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

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Message-ID: <20200210143717.GO7685@sirena.org.uk>
Date:   Mon, 10 Feb 2020 14:37:17 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     Takashi Iwai <tiwai@...e.de>, 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 Mon, Feb 10, 2020 at 04:28:44PM +0200, Peter Ujfalusi wrote:
> On 10/02/2020 16.21, Takashi Iwai wrote:

> >>  	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.

I think Takashi's point here (and a query I have) is that we end up with

	delay = runtime->delay;
	delay += stuff;
	runtime->delay += delay;

which is equivalent to

	runtime->delay = (runtime->delay * 2) + stuff;

and that's a bit surprising.

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