It's nicer to do the check in startup() because then the application open() will fail cleanly. We could delay until prepare - which is the point we really need the hardware to be accessible - and hope the hardware enumerated and initialized by that time. But that's not so nice from the app point of view.
Another way to avoid problems is to rely on the codec component .probe to check if the SoundWire device is initialized before registering a card.
I just tried with a system where the ACPI info exposes a codec which is not connected, it fails nicely. That avoids the pitfalls of creating a card which isn't functional since all dependencies are not met.
[ 64.616530] snd_soc_sof_sdw:mc_probe: sof_sdw sof_sdw: Entry [ 64.616549] snd_soc_sof_sdw:log_quirks: sof_sdw sof_sdw: quirk SOF_SDW_PCH_DMIC enabled [ 64.616559] snd_soc_sof_sdw:sof_card_dai_links_create: sof_sdw sof_sdw: sdw 2, ssp 0, dmic 2, hdmi 0 [ 64.616587] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create dai link SDW0-Playback, id 0 [ 64.616600] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create dai link SDW0-Capture, id 1 [ 64.616607] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create dai link dmic01, id 2 [ 64.616614] snd_soc_sof_sdw:init_dai_link: sof_sdw sof_sdw: create dai link dmic16k, id 3 [ 69.757115] rt5682 sdw:0:025d:5682:00: Initialization not complete, timed out [ 69.757128] rt5682 sdw:0:025d:5682:00: ASoC: error at snd_soc_component_probe on sdw:0:025d:5682:00: -110 [ 69.757224] sof_sdw sof_sdw: ASoC: failed to instantiate card -110 [ 69.757734] sof_sdw sof_sdw: snd_soc_register_card failed -110
see https://elixir.bootlin.com/linux/latest/source/sound/soc/codecs/rt5682.c#L29...
I think this is compatible with the device model and bind/unbind, but it could be improved with the removal of the wait if we had a way to return -EPROBEDEFER, and have a mechanism to force the deferred probe work to be triggered when a device actually shows up. It's a generic problem that the probe cannot always be a synchronous function but may complete 'later'.
I see what you've done in your patch, but I had already experimented with this idea and found that the wait_for_completion() can deadlock the Soundwire core.
That's not good. Do you have any logs or explanation on what the root-cause of this deadlock might be? If something's broken, we might as well fix it.