linux-kernel - Question about simple-audio-card with dummy CODEC

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Message-ID: <CAHCN7xKQ549ExO83Yur=_MPv3aYFdR0nfzUSuEXO74OddAr5gQ@mail.gmail.com>
Date:   Sat, 26 Aug 2023 23:15:30 -0500
From:   Adam Ford <aford173@...il.com>
To:     Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        ALSA Development Mailing List <alsa-devel@...a-project.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Shengjiu Wang <shengjiu.wang@...il.com>,
        Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <festevam@...il.com>
Subject: Question about simple-audio-card with dummy CODEC

I want to use a PDM microphone on a variety of i.MX8M (Mini/Nano/Plus)
boards with the simple-audio-card driver.  I can get this device to
enumerate when I use "fsl,imx-audio-card" and it appears that it
connects to a dummy codec:

sysdefault:CARD=imxaudiomicfil
    imx-audio-micfil, micfil hifi snd-soc-dummy-dai-0
    Default Audio Device

When I attempt to use "simple-audio-card" with just the
simple-audio-card,cpu  sound-dai node pointing to the micfil cpu
without a codec node, nothing appears in arecord -L, and I don't see
errors.

I am guessing the simple-audio-card driver is expecting a CODEC, so
hacked soc-utils.c to give soc_dummy_driver an of_match_table and
assigned it a corresponding 'compatible' name. I then routed the codec
node on the simple-audio-card to the dummy CODEC and it enumerated
just like it did with the custom fsl,imx-audio-card driver:

sysdefault:CARD=soundpdm
    sound-pdm, 30ca0000.audio-controller-snd-soc-dummy-dai snd-soc-dummy-dai-0
    Default Audio Device

I am asking what is the best way to let simple-audio-card  use a dummy
codec?  If the dummy codec is the better option, what compatible name
should I use?

adam