linux-kernel - Re: [PATCH v26 16/33] ASoC: doc: Add documentation for SOC USB

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Message-ID: <522dd841-2060-4e7c-b8ce-7e9ea2fa8498@linux.intel.com>
Date: Fri, 30 Aug 2024 11:03:16 +0200
From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To: Wesley Cheng <quic_wcheng@...cinc.com>, srinivas.kandagatla@...aro.org,
 mathias.nyman@...el.com, perex@...ex.cz, conor+dt@...nel.org,
 dmitry.torokhov@...il.com, corbet@....net, broonie@...nel.org,
 lgirdwood@...il.com, tiwai@...e.com, krzk+dt@...nel.org,
 Thinh.Nguyen@...opsys.com, bgoswami@...cinc.com, robh@...nel.org,
 gregkh@...uxfoundation.org
Cc: linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 linux-sound@...r.kernel.org, linux-input@...r.kernel.org,
 linux-usb@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 linux-doc@...r.kernel.org, alsa-devel@...a-project.org
Subject: Re: [PATCH v26 16/33] ASoC: doc: Add documentation for SOC USB


> diff --git a/Documentation/sound/soc/index.rst b/Documentation/sound/soc/index.rst
> index e57df2dab2fd..8bed8f8f48da 100644
> --- a/Documentation/sound/soc/index.rst
> +++ b/Documentation/sound/soc/index.rst
> @@ -18,3 +18,4 @@ The documentation is spilt into the following sections:-
>     jack
>     dpcm
>     codec-to-codec
> +   usb
> diff --git a/Documentation/sound/soc/usb.rst b/Documentation/sound/soc/usb.rst
> new file mode 100644
> index 000000000000..bd3d9eb86b07
> --- /dev/null
> +++ b/Documentation/sound/soc/usb.rst
> @@ -0,0 +1,429 @@
> +================
> +ASoC USB support
> +================
> +
> +Overview
> +========
> +In order to leverage the existing USB sound device support in ALSA, the
> +ASoC USB APIs are introduced to allow for the entities to communicate
> +with one another.

nit-pick: entities is rather vague and an overloaded term in USB audio.
Maybe "allow the subsystems to exchange configuration information"

> +One potential use case would be to support USB audio offloading, which is
> +an implementation that allows for an external DSP on the SoC to handle the

nit-pick: not sure about the reference to an 'external DSP'. "external"
would usually to a stand-alone device and there's no real need for DSP
capabilities for USB offload, e.g. a regular embedded core would do just
fine.

maybe "allows for an alternate power-optimized path in the audio
subsystem to handle the transfer of audio data over the USB bus"

> +transfer of audio data over the USB bus.  This would let the main
> +processor to stay in lower power modes for longer duration.  The following
> +is an example design of how the ASoC and ALSA pieces can be connected
> +together to achieve this:

> +	int snd_soc_usb_update_offload_route(struct device *dev, int card, int pcm,
> +					     int direction, long *route)
> +..
> +
> +  - ``dev``: USB device to look up offload path mapping
> +  - ``card``: USB sound card index
> +  - ``pcm``: USB sound PCM device index
> +  - ``direction``: direction to fetch offload routing information
> +  - ``route``: mapping of sound card and pcm indexes for the offload path

nit-pick: again explain how the card and pcm indices are handled.


> +--------------------------------
> +USB devices can be hotplugged into the USB root hub at any point in time.

"root hub" really?

Is this really required? I would think the entire framework would work
just fine if the device is connected to any hub at any level, not just
"the" root hub.