Re: [RFC PATCH 2/4] ALSA: core: add .notify callback for pcm ops - Pierre-Louis Bossart

From: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
To: Takashi Iwai <tiwai@suse.de>
Cc: alsa-devel@alsa-project.org
Subject: Re: [RFC PATCH 2/4] ALSA: core: add .notify callback for	pcm ops
Date: Wed, 08 Jul 2015 19:10:32 +0200	[thread overview]
Message-ID: <559D5988.70206@linux.intel.com> (raw)
In-Reply-To: <s5hlheqhghu.wl-tiwai@suse.de>

>> This is only enabled when the NO_REWIND hardware flag is used,
>> so that the low-level driver/hardware to opportunistically pre-fetch
>> data.
>>
>> FIXME: should we rely on .ack for this?
>> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
>
> Hmm, OK, so the forward is allowed but with workarounds...
> But then why rewind won't work in a similar way?  DSP might be able to
> cancel some of inflight data.

Nope, this is explicitly not supported, so unfortunately if we want to 
optimize for power and let hardware fetch data when it's most 
appropriate rewinds need to be disabled.

> In other words, I see no reason to strict notify callback only for
> no_rewinds.  This is an optional ops in anyway.

It's fine to remove the check. I added this based on internal review 
comments but I agree with your point.

> Also, I find the name "notify" a bit too ambiguous.  In this case,
> it's notifying the applptr change.  So, a name related with the
> function would be more understandable.

The first open I had was to know if we could use .ack for this? if a 
different callback is needed, we can use 'appl_ptr_update' instead of 
'notify'

>
>
> thanks,
>
> Takashi
>
>> ---
>>   include/sound/pcm.h     |  2 ++
>>   sound/core/pcm_native.c | 18 +++++++++++++++++-
>>   2 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/sound/pcm.h b/include/sound/pcm.h
>> index 25310b7..d5eff03 100644
>> --- a/include/sound/pcm.h
>> +++ b/include/sound/pcm.h
>> @@ -87,6 +87,8 @@ struct snd_pcm_ops {
>>   			     unsigned long offset);
>>   	int (*mmap)(struct snd_pcm_substream *substream, struct vm_area_struct *vma);
>>   	int (*ack)(struct snd_pcm_substream *substream);
>> +	int (*notify)(struct snd_pcm_substream *substream);
>> +	/* FIXME: what's the difference between ack and notify ? */
>>   };
>>
>>   /*
>> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
>> index a70e52d..dd519b8 100644
>> --- a/sound/core/pcm_native.c
>> +++ b/sound/core/pcm_native.c
>> @@ -2565,6 +2565,10 @@ static snd_pcm_sframes_t snd_pcm_playback_forward(struct snd_pcm_substream *subs
>>   		appl_ptr -= runtime->boundary;
>>   	runtime->control->appl_ptr = appl_ptr;
>>   	ret = frames;
>> +
>> +	if (runtime->no_rewinds && substream->ops->notify)
>> +		substream->ops->notify(substream);
>> +
>>    __end:
>>   	snd_pcm_stream_unlock_irq(substream);
>>   	return ret;
>> @@ -2614,6 +2618,10 @@ static snd_pcm_sframes_t snd_pcm_capture_forward(struct snd_pcm_substream *subst
>>   		appl_ptr -= runtime->boundary;
>>   	runtime->control->appl_ptr = appl_ptr;
>>   	ret = frames;
>> +
>> +	if (runtime->no_rewinds && substream->ops->notify)
>> +		substream->ops->notify(substream);
>> +
>>    __end:
>>   	snd_pcm_stream_unlock_irq(substream);
>>   	return ret;
>> @@ -2713,8 +2721,16 @@ static int snd_pcm_sync_ptr(struct snd_pcm_substream *substream,
>>   			return err;
>>   	}
>>   	snd_pcm_stream_lock_irq(substream);
>> -	if (!(sync_ptr.flags & SNDRV_PCM_SYNC_PTR_APPL))
>> +	if (!(sync_ptr.flags & SNDRV_PCM_SYNC_PTR_APPL)) {
>> +		/* FIXME: this code is used by mmap_commit, should it handle boundary
>> +		 *   wrap-around as done for read/write in pcm_lib.c
>> +		 */
>> +
>>   		control->appl_ptr = sync_ptr.c.control.appl_ptr;
>> +		/* if supported, let low-level driver know about appl_ptr change */
>> +		if (runtime->no_rewinds && substream->ops->notify)
>> +			substream->ops->notify(substream);
>> +	}
>>   	else
>>   		sync_ptr.c.control.appl_ptr = control->appl_ptr;
>>   	if (!(sync_ptr.flags & SNDRV_PCM_SYNC_PTR_AVAIL_MIN))
>> --
>> 1.9.1
>>
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>>
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