Messages in this thread |  | | Date | Wed, 19 Sep 2018 11:52:33 +0200 | From | Takashi Iwai <> | Subject | Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint |
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On Wed, 19 Sep 2018 11:42:22 +0200, Stefan Wahren wrote: > > Hi Takashi, > > Am 04.09.2018 um 17:58 schrieb Takashi Iwai: > > It seems that the resolution of vc04 callback is in 10 msec; i.e. the > > minimal period size is also 10 msec. > > > > This patch adds the corresponding hw constraint. > > > > Signed-off-by: Takashi Iwai <tiwai@suse.de> > > --- > > drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c > > index 9659c25b9f9d..6d89db6e14e4 100644 > > --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c > > +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c > > @@ -145,6 +145,11 @@ static int snd_bcm2835_playback_open_generic( > > SNDRV_PCM_HW_PARAM_PERIOD_BYTES, > > 16); > > > > + /* position update is in 10ms order */ > > + snd_pcm_hw_constraint_minmax(runtime, > > + SNDRV_PCM_HW_PARAM_PERIOD_TIME, > > + 10 * 1000, UINT_MAX); > > + > > chip->alsa_stream[idx] = alsa_stream; > > > > chip->opened |= (1 << idx); > > in the Foundation Kernel (Downstream) there is a patch to interpolate > the audio delay. So my questions is, does your patch above makes the > following patch obsolete?
Through a quick glance, no, my patch is orthogonal to this.
My patch adds a PCM hw constraint so that the period size won't go below 10ms, while the downstream patch provides the additional delay value that is calculated from the system clock.
> [PATCH] bcm2835: interpolate audio delay > > It appears the GPU only sends us a message all 10ms to update > the playback progress. Other than this, the playback position > (what SNDRV_PCM_IOCTL_DELAY will return) is not updated at all. > Userspace will see jitter up to 10ms in the audio position. > > Make this a bit nicer for userspace by interpolating the > position using the CPU clock. > > I'm not sure if setting snd_pcm_runtime.delay is the right > approach for this. Or if there is maybe an already existing > mechanism for position interpolation in the ALSA core.
That's OK, as long as the computation is accurate enough (at least not exceed the actual position) and is light-weight.
> I only set SNDRV_PCM_INFO_BATCH because this appears to remove > at least one situation snd_pcm_runtime.delay is used, so I have > to worry less in which place I have to update this field, or > how it interacts with the rest of ALSA.
Actually, this SNDRV_PCM_INFO_BATCH addition should be a separate patch. It has nothing to do with the runtime->delay calculation. (And, this "one situation" is likely called PulseAudio :)
> In the future, it might be nice to use VC_AUDIO_MSG_TYPE_LATENCY. > One problem is that it requires sending a videocore message, and > waiting for a reply, which could make the implementation much > harder due to locking and synchronization requirements.
This can be now easy with my patch series. By switching to non-atomic operation, we can issue the vc04 command inside the pointer callback, too.
thanks,
Takashi
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