From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Alexander E. Patrakov" Subject: Re: Master Plan on rewinding Date: Tue, 09 Sep 2014 14:55:42 +0600 Message-ID: <540EC08E.4000906@gmail.com> References: <540C76E0.9050808@gmail.com> <540CC53B.7080204@ladisch.de> <540D5B46.3020904@gmail.com> <540EBD9A.9000009@ladisch.de> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: Received: from mail-lb0-f169.google.com (mail-lb0-f169.google.com [209.85.217.169]) by alsa0.perex.cz (Postfix) with ESMTP id BDC6F2615BF for ; Tue, 9 Sep 2014 10:55:44 +0200 (CEST) Received: by mail-lb0-f169.google.com with SMTP id p9so7512049lbv.0 for ; Tue, 09 Sep 2014 01:55:44 -0700 (PDT) In-Reply-To: <540EBD9A.9000009@ladisch.de> 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: Clemens Ladisch , ALSA Development Mailing List , Takashi Iwai , David Henningsson , Takashi Sakamoto List-Id: alsa-devel@alsa-project.org 09.09.2014 14:43, Clemens Ladisch wrote: > Alexander E. Patrakov wrote: >> |---------|---------P----h----p---------|-a-------|---------| >> >> So, what should alsa-lib return for snd_pcm_avail() and snd_pcm_rewind()? >> The driver only knows that "P" is already used, can infer that "p" isn't >> used yet, and knows nothing about samples in the middle. > Indeed. However, the DMA pointer moves asynchronously, so it is possible > that it has already moved beyond p when snd_pcm_rewindable() returns. > For the samples between P and p, the risk is larger than for those after > p, but p is not a boundary where the risk abruptly decreases. > > It would make sense to report the pointer update granularity, but not > to adjust the return value of snd_pcm_avail/rewindable(). OK, I understand your viewpoint, and the phrase "some indicator of the actual rewind granularity and/or safeguard ... should be enough for PA to be able to pick a suitable default latency" from David indicates that he has a similar opinion. Now the remaining question is: can the proposed heuristic (minimum period size for a given sample rate, number of channels and sample format) be useful as an upper-bound approximation of the pointer update granularity for cards that are "rewindable even further than the nearest period"? -- Alexander E. Patrakov