On 05/15/2013 01:22 PM, Jaroslav Kysela wrote: > Date 15.5.2013 13:03, David Henningsson wrote: >> On 05/15/2013 12:53 PM, Jaroslav Kysela wrote: >>> Date 15.5.2013 12:48, Takashi Iwai wrote: >>>> At Wed, 15 May 2013 12:26:51 +0200, >>>> Jaroslav Kysela wrote: >>>>> >>>>> Date 15.5.2013 11:55, Arun Raghavan wrote: >>>>>> Hello, >>>>>> A number of users have intermittently(?) been hitting a crash in >>>>>> alsa-lib 1.0.27 [1, 2] related to the softvol plugin. I'm not able to >>>>>> reproduce this reliably, so can't find an easy way to debug/fix. >>>>> >>>>> The problem is that the offsets are not in sync in this case [1]: >>>>> >>>>> src_offset = 38560 >>>>> dst_offset = 38568 >>>>> frames = 16374 >>>>> >>>>> Could you reproduce this bug in any way? At least snd_pcm_dump() before >>>>> the failing snd_pcm_mmap_commit() call might help to determine what was >>>>> the status before the assert() was entered. >>>> >>>> Yep. And this path is actually with volume 0dB, that is, a simply >>>> passthrough in softvol. Thus the bug may hit essentially any >>>> plugins, not specifically softvol. >>>> >>>> >>>>>> However, this raises a tangential question - why do we need softvol to >>>>>> be plugged for 'front' at all? David explained to me that this is to >>>>>> guarantee the existence of a PCM control. Perhaps I don't fully >>>>>> understand this, because I'm unconvinced by the reason. Could someone >>>>>> explain/refute? >>>>>> >>>>>> This is especially bad for us, from PulseAudio's perspective, because we >>>>>> aren't getting a zero-copy path. >>>>> >>>>> If the softvol is set to 0dB (no attenuation or gain), then the ring >>>>> buffer pointers are moved without any sample processing, so the >>>>> zero-copy functionality is kept. >>>> >>>> Yeah, a sort of. The mmap is cleared in the slave PCM, so actually >>>> there will be copy operations in underlying layers even though softvol >>>> itself does zero copy. >>>> >>>> Actually it makes no sense to keep softvol for PA, but the problem is >>>> always the regression. There are certainly users without PA, which >>>> might still rely on the softvol for such hardware without the amp >>>> control. >>>> >>>> Maybe We can add some flag to indicate whether to handle softvol or >>>> not, e.g. defaults.pcm.skip_softvol, and let PA set this in its config >>>> space. Setting a config item itself would break anything, so it'll >>>> still work with old alsa-lib (but with softvol). >>> >>> We have already SND_PCM_NO_SOFTVOL open mode for this purpose, so I >>> wonder, why PA does not use it.. >> >> The problem is knowing whether PCM is a softvol or not. In some cases, >> we need to set PCM to control hardware volume. >> >> Maybe, if we could figure this out somehow, we could ignore the PCM >> mixer control (or possibly set it to zero) in case PCM is a softvol, >> and actually use it if PCM is not a softvol. >> >> It does not look like this is currently possible from the simple mixer >> interface, but I might be missing something? > > It is not possible. Perhaps, we may create a new dummy mixer control (in > an inactive state) which will identify the presence of the softvol > plugin, like: > > "Softvol PCM Playback Volume" - full name for the raw control API > "Softvol PCM" - simple mixer name Or perhaps add a SND_CTL_NO_SOFTVOL flag that can be used in the call to snd_mixer_open / snd_ctl_open? That would make it somewhat consistent with the approach recommended for snd_pcm_open. -- David Henningsson, Canonical Ltd. https://launchpad.net/~diwic
