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From: Clemens Ladisch <clemens@ladisch.de>
To: Alan Young <Alan.Young@IEE.org>,
	Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>,
	Takashi Iwai <tiwai@suse.de>
Cc: Raymond Yau <superquad.vortex2@gmail.com>, alsa-devel@alsa-project.org
Subject: Re: Improving status timestamp accuracy
Date: Tue, 2 Aug 2016 09:55:07 +0200	[thread overview]
Message-ID: <35f9d6de-f11f-7977-7b6f-69d2f3e1e73c@ladisch.de> (raw)
In-Reply-To: <57A04BFE.9030704@IEE.org>

Alan Young wrote:
> Stepping back for a moment, the delay calculation essentially
> consists of two parts:
>
> 1. How much data is still in the ring buffer.
> 2. How much data has been removed from the ring buffer but not yet
>    played out.
> [...]
> The more that I think about it the more it seems to me that using
> a time-based estimate for position (hw_ptr), outside of an interrupt
> callback, will always be more accurate than that returned by
> substream->ops->pointer().

Please note that "hw_ptr" (or "avail" in the API) specifies the DMA
position, i.e., it is guaranteed that the samples up to this position
have been handled by the DMA controller and can now be accessed by the
program.  An estimate that gets this wrong can lead to data corruption.

Using an estimate for the delay is perfectly fine.


Regards,
Clemens

  reply	other threads:[~2016-08-02  7:55 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-04  9:31 Improving status timestamp accuracy Alan Young
2016-06-04 10:17 ` Clemens Ladisch
2016-06-04 10:43   ` Alan Young
2016-06-04 15:59     ` Clemens Ladisch
2016-06-04 16:20       ` Alan Young
2016-06-05 16:27         ` Clemens Ladisch
2016-06-05 16:32           ` Alan Young
2016-06-05  1:14 ` Raymond Yau
2016-06-05 10:33   ` Alan Young
2016-06-06  1:24     ` Raymond Yau
2016-06-06  9:40       ` Alan Young
2016-06-06  8:34     ` Takashi Iwai
2016-06-06  9:42       ` Alan Young
2016-06-06 14:53         ` Pierre-Louis Bossart
2016-06-07  6:44           ` Alan Young
2016-06-07 18:01             ` Pierre-Louis Bossart
2016-07-08 15:03             ` Alan Young
2016-07-15 20:13               ` Pierre-Louis Bossart
2016-07-19 15:33                 ` Alan Young
2016-07-19 15:58                   ` Pierre-Louis Bossart
2016-07-20  6:59                     ` Alan Young
2016-08-01 21:56                       ` Pierre-Louis Bossart
2016-08-02  7:30                         ` Alan Young
2016-08-02  7:55                           ` Clemens Ladisch [this message]
2016-08-02 16:25                           ` Pierre-Louis Bossart

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