linux-kernel - Re: [PATCH v2 0/3] USB Audio Gadget refactoring

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Date:	Tue, 16 Aug 2016 17:49:52 +0800
From:	Peter Chen <hzpeterchen@...il.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	Ruslan Bilovol <ruslan.bilovol@...il.com>,
	Felipe Balbi <balbi@...nel.org>,
	Daniel Mack <zonque@...il.com>,
	Jassi Brar <jassisinghbrar@...il.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] USB Audio Gadget refactoring

On Tue, Aug 16, 2016 at 11:32:55AM +0200, Clemens Ladisch wrote:
> Peter Chen wrote:
> > I find UAC2 (UAC1 is ok)  support is not well with the latest mainline
> > kernel w/o your patch set. The windows7 can't install the driver
> > successfully
> 
> Windows does not have UAC2 support.
> 

Thanks, before windows7 or all windows versions have no UAC2 support?

> > and the playback shows underrun (using local codec)
> > using Linux host.
> 
> > # arecord -f dat -t wav -D hw:1,0 | aplay -D hw:0,0 &
> 
> The clocks of the two devices are not synchronized.
> 
> In the ALSA API, a PCM device is assumed to have its own clock, so it is
> not possible to synchronize the USB gadget to the actual sound device
> without some separate mechanism (like the old uac1 gadget probably has).
> 

The reason for immediate underrun is the small USB request number.
The default request number of USB is 2 for UAC2, but 256 for
UAC1. I have a internal 4.1.y version which works fine for UAC2 with
only two requests, maybe ALSA has changed some recently.

Yes, we need feedback endpoint to adjust sample rate for mismatch
clocks between USB and codec.

-- 

Best Regards,
Peter Chen