linux-kernel - Re: [PATCH v2] ALSA: dmaengine_pcm: Consider DMA cache caused delay in pointer callback

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Date:   Mon, 10 Feb 2020 17:26:55 +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 v2] ALSA: dmaengine_pcm: Consider DMA cache caused delay
 in pointer callback



On 10/02/2020 17.23, Takashi Iwai wrote:
> On Mon, 10 Feb 2020 16:14:02 +0100,
> Peter Ujfalusi wrote:
>>
>> Some DMA engines can have big FIFOs which adds to the latency.
>> The DMAengine framework can report the FIFO utilization in bytes. Use this
>> information for the delay reporting.
>>
>> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
>> ---
>> Hi,
>>
>> Changes since v1:
>> - use bytes_to_frames() for the DMA delay calculation
>> - Drop changes to soc-pcm
>>
>> 5.6-rc1 now have support for reporting the DMA cached data.
>> With this patch we can include it to the delay calculation.
>> The first DMA driver which reports this is the TI K3 UDMA driver.
>>
>> Regards,
>> Peter
>>
>>  sound/core/pcm_dmaengine.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/sound/core/pcm_dmaengine.c b/sound/core/pcm_dmaengine.c
>> index 5749a8a49784..d8be7b488162 100644
>> --- a/sound/core/pcm_dmaengine.c
>> +++ b/sound/core/pcm_dmaengine.c
>> @@ -247,9 +247,14 @@ snd_pcm_uframes_t snd_dmaengine_pcm_pointer(struct snd_pcm_substream *substream)
>>  
>>  	status = dmaengine_tx_status(prtd->dma_chan, prtd->cookie, &state);
>>  	if (status == DMA_IN_PROGRESS || status == DMA_PAUSED) {
>> +		struct snd_pcm_runtime *runtime = substream->runtime;
>> +
>>  		buf_size = snd_pcm_lib_buffer_bytes(substream);
>>  		if (state.residue > 0 && state.residue <= buf_size)
>>  			pos = buf_size - state.residue;
>> +
>> +		runtime->delay = bytes_to_frames(runtime,
>> +						 state.in_flight_bytes);
> 
> Another call of bytes_to_frames() below...
> 
>>  	}
>>  
>>  	return bytes_to_frames(substream->runtime, pos);
> 
> ... refers to substream->runtime.
> Better to align both places, either runtime or substream->runtime.

Sure, I'll use the runtime as with substream->runtime the delay part is
not nicely wrapping.

> With that minor nitpick, the change looks good:
> Reviewed-by: Takashi Iwai <tiwai@...e.de>
> 
> 
> thanks,
> 
> Takashi
> 

- Péter

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