Date:   Mon, 05 Jun 2017 12:22:13 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Ruslan Bilovol <ruslan.bilovol@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>
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\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "linux-kernel\@vger.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:
>> 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.
>
> It is on LKML (which is down for me) [1] or alternative archive [2]
>
>>
>>> 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?
>
> In first RFC ([1],[2]) I did exactly what you wrote here (removed
> old uac1 implementation and replaced it by new one) but got feedback
> that it will break things for existing f_uac1 legacy users and it's better to
> have separate implementation.
>
> I'm OK with dropping legacy f_uac1 implementation.
>
> Another idea I was thinking about is to implement simple in-kernel
> driver which will do the same as existing alsaloop tool userspace
> tool does (so legacy users will need to load two kernel modules
> and get same functionality). But this seems to be a wrong way,
> since It known that Linux kernel community doesn't like to take drivers
> with same functionality as existing userspace tools already have.
>
> So bottom line: since I'm not a legacy f_uac1 user, there is no
> difference for me how to handle it - remove legacy f_uac1 completely,
> rename it to f_uac1_legacy or add separate f_uac1_acard function.
>
> So if dropping of legacy f_uac1 implementation is OK for you,
> I can do it quickly in next patchset.

Personally, I don't want duplicated functionality and I think the
virtual sound card approach is much better. Then again, removing
functionality we already support is kind of odd.

Greg, Alan, what do you guys think? Do we keep a duplicated function
around or do we just tell people to rely on alsaloop? Personally, I
think we're better off with the flexibility of the virtual sound card,
what's your take?

-- 
balbi

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