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Subject: Re: [PATCH] staging: bcm2835-audio: interpolate audio delay
To:     Mike Brady <mikebrady@eircom.net>
Cc:     eric@anholt.net, stefan.wahren@i2se.com,
        gregkh@linuxfoundation.org, f.fainelli@gmail.com, tiwai@suse.de,
        nishka.dasgupta_ug18@ashoka.edu.in, julia.lawall@lip6.fr,
        linux-rpi-kernel@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, devel@driverdev.osuosl.org,
        linux-kernel@vger.kernel.org
References: <20181018105715.GA6660@ubuntu>
From:   Kirill Marinushkin <k.marinushkin@gmail.com>
Message-ID: <22867d63-12f1-31f7-acb6-ce9a58fcbe71@gmail.com>
Date:   Thu, 18 Oct 2018 21:48:36 +0200
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Hello Mike,

On 10/18/18 12:57, Mike Brady wrote:
> +	ktime_t now = ktime_get();
> +
> +	/* Give userspace better delay reporting by interpolating between GPU
> +	 * notifications, assuming audio speed is close enough to the clock
> +	 * used for ktime
> +	 */
> +
> +	if ((ktime_to_ns(alsa_stream->interpolate_start)) &&
> +	    (ktime_compare(alsa_stream->interpolate_start, now) < 0)) {
> +		u64 interval =
> +			(ktime_to_ns(ktime_sub(now,
> +				alsa_stream->interpolate_start)));
> +		u64 frames_output_in_interval =
> +			div_u64((interval * runtime->rate), 1000000000);
> +		snd_pcm_sframes_t frames_output_in_interval_sized =
> +			-frames_output_in_interval;
> +		runtime->delay = frames_output_in_interval_sized;
> +	}

This doesn't look like a good solution for me. More like a workaround. What is
the root cause of the delay?

Best Regards,
Kirill