linux-kernel - Re: [PATCH] ASoC: dmaengine_pcm: Consider DMA cache caused delay in pointer callback

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Date:   Mon, 10 Feb 2020 17:01:19 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     <broonie@...nel.org>, <lgirdwood@...il.com>, <tiwai@...e.com>,
        <perex@...ex.cz>, <lars@...afoo.de>, <alsa-devel@...a-project.org>,
        <vkoul@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ASoC: dmaengine_pcm: Consider DMA cache caused delay in
 pointer callback



On 10/02/2020 16.38, Takashi Iwai wrote:
> On Mon, 10 Feb 2020 15:28:44 +0100,
> Peter Ujfalusi wrote:
>>
>> Hi Takashi,
>>
>>>> --- a/sound/soc/soc-pcm.c
>>>> +++ b/sound/soc/soc-pcm.c
>>>> @@ -1151,7 +1151,7 @@ static snd_pcm_uframes_t soc_pcm_pointer(struct snd_pcm_substream *substream)
>>>>  	}
>>>>  	delay += codec_delay;
>>>>  
>>>> -	runtime->delay = delay;
>>>> +	runtime->delay += delay;
>>>
>>> Is it correct?
>>> delay already takes runtime->delay as its basis, so it'll result in a
>>> double.
>>
>> The delay here is coming from the DAI and the codec.
>> The runtime->delay hold the PCM (DMA) caused delay.
> 
> Well, let's take a look at soc_pcm_pointer():
> 
> 	/* clearing the previous total delay */
> 	runtime->delay = 0;
> 
> 	offset = snd_soc_pcm_component_pointer(substream);
> 
> 	/* base delay if assigned in pointer callback */
> 	delay = runtime->delay;
> 
> 	delay += snd_soc_dai_delay(cpu_dai, substream);
> 
> 	for_each_rtd_codec_dai(rtd, i, codec_dai) {
> 		codec_delay = max(codec_delay,
> 				  snd_soc_dai_delay(codec_dai, substream));
> 	}
> 	delay += codec_delay;
> 
> 	runtime->delay = delay;
> 
> So, the code reads the current runtime->delay and saves it as delay
> variable.  Then it adds the max delay from codec DAIs, and stores back
> to runtime->delay.
> 
> If we change the last line to
> 	runtime->delay += delay;
> it'll add to the already existing value again, so it'll be doubly if
> runtime->delay was non-zero beforehand.

Yes, you are right.
The change is added by
9fb4c2bf130b ASoC: soc-pcm: Use delay set in component pointer function

which I have missed, apparently.

> That said, judging from the code, I believe the current soc-pcm.c code
> needs no change.

Yes, there is no need to change soc-pcm.

> 
> 
> thanks,
> 
> Takashi
> 

- Péter

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