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

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Message-ID: <s5ha75qfoum.wl-tiwai@suse.de>
Date:   Mon, 10 Feb 2020 16:23:29 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
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 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.

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


thanks,

Takashi