From: David Henningsson <david.henningsson@canonical.com>
To: "Alexander E. Patrakov" <patrakov@gmail.com>,
ALSA Development Mailing List <alsa-devel@alsa-project.org>,
Takashi Iwai <tiwai@suse.de>,
Takashi Sakamoto <o-takashi@sakamocchi.jp>
Subject: Re: Master Plan on rewinding
Date: Mon, 08 Sep 2014 09:59:00 +0200 [thread overview]
Message-ID: <540D61C4.90206@canonical.com> (raw)
In-Reply-To: <540C76E0.9050808@gmail.com>
On 2014-09-07 17:16, Alexander E. Patrakov wrote:
> Hello.
>
> (TL;DR: nothing really new except the strawman proposal about threads
> and the note about interaction of variable sample rate with rewindability)
So, having looked this through another time, it looks like we have three
categories of ALSA devices, from rewindability point of view:
1) Not rewindable at all.
2) Rewindable down to the period size.
3) Rewindable even further than the nearest period, down to DMA
transfer sizes or something else. This also requires the .pointer to
have better granularity than the period size.
So I think any is_seekable() call or flag should indicate whether the
device is case 1), 2) or 3). And for case 3) perhaps also some indicator
of the actual rewind granularity and/or safeguard. This should be enough
for PA to be able to pick a suitable default latency.
Case 1) is simple. Just let snd_pcm_rewindable return 0 and
snd_pcm_rewind fail. If it doesn't, just fix it. (The extplug problem
could be solved by PA having ifdefs depending on alsa-lib version,
rather than making snd_pcm_rewind and snd_pcm_rewindable behave
inconsistent.)
For case 2) you seem to suggest to emulate case 3) by using either a
low-latency thread, or by increasing the number of interrupts from the
hardware. Either method will inevitably increase power consumption, and
the former might also increase the risk of glitches. Therefore I think
this is replacing something bad with something worse, because I would
value low power consumption higher than better rewinding.
Could we enable this functionality by explicit request by the
application? Probably. E g, if the application sets a low period size
but also sets the "disable period interrupts" flag, that could be an
indicator that it wants lots of interrupts just to update the pointer,
but nothing else. Maybe that's even the behaviour today (haven't checked).
The low-latency thread approach could be implemented by a separate
ioplug layer, so that people who want it could open
"flexiblerewind:plug:hw:0" instead of "plug:hw:0".
However, with my PA hat on, I would still say no to having PA use either
of those by default, for power consumption reasons.
With that in mind I suggest, just as you, that we add
.rewind/.rewindable callbacks to the ioplug layer. Any ioplug using
.transfer needs to implement that if it wants to support rewinding,
otherwise that ioplug would fall into category 1).
Does that make sense? It was a long email, so I might have missed
something. :-)
--
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic
next prev parent reply other threads:[~2014-09-08 7:58 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-07 15:16 Master Plan on rewinding Alexander E. Patrakov
2014-09-07 18:38 ` Tanu Kaskinen
2014-09-07 19:05 ` Alexander E. Patrakov
2014-09-07 20:51 ` Clemens Ladisch
2014-09-08 3:06 ` Raymond Yau
2014-09-08 7:31 ` Alexander E. Patrakov
2014-09-09 8:43 ` Clemens Ladisch
2014-09-09 8:55 ` Alexander E. Patrakov
2014-09-09 9:08 ` David Henningsson
2014-09-09 9:31 ` Alexander E. Patrakov
2014-09-21 2:02 ` Raymond Yau
2014-09-22 13:20 ` Lars-Peter Clausen
2014-09-22 13:36 ` Alexander E. Patrakov
2014-09-22 13:44 ` Lars-Peter Clausen
2014-09-23 8:29 ` Raymond Yau
2014-09-23 10:22 ` Alexander E. Patrakov
2014-09-09 13:45 ` Clemens Ladisch
2014-09-09 15:55 ` Alexander E. Patrakov
2014-09-09 16:09 ` Takashi Iwai
2014-09-07 23:12 ` David Henningsson
2014-09-09 19:56 ` Pierre-Louis Bossart
2014-09-10 5:38 ` Alexander E. Patrakov
2014-09-08 7:34 ` Lars-Peter Clausen
2014-09-08 7:59 ` David Henningsson [this message]
2014-09-08 8:46 ` Alexander E. Patrakov
2014-09-08 9:26 ` David Henningsson
2014-09-08 10:21 ` Alexander E. Patrakov
2014-09-09 8:43 ` Clemens Ladisch
2014-09-11 3:49 ` Raymond Yau
2014-09-11 4:19 ` A. C. Censi
2014-09-13 9:15 ` Raymond Yau
2014-09-11 5:28 ` Alexander E. Patrakov
2014-09-11 6:21 ` Raymond Yau
2014-09-13 8:57 ` Raymond Yau
2014-09-13 10:43 ` Alexander E. Patrakov
2014-09-13 11:33 ` Raymond Yau
2014-09-13 11:36 ` Alexander E. Patrakov
2014-09-13 18:35 ` Alexander E. Patrakov
2014-09-14 11:37 ` Raymond Yau
2014-09-14 12:07 ` Alexander E. Patrakov
2014-09-15 2:43 ` Raymond Yau
2014-09-15 9:19 ` Takashi Iwai
2014-09-15 9:58 ` Alexander E. Patrakov
2014-09-15 10:08 ` Takashi Iwai
2014-09-15 17:01 ` Pierre-Louis Bossart
2014-09-15 17:14 ` Alexander E. Patrakov
2014-09-15 18:08 ` Takashi Iwai
2014-09-18 1:15 ` Raymond Yau
2014-09-21 9:22 ` Alexander E. Patrakov
2014-09-21 9:53 ` Clemens Ladisch
2014-09-21 10:56 ` Alexander E. Patrakov
2014-09-22 3:27 ` Raymond Yau
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