24 @@ static bool is_hdmi_pcm_attached(struct hdac_device *hdac, int pcm_idx)
 static int generic_hdmi_build_pcms(struct hda_codec *codec)
 {
 	struct hdmi_spec *spec = codec->spec;
-	int idx;
+	int idx, pcm_num;
 
 	/*
 	 * for non-mst mode, pcm number is the same as before
-	 * for DP MST mode, pcm number is (nid number + dev_num - 1)
-	 *  dev_num is the device entry number in a pin
-	 *
+	 * for DP MST mode without extra PCM, pcm number is same
+	 * for DP MST mode with extra PCMs, pcm number is
+	 *  (nid number + dev_num - 1)
+	 * dev_num is the device entry number in a pin
 	 */
-	for (idx = 0; idx < spec->num_nids + spec->dev_num - 1; idx++) {
+
+	if (codec->mst_no_extra_pcms)
+		pcm_num = spec->num_nids;
+	else
+		pcm_num = spec->num_nids + spec->dev_num - 1;
+
+	codec_dbg(codec, "hdmi: pcm_num set to %d\n", pcm_num);
+
+	for (idx = 0; idx < pcm_num; idx++) {
 		struct hda_pcm *info;
 		struct hda_pcm_stream *pstr;