From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Mack Subject: Re: USB Implicit Feedback Date: Sat, 06 Sep 2014 13:56:02 +0200 Message-ID: <540AF652.8080501@zonque.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail.zonque.de (svenfoo.org [82.94.215.22]) by alsa0.perex.cz (Postfix) with ESMTP id E17E1264EFF for ; Sat, 6 Sep 2014 13:56:03 +0200 (CEST) In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: alsa-devel-bounces@alsa-project.org Sender: alsa-devel-bounces@alsa-project.org To: Pierre C , alsa-devel@alsa-project.org List-Id: alsa-devel@alsa-project.org On 09/06/2014 08:53 AM, Pierre C wrote: > I'm implementing a USB Audio Class 2 device using a Cortex-M4 with high > speed USB. So far it works well, the device is recognized by ALSA, > playback, capture, sample rate change, and all controls like volume and > mute work. I'm using explicit feedback. > > I'd like to use implicit feedback instead, Why? > and had some problems... > > With implicit feedback, when playback starts, ALSA should start capture > automatically (even if there are no applications requesting capture at the > moment) to use the number of capture samples as feedback. But it doesn't. > > So, I'd like to know : > > - Is it actually supported ? Yes. > - If it is, what descriptors should I use to make it work ? The usage mask of bmAttributes of the endpoint should be USB_ENDPOINT_USAGE_IMPLICIT_FB, along with some other constraints. See set_sync_endpoint() in sound/usb/pcm.c Note, however, that we only implemented implicit feedback mode in the driver because there are devices out there which use such modes. If you're doing the device-side yourself, I'd actually recommend going for explicit feedback, especially if that already works for you. Any particular reason why you want to change that? Daniel