linux-kernel - Re: [PATCH v2] usb: gadget: f_uac2: Expose all string descriptors through configfs.

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Date: Tue, 23 Apr 2024 17:22:50 +0000
From: Chris Wulff <Chris.Wulff@...mp.com>
To: Pavel Hofman <pavel.hofman@...tera.com>,
        "linux-usb@...r.kernel.org"
	<linux-usb@...r.kernel.org>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        James Gruber
	<jimmyjgruber@...il.com>, Lee Jones <lee@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2] usb: gadget: f_uac2: Expose all string descriptors
 through configfs.

> From: Pavel Hofman <pavel.hofman@...tera.com>
> Sent: Tuesday, April 23, 2024 11:38 AM

> > +             p_it_name               playback input terminal name
> > +             p_ot_name               playback output terminal name
> > +             p_fu_name               playback function unit name
> > +             p_alt0_name             playback alt mode 0 name
> > +             p_alt1_name             playback alt mode 1 name
>
> Nacked-by: Pavel Hofman <pavel.hofman@...tera.com>
>
> I am not sure adding a numbered parameter for every additional alt mode
> is a way to go for the future. I am not that much concerned about UAC1,
> but IMO (at least) in UAC2 the configuration method should be flexible
> for more alt setttings. I can see use cases with many more altsettings.
> 
> My proposal for adding more alt settings
> https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/35be4668-58d3-894a-72cf-de1afaacae45@ivitera.com/__;!!HBnMciuwfVSXJQ!TYg7j7-fh3eZAzPfiONi2lo54mf2qsWtpG0nwdaQwSqd1nGdKkTDN8o6_lSIWlWPtHoc-2Nz1KCbRhiXJnzXO8Ku1w$
> suggested using lists to existing parameters where each item would
> correspond to the alt setting of the same index (+1). That would allow
> using more altsettings easily, without having to add parameters to the
> source code and adding configfs params. I received no feedback. I do not
> push the param list proposal, but I am convinced an acceptable solution
> should be discussed thoroughly by the UAC2 gadget stakeholders.
>
> I am afraid that once p_alt1_name/c_alt1_name params are accepted, there
> will be no way back because subsequent removal of configfs params could
> be viewed as a regression for users.

I have been thinking about this as well. The alt names are slightly different than the rest of the settings
since they also include alt mode 0. I was thinking p/c_alt1_name could be expanded to the array so 
that the entries line up with the other settings and don't have an extra entry for alt 0. Perhaps a different
name would make more sense.

Along those lines, I didn't see any gadget drivers using an array of strings for anything, which is also why
I didn't try to do anything here that merged alt0/1 names into an array. If we were to do an array of strings
I'm not sure what the best separator would be. Maybe ";"? The rates array uses ",".

This patch only exposes the existing strings to make them configurable, but I don't want to do anything
that would preclude a nice interface for extra alt modes.

  -- Chris Wulff