Re: About component->card

[Takashi Iwai <tiwai@suse.de>]

On Wed, 17 Dec 2025 03:39:27 +0100,
Kuninori Morimoto wrote:
> 
> 
> Hi Mark, Lars-Peter
> 
> I would like to confirm/suggest about restriction of component->card.
> See also this commit
> 
> 	45655ec69cb954d7fa594054bec33d6d5b99f8d5
> 	("ASoC: soc-core.c: enable multi Component")
> 
> We might have complex system, like this
> 
> 	+-- Basic Board ---------+
> 	|+--------+      +------+|
> 	|| CPU ch0| <--> |CodecA||
> 	||     ch1| <-+  +------+|
> 	|+--------+   |          |
> 	+-------------|----------+
> 	+-- expansion board -----+
> 	|             |  +------+|
> 	|             +->|CodecB||
> 	|                +------+|
> 	+------------------------+
> 
> We would like to implement it as below, it is intuitive.
> This means 1 Component connects to multi Cards.
> 
> 	 +-driver------+
> 	 |+-component-+|
> 	+-card0-------------------------+
> 	|||           || +-driver------+|
> 	|||           || |+-component-+||
> 	|||    ch0 dai|<=>|dai        |||
> 	|||           || |+-----------+||
> 	|||           || +-------------+|
> 	+-------------------------------+
> 	 ||           ||
> 	+-card1-------------------------+
> 	|||           || +-driver------+|
> 	|||           || |+-component-+||
> 	|||    ch1 dai|<=>|dai        |||
> 	|||           || |+-----------+||
> 	|||           || +-------------+|
> 	+-------------------------------+
> 	 |+-----------+|
> 	 +-------------+
> 
> But, we can't this today, beucase we have restriction (A) that 1 Component
> can't connect to multi Cards, because of component->card.
> 
> 	static int soc_probe_component(...)
> 	{
> 		...
> 		if (component->card) {
> (A)			if (component->card != card) {
> 				dev_err(...);
> 				return -ENODEV;
> 			}
> 			return 0;
> 		}
> 		...
> 	}
> 
> I haven't fully analyzed it, but I guess this restriction came from
> each driver want to access to card, but FW doesn't hand over card pointer ?
> 
> 	-- ASoC FW --
> 
> 	/* ASoC hand over component pointer only */
> 	ops->callback(component);
> 
> 	-- driver --
> 
> 	/*
> 	 * It gets component pointer only, but want to access
> 	 * to card, too. component->card is added for it.
> 	 */
> 	callback(component)
> 	{
> 		struct snd_soc_card *card = component->card;
> 		...
> 	}
> 
> If this is the reason why restriction was created, do you think we can
> remove it if ASoC FW hand over both card and component pointer ?
> 
> 	-- ASoC FW --
> 
> 	/* ASoC hand over both card and component pointer */
> 	ops->callback(card, component);
> 	              ^^^^
> 	-- driver --
> 
> 	/*
> 	 * It gets both card and component pointer.
> 	 * component->card is no longer needed ?
> 	 */
> 	callback(card, component)
> 	{        ^^^^
> 		 ...
> 	}
> 
> But are there any other reasons ?
> 
> I think it is not strange that DAI has Component pointer.
> It is fixed.
> 
> 	dai->component;
> 
> But, Component might be connected to multi Cards, we shouldn't fix it
> 
> 	component->card;
> 
> If above was sane, I would like to cleanup it.
> I think ASoC need to hand over Card and DAI pointer (instead of Component) ?
> But what do you think ?

Well, managing multiple cards would lead to a complete mess of
locking, I'm afraid.  And, conceptually the card is the top-level
abstraction, so it'd be unnatural to go beyond that in the ASoC
component.

Of course, we may use multiple cards, but its handling needs really a
careful manner, so it won't be that trivial, especially if DPCM plays
a role.


thanks,

Takashi