Date: Sun, 03 Feb 2013 12:43:43 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: Alexander Motin <mav@FreeBSD.org> Cc: freebsd-multimedia@FreeBSD.org Subject: Re: sound(4) vs alsa oss plugin Message-ID: <510E3F5F.8090104@FreeBSD.org> In-Reply-To: <5108111F.3080205@FreeBSD.org> References: <5103A438.9010806@FreeBSD.org> <5108111F.3080205@FreeBSD.org>
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on 29/01/2013 20:12 Alexander Motin said the following: > I am not sure how it supposed to work in Linux, but IMHO it is obvious > that single ptr field is not sufficient in such conditions to identify > full buffer wrap. Additional checks for bytes or blocks fields could > help, but that is user-level side. I am not sure what kernel can do > about it. I have a very nasty local kernel-side hack that "fixes up" 'ptr' wrap-around based on a check of 'blocks'. But root causes are, of course: - an application using sleep and GET*PTR for interacting with OSS - ALSA OSS using just 'ptr' - a 10 millisecond sleep taking e.g. 100 milliseconds once in a while (or under some circumstances) > From the other side, case when buffer wrapped completely is IMHO already > fatal situation. It means that we are already late -- hardware already > started to either play or overwrite previous buffer. That is probably > not much better then if we would woke up some samples later. Well, some sound distortion/corruption is very bad, but completely blocked sound is even worse. I have the following patch for ALSA OSS, but haven't tested it yet: diff --git a/oss/pcm_oss.c b/oss/pcm_oss.c index d43b44c..4729962 100644 --- a/oss/pcm_oss.c +++ b/oss/pcm_oss.c @@ -83,7 +83,13 @@ static snd_pcm_sframes_t oss_pointer(snd_pcm_ioplug_t *io) fprintf(stderr, "*** OSS: oss_pointer error\n"); return 0; } - ptr = snd_pcm_bytes_to_frames(io->pcm, info.ptr); + + /* + * Note that the following calculations will produce a temporary glitch + * when info.bytes wraps around UINT_MAX or INT_MAX depending on type + * of info.bytes in an OSS implementation. + */ + ptr = snd_pcm_bytes_to_frames(io->pcm, info.bytes) % io->buffer_size; return ptr; } -- Andriy Gapon
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