Re: [PATCH v3 0/7] ALSA: aloop: Support sound timer as clock source instead of jiffies

[Takashi Iwai <tiwai@suse.de>]

On Mon, 11 Nov 2019 12:08:39 +0100,
Andrew Gabbasov wrote:
> 
> This patch set is an updated version of patches by Timo Wischer:
> https://mailman.alsa-project.org/pipermail/alsa-devel/2019-March/146871.html
> 
> This patch set is required for forwarding audio data between a HW sound
> card and an aloop device without the usage of an asynchronous sample rate
> converter.
> 
> Most of sound and timers related code is kept the same as in previous set.
> The code, related to snd_pcm_link() functionality and its using for
> timer source setting, is removed (as rejected earlier). The changes in this
> update are mainly related to the parameters handling and some cleanup.
> 
> The timer source can be initially selected by "timer_source" kernel module
> parameter. It is supposed to have the following format:
>     [<pref>:](<card name>|<card idx>)[{.,}<dev idx>[{.,}<subdev idx>]]
> For example: "hw:I82801AAICH.1.0", or "1.1", or just "I82801AAICH".
> (Prefix is ignored, just allowed here to be able to use the strings,
> the user got used to).
> Although the parsing function recognizes both '.' and ',' as a separator,
> module parameters handling routines use ',' to separate parameters for
> different module instances (array elements), so we have to use '.'
> to separate device and subdevice numbers from the card name or number
> in module parameters.
> Empty string indicates using jiffies as a timer source.
> 
> Besides "static" selection of timer source at module load time,
> it is possible to dynamically change it via sound "info" interface
> (using "/proc/asound/<card>/timer_source" file in read-write mode.
> The contents of this file is used as a timer source string for
> a particular loopback card, e.g. "hw:0,0,0" (and here ',' can be used
> as a separator).
> 
> The timer source string value can be changed at any time, but it is
> latched by PCM substream open callback "loopback_open()" (the first
> one for a particular cable). At this point it is actually used,
> that is the string is parsed, and the timer is looked up and opened.
> This seems to be a good trade-off between flexibility of updates and
> synchronizations or racing complexity.
> 
> The timer source is set for a loopback card (the same as initial setting
> by module parameter), but every cable uses the value, current at the moment
> of opening. Theoretically, it's possible to set the timer source for each
> cable independently (via separate files), but it would be inconsistent
> with the initial setting via module parameters on a per-card basis.
> 
> v2:
> https://mailman.alsa-project.org/pipermail/alsa-devel/2019-November/157961.html
> 
> v3:
> - Change sound card lookup to use snd_card_ref() and avoid direct access
>   to sound cards array
> - Squash commits on returning error codes for timer start and stop
> - Some locking related fixes
> - Some code cleanup

The patch won't work with the latest ALSA timer code found in my
for-next branch due to the API changes.  Essentially you need to
rewrite as:
	timeri = snd_timer_instance_new(...);
	if (!timeri)
		no_memory_error;
	timeri->flags |= ...;
	timeri->ccallback = ...;
	....
	err = snd_timer_open(timeri, ....);
	if (err < 0)
		error;


thanks,

Takashi