linux-kernel - Re: [PATCH v4 0/3] USB Audio Gadget refactoring

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Date:   Fri, 02 Jun 2017 12:42:16 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Ruslan Bilovol <ruslan.bilovol@...il.com>
Cc:     Daniel Mack <zonque@...il.com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Clemens Ladisch <clemens@...isch.de>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Chen <hzpeterchen@...il.com>,
        Julian Scheel <julian@...st.de>, linux-usb@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/3] USB Audio Gadget refactoring


Hi,

Ruslan Bilovol <ruslan.bilovol@...il.com> writes:
> I came to this patch series when wanted to do two things:
>  - use UAC1 as virtual ALSA sound card on gadget side,
>    just like UAC2 is used so it's possible to do rate
>    resampling
>  - have both playback/capture support in UAC1
>
> Since I wanted to have same behavior for both UAC1/UAC2,
> obviously I've got an utility part (u_audio.c) for
> virtual ALSA sound card handling like we have
> for ethernet(u_ether) or serial(u_serial) functions.
> Function-specific parts (f_uac1/f_uac2) became almost 
> as storage for class-specific USB descriptors, some
> boilerplate for configfs, binding and few USB
> config request handling.
>
> Originally in RFC [1] I've posted before, there was
> major change to f_uac1 after that it couldn't do
> direct play to existing ALSA sound card anymore,
> representing audio on gadget side as virtual
> ALSA sound card where audio streams are simply
> sinked to and sourced from it, so it may break
> current usecase for some people (and that's why
> it was RFC).
>
> During RFC discussion, it was agreed to not touch
> existing f_uac1 implementation and create new one
> instead. This patchset (v4) introduced new function
> named f_uac1_acard and doesn't touch current f_uac1
> implementation, so people still can use old behavior

Do you have a pointer to the original RFC discussion where this was
discussed? If we really *must* keep the old implementation, I would
rather rename that to f_uac1_legacy. Still, I find it unlikely that
anybody will care about the old implementation.

> Now, it's possible to use existing user-space
> applications for audio routing between Audio Gadget
> and real sound card. I personally use alsaloop tool
> from alsautils and have ability to create PCM
> loopback between two different ALSA cards using
> rate resampling, which was not possible with previous
> "direct play to ALSA card" approach in f_uac1. 

this is really good result and will actually make it a lot easier for
testing things out.

> While here, also dropped redundant platform
> driver/device creation in f_uac2 driver (as well as
> didn't add "never implemented" volume/mute functionality
> in f_uac1 to f_uac1_acard) that made this work even
> easier to do.
>
> This series is tested with both legacy g_audio.ko and
> modern configfs approaches under Ubuntu 14.04 (UAC1 and
> UAC2) and under Windows7 x64 (UAC1 only) having
> perfect results in all cases.
>
> Comments, testing are welcome.
>
> v4 changes:
>  - renamed f_uac1_newapi to f_uac1_acard that is
>    more meaningful

I really don't get why you wanna keep both f_uac1 and f_uac1_acard. Why
do we need to maintain the old uac1 implementation? Why two separate
files?

-- 
balbi

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