@bardliao why we get BE here ? this function only deal with FE in topology.

@RanderWang If soc_check_tplg_fes find a matched machine (like sof_sdw), it will overwrite all preset links. See for_each_card_prelinks(card, i, dai_link)

so soc_check_tplg_fes doesn't only check fes. why force dpcm_playback & dpcm_capture to 1 ? I don't get it.

@RanderWang @bardliao i think I'm at fault for this

commit 218fe9b7ec7f32c10a07539365488d80af7b0084
Author: Daniel Baluta <daniel.baluta@nxp.com>
Date:   Wed Dec 4 17:13:33 2019 +0200

    ASoC: soc-core: Set dpcm_playback / dpcm_capture
    
    When converting a normal link to a DPCM link we need
    to set dpcm_playback / dpcm_capture otherwise playback/capture
    streams will not be created resulting in errors like this:
    
    [   36.039111]  sai1-wm8960-hifi: ASoC: no backend playback stream
    
    Fixes: a655de808cbde ("ASoC: core: Allow topology to override machine driver FE DAI link config")

I think the proper fix would be to check if the original link has playback OR capture capability and update only that capability instead of updating both playback and capture.

this is unrelated. But looking at the code I'd rather remove the goto by using

	if (strcmp(component->driver->ignore_machine, card->dev->driver->name) &&
	    strcmp(component->driver->ignore_machine, dev_name(card->dev)))
		continue;

in a separate PR of course

not for this PR, but in line 1649/1650 above card->dai_link[i].name should be dai_link->name

At this moment in code we can reach if dai_link is a normal link (no_pcm = 0, dynamic=0) or a BE link (no_pcm=1).

If the link is already BE, we should do nothing.
if the link is normal link, we should convert it to BE.
-> no_pcm = 1
-> now here is the challenge as dai_link doesn't have a .capture or .playback field.

It has:

-> playback_only=1
   -> .dpcm_playback = 1
   -> dpcm_capture = 0
-> capture_only =1
   -> .dpcm_playback = 0
   -> .dpcm_capture =1
-> BUT what does capture_only = 0 and playback_only = 0 means, logically it could be either bot playback/capture supported or none.

BUT what does capture_only = 0 and playback_only = 0 means, logically it could be either bot playback/capture supported or none.

I thought about the capture_only and playback_only flag, too. But it seems it is set by a few machine driver only. I am not sure if we can relay on the capture_only and playback_only flag or not. The other question is that is there any link supports none of playback and capture?

@bardliao If I would have to guess I would say that not-supporting playback/capture makes no sense. So, I think we can infer that by default if capture_only is not set and playback_only is not set then both playback/capture are supported.
I have sent an email to alsa mailing list to clarify this situation:

https://mailman.alsa-project.org/pipermail/alsa-devel/2020-May/168352.html

I think the best way to fix this is as follows:

diff --git a/sound/soc/soc-core.c b/sound/soc/soc-core.c
index 8321e75ff244..464004648bd0 100644
--- a/sound/soc/soc-core.c
+++ b/sound/soc/soc-core.c
@@ -1722,9 +1722,18 @@ static void soc_check_tplg_fes(struct snd_soc_card *card)
                        dai_link->platforms->name = component->name;
 
                        /* convert non BE into BE */
-                       dai_link->no_pcm = 1;
-                       dai_link->dpcm_playback = 1;
-                       dai_link->dpcm_capture = 1;
+                       if (!dai_link->no_pcm) {
+
+                               dai_link->no_pcm = 1;
+
+                               dai_link->dpcm_playback = 1;
+                               dai_link->dpcm_capture = 1;
+
+                               if (dai_link->playback_only)
+                                       dai_link->dpcm_capture = 0;
+                               if (dai_link->capture_only)
+                                       dai_link->dpcm_playback = 0;
+                       }

@dbaluta I asked the question on alsa-devel on why we have separate flags for dpcm and non-dpcm solutions. I think it's a complete mess. We should have .pcm_playback and .pcm_capture flags only, you can infer playback_only and capture_only when only one of the two is set.