linux-kernel - Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint

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Date:   Wed, 19 Sep 2018 14:41:28 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Takashi Iwai <tiwai@...e.de>, Mike Brady <mikebrady@...com.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Eric Anholt <eric@...olt.net>,
        linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Phil Elwell <phil@...pberrypi.org>
Subject: Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint

Hi,

[add Phil and Mike]

Am 19.09.2018 um 11:52 schrieb Takashi Iwai:
> 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@...e.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.

thanks for your explanation. So your patch must be reverted with
implementation of interpolate audio delay.

>
>> [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.

I think we should try to implement this later.

@Mike: Do you want to write a patch series which upstream "interpolate
audio delay" and addresses Takashi's comments?

I would help you, in case you have questions about setup a Raspberry Pi
with Mainline kernel or patch submission.

Regards
Stefan

>
>
> thanks,
>
> Takashi