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>