Date: Sat, 26 Jan 2013 14:04:24 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: Clemens Ladisch <clemens@ladisch.de> Cc: freebsd-multimedia@FreeBSD.org Subject: Re: sound(4) vs alsa oss plugin Message-ID: <5103C648.4090103@FreeBSD.org> In-Reply-To: <5103C03D.4040201@ladisch.de> References: <5103A438.9010806@FreeBSD.org> <5103C03D.4040201@ladisch.de>
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on 26/01/2013 13:38 Clemens Ladisch said the following: > Andriy Gapon wrote: >> For some reason ALSA OSS plugin developers decided to use SNDCTL_DSP_GETOPTR and >> SNDCTL_DSP_GETIPTR to track state of playback/recording. > > The reason might be compatibility with all the other OSS implementations > out there. Possible... Although opensound documentation has a big fat warning: This ioctl call should only be used when using mmap(). In normal applications it's practically useless. >> Of all information returned by these ioctl they use just 'ptr' to track the >> progress by hardware. >> >> Now I see that under a "right mix" of circumstances the following is possible: >> - a buffer is completely empty for recording or completely full for playback >> - an application / ALSA checks the buffer pointer and gets some value P >> - the application sleeps for some time, perhaps oversleeps due to system load or >> something else >> - the buffer is completely filled or drained depending on the direction >> - the application / ALSA checks the buffer pointer and gets the same value P >> - the application assumes that the buffer is in the old state and waits further >> - sound(4) code knows that the buffer is in the new state and does nothing >> >> So, couple of questions: >> - apparently this used to work (works?) in Linux - how? > > What evidence do you have that this particular case is handled correctly > in Linux? :o) No real evidence. Just an assumption that since it's done this way and hasn't been changed in many years, then it must work (have been working when OSS was in use) :-) Also, given that ALSA comes from Linux, I assumed that ALSA OSS pluging was developed and tested on Linux and thus primarily with Linux OSS implementation in mind. >> - is there any trick that we can do in our sound(4) code to prevent this 'ptr' >> full cycle from happening? > > Not really. It's the logic in the ALSA OSS plugin that should be fixed > to also use the 'bytes' field. OK. >> Ideally, it's the logic in the ALSA OSS plugin that should be fixed, but... >> The upstream is not very responsive. > > Not very active. Patches will be accepted. (As long as they don't > break with all the other OSS implementations ...) Great to hear. I will try to put together a patch. Are you one of the ALSA developers? Should I contact you in addition to the mailing list? -- Andriy Gapon
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