From mboxrd@z Thu Jan  1 00:00:00 1970
From: Clemens Ladisch <clemens@ladisch.de>
Subject: Re: Some questions about userspace control elements
Date: Fri, 06 Feb 2015 10:38:48 +0100
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To: Takashi Sakamoto <o-takashi@sakamocchi.jp>, "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>
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Takashi Sakamoto wrote:
> On Feb 5 2015 08:51, Takashi Sakamoto wrote:
>> sound/core/control.c:1188
>> {{{
>> kctl.count = info->owner ? info->owner : 1;
>> }}}
>>
>> In this code, the value in struct snd_ctl_elem_info.owner is assigned to
>> struct snd_kcontrol.count. The meaning of these two member is completely
>> different but assigned.

There is hardware that has many identical controls.  To save memory,
ALSA treats controls with .count > 1 as if there were multiple controls.

This optimization is not visible in the userspace API.

The owner field is used because there is no other field to set the
count.

> When any userspace control element are added, the value of struct
> snd_ctl_elem_info.owner is zero because there's no API to set this
> member in userspace

So far, setting this field has not been needed.  An API could be added.

>> I guess that the reason is to limit the number of event generated when
>> the control element set is operated.

If a driver needs twenty identical controls, it could create one control
with count=20, or twenty controls with count=1.  In both cases, twenty
events are sent.


Regards,
Clemens